Drilling Problems

Contents

Chapter page

I D r il l in g P r o b l e m s a n d t h e ir R e m e d y 1

II Rig Mud Hydraulics 59

H I Factors Affecting Bit Perform ance*

81

IV Straight and Directional Oilwell Drilling, and Deviation Control

94

V Oilwell Cementing 159

R e f e r e n c e s 200

Chapter 1

Drilling Problems and their Remedy

1.1 IntroductionProblems associated with the drilling of oil and gas wells are largely due to the

disturbance of earth stresses around the borehole caused by the creation of the hole itself and by drilling mud/formation interaction. Earth stresses, together with formation (pore) pressure, attempt to re-establish previous equilibrium by forcing strata to move toward the borehole. Thus, a hole is kept open (or stable) by maintaining a balance between earth stresses and pore pressure on one side and wellbore mud pressure and mud chemical composition on the other side. Any time this balance is disturbed, hole problems may be encountered. Hole problems can be classified under three types of drilling wells, namely: v < •

1. Vertical well drilling.2. Deviated or directional well drilling,3. Horizontal well drilling.

1.2 Special Problems during Vertical Well Drilling

are:Drilling problems that may be encountered during drilling vertical oil and gas wells

1. Ix>st circulation of drilling fluid. 2. Controlling hole deviations. r '3. Sticking and torquing pipe. 4. Blowouts of fluids or gases. 5 ! '5. Sloughing or caving shales. 6. Bridging. ' , r >7. Twist-off. 8. Going back to bottom.9. Vertical well drilling. HO. Pipe washout.11. Strapping pipe to get accurate tally.

1.3 lost Circulation Problems

Lost circulation is defined as the partial oi complete loss of drilling fluid during drilling, circulating or running casing or loss of cement during cementing. Lost circulation occurs when the hydrostatic pressure of mud exceeds the breaking strength o f the formation, which creates cracks along which the fluid will flow. For lost circulation to occur, the size of the pore openings of the induced fractures must be larger than the size of the mud particles. In practice, the size of openings that can cause lost circulation is in the range 0.1 — 1.0 mm.

However, circulation may be lost if;

1. The total pressure exerted on a formation exceeds the formation pressure.2. The openings in the formation are about three l imes as large as the biggest particles

present in the mud in substantial quantity. 7,

Accordingly, the formations where lost circulation may occur can be subdivided into three* categories:....... ............................ - w, •• .

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2 Chapter I

1. Coarse permeable, unconsolidated formations.2. Vugular and cavernous formations.3. Fractured, faulted, jointed and fissure formations.

}. Formations with natural fractures.U. Formations with induced fractures.

Coarsely permeable formations, such as sands, gravels, conglomerates vary considerably in permeability. Their ability to take drilling fluids depends upon the ratio between the size of pore openings and the size of the solid particles present in the mud. Also, vugs and caverns are usually found in limestone and circulation losses in cavernous formations are usually predictable.

Formations may have some natural fractures. However, these frifctu’rks^can't always take drilling fluid. If fractures are not opened wide enough, they will not start taking mud. Even if formations do not have fractures, the latter can be created in the course of drilling operations if pressure applied to a formation is high enough to break it.

1.34 Prevention Measures

It is more economical to prevent circulation losses than to restore circulation after it was lost. As it has been stated previously, two conditions are necessary to get circulation lost:

1. Formation with openings large enough to take the mud solids.2. Pressure overbalance.

Since formation properties cannot be controlled, preventive measures should be directed to avoid any unnecessary pressure overbalance that may initiate circulation losses and cause sometimes formagiah fracturing. Also, hydrostatic pressure is usually maintained at a level sufficient to slightly overbalance reservoir pressure. However, pressure applied to formation may increase in this process of drilling operations. Reasons for pressure increase and preventive measures to be taken are given in Table (1.1).

1.3.2 Methods o f Restoring Circulation L

If despite the preventive measures taken circulation is lost, some simple methods may be tried to restore circulation before other more radical methods are used. These methods are as fellows: : <

1. Waiting period: is one of the best methods, which can be recommended in all instances.The well should be left quiet for 6 8 hours. This allows the hole to heal itself. Solidspresent in the mud probably enter the thief zone, filtration cake is deposited and mud gellation takes place. All these factors contribute to restore the circulation. The bit should be pulled to a point before the well is left quiet.

2. Reduction o f mud density: by diluting with water or by adding oil that decreases pressure of the mud column and may be sufficient to restore circulation,

3. Deliberate mud gellation: by treatment with lime, cement, gypsum or even salt may help to restore circulation in case it is lost in shallow coarse permeable sands or gravels.

4. Reduction of pump pressure by decreasing the rate ,of circulation or chemical treatment to cut down mud yield point may restore circulation due to the corresponding decrease of pressure losses in the annulât space.

Dr. M.S. Farahat

Drilling Problems 3

Table 1.1: Reasons for pressure increase and preventive measures to avoid lost circulation of drilling fluid.

Cases o f Pressure Buildup Reasons Preventive m easures

1. Increase of hydrostatic pressure.

- High solids content - Dilution with water, or removing extra solids Jby desanders, desilters, apd shale shakers.

2. High pressures at the moment of starting circulation.

- High gel strength of the mud.

- Pumps start too rapidly.

- Chemical treatment to- keep the gel strength low

- Careful and slow start of the. pumps. ri.

3. High pressure losses in the annular space

- High plastic viscosity and yield point of the mud.

- Narrowing the annular space due to thick filter cake

—Control of rheological properties o f the mu$- maintaining them at a low level.

- Proper contact of filtration properties of the mud.

4. High pressure surges caused by moving the

' drill string down.

- High viscosity and gel strength of the mud.

- Too fast running of the drill string down the hole.

- Narrowing the annular space due to thick filter cake.

- Balling up the bit and drill collars due to thick fitter cake and accumulation of cavings in the hole

.

- Proper control of mud flow properties, maintaining low viscosity and gel strength.

- Careful running down the drillstring at a limited velocity.

Maintenance of good filtration properties: low water loss and thin fitter cake.

- Maintenance o f low water loss.

- Application of emulsion muds.

- Effective cleaning of the [ hole - -'K...

If circulation is not restored by these measures, the more radical ones,, should be applied. However before taking further steps the lost circulation zone should be located as accurately as possible and evaluated.

1.3.3 Methods o f Locating Lost Circulation Zones

Usually when losses occur during drilling, lost circulation material is spotted across the suspect zone to combat fluid losses. However, in severe lost circulation cases the location of the “thief’ (or lost circulation) zone must be deteimined prior to combating fluid losses. _ There are a number of established methods used for this purpose, including temperature survey, radioactive trace survey and spinner survey.

Dr. M.S. Farobat

4 Ch apter /

1.3.3.1. Temperature survey.

A temperature recording device is run in hole on a wire to provide a record of temperature against depth. Under normal conditions, a constant increase in temperature with increasing depth is observed. This trend (as shown in Figure 1.1) is recorded under static conditions to provide a base log. A quantity of cool mud is then pumped in the hole and another survey is made. The cool mud will cause the device to recorded a lower temperature than previously recorded, down to the “thief* zone where mud is lost. Below the thiefzone, the mud level is static and its temperature is higher than the mud flowing in the thief zones.

It follows that the new temperature log will show an anomaly across the thief zone, and the location of this zone can be determined by reading the depth at which the temperature line changes its gradient. Figure 1.1 shows two logs, one under static conditions and the other with flowing mud. Location of the lost circulation zone is clearly seen,

1.3.3.2. Radioactive tracer survey

" A gamma-ray log is first run to establish the normal radioactivity of formation in hole and act a basis for comparison. A small quantity of radioactive material is then displaced into the hole around the area where the thief zone is expected. A second gamma-ray log is run and compared with the base log. The point of loss of the new log, where the radioactive material is lost, is the thief formation.

1.3.3.3. Spinner survey

A spinner attached to the end of a cable is run in hole to the place where loss of circulation is suspected. The spinner will rotate in the presence of any vertical-motion of mud such as encountered near a “thief* zone. The speed of the rotor is recorded on a film as a series of dashes and spaces.

The spinner survey method was found to be ineffective when large quantities of sealing (lost circulation) is used in the mud.

1,3.4 Severity of tost circulation *

Before taking any corrective measures the type of the loss and the verity should be determined. The type of the thief zone can be best determined from lithology although some features may help to identify the type of loss.1. A circulation loss to unconsolidated formations is accompanied with gradual lowering of

mud level in pits. If drilling is continued the loss may become complete.2. Circulation lost to formations with natural fractures is evidenced by gradual lowering of

mud in pits. If drilling is continued and more fractures are exposed, complete loss of returns may be experienced.

TMnfWratur«

Dr. M.S.Farahat

Drilling Problems 5

3. Circulation loss to induced fractures may occur in any type rock. However it would beexpected in formation with characteristically weak planes. This types of lost returns is more likely to happen if the density of mud exceeds 1.25 gm/cc. ;

4. Lost circulation to cavernous formations is normally occurring in limestone sections. Circulation may be lost suddenly and completely Before circulation is lost drilling may be “rough” and the bit may drop from several centimeters to several meters just proceeding the loss.

The severity of the loss is best determined by the amount of loss and the static level of the mud column in the well.

One of the classifications of lost circulation zones by severity of the loss subdivides them into five categories:

1. Seeping loss 0.2 - 2.0 cu m. /hr.2. Partial loss 2.0 - 8.0 cu m./hr.3. Complete loss-with the static mud level at the depth of 60-150m.4. Partial or complete loss to deep induced fractures.5. Severe complete loss -with the static mud level at the depth 150-300 m. the'loss to

honicomb, fractured or cavernous formations with water moving within or into the loss zone.

Several methods of restoring circulation exist and can be used. However a ’technique selected to combat lost circulation should be correlated to severity of the loss otherwise if an improper technique is selected it will result only in wasting materials, time and money.

1,3.5 Combating lost circulation

Loss of circulation has a number of determined effects, which can be summarized as follows: (a) loss o f drilling mud and costly constituents; (b) loss of drilling time; (c) plugging of potentially productive zones; (d) blowouts resulting from the decrease in hydrostatic pressure subjected to formations other than the thief zone; (e) excessive inflow of water; and (f) excessive caving of formation.

r ■ Loss of circulation can be reduced or cured by one of the following methods: ?’i’■ '< " . . , •• ~V- ' “- "if

1, Reducing mud weight until the hydrostatic pressure o f mud is equal to the/formation pressure.

2. Spotting of a pill of mud containing a high concentration of bridging materials against the thief zone. Bridging or lost circulation material s may also be used as additives in the circulating mud during drilling of formations susceptible to loss of circulation.

Lost circulation materials can classified as fibers, flakes, granular material, and a mixture of all three. The fibers include plant fibers (such as hay or wood shavings), glass fibers, mineral fibers and leather. The flakes include cellophane, mica, cotton seed hulls, nut hulls, etc. Granular material includes ground rubber tyres, crushed rock, ground asphalt, asbestos, etc.

The fibers and flakes were found to be effective with low mud weight, while granular materials are best suited to weighted muds. Blends of mica and cellophane, and fibrous, flaky or ground materials are particularly effective a gradation of size which can build up an effective seal.

Lost circulation material is normally mixed with a sufficient quantity of mud to prepare a pill which can be pumped to the lost circulation zone. The pill is spotted against- the thief zone and gradually squeezed into the formation while the mud level in the annulus is continuously observed, if the mud level is still falling when the complete pill is

Dr. M.S, Farah.it

* Chapter /

squeezed into the zone, another pill is prepared and the procedure is repeated until the ' loss of circulation is stopped.

For severe lost circulation, the bridging materials form part of the mud additives and must, therefore, be capable of being pumped through the hole without causing severe pressure losses.

The severity of any lost circulation problem is related to the width and length of fractures created by the excessive hydrostatic pressure. Thus, for lost circulation to be cured, the openings or the fractures must be tightly packed with lost circulation materials. This can be achieved only if the lost circulation material contains a gradation of size such that large particles form bridges through the pore or fractures and small particles pack off the spaces between the large particles. Such a size distribution will produce an effective seal.

The performance oflost circulation materials is pressure-dependent; a good seal at 1000-psi differential pressure may fail at a higher pressure of, say 2000 psi.

When lost circulation materials are used as part of the circulating mud additives, the shale shakers should be by-passed, to prevent the loss of these materials through the shakers. >

3. Spotting of bentonite-diesel oil or cement-diesel oil plugs across the thief zones. Several plugs may be required in die case of bentonite plugs loss of circulation is spotting.

When spotting cement plugs, a wait-on-cement (WOC) time must be allowed for prior to resuming drilling, to permit the cement to set and Seal the pore and fracture space of the thief zone. Cement plugs are normally spotted as a last resort when everything else fails. Bentonite or cement plugs are spotted with an open-ended drill pipe (OEDP).

4. Adoption of special drilling methods such as blind drilling, drilling with uirldertralance or drilling with air. Blind drilling refers to drilling without returns at the surface, soithat the generated cuttings are used to seal off the fractures of the thief zone. The hydraulic programme must be adjusted so that the cuttings have sufficient annular velocity to reach the thief zones. For this technique to be effective, a plentiful supply of water is required to replace the mud lost to the formation.

Example 1.1During drilling of an 8-1/2 in hole at 8000 ft, a complete loss of circulation was

observed. Drilling was stopped and the mud level in the annulus was observed to fall rapidly. The well was filled with water o f 62 pcf density until the annular level remained stationary. If the volume of water used was 65.7 bbl and mud density 75 pcf, determine the formation pressure and the new mud weight required to balance the formation pressure. Assume the intermediate casing to be 9-5/8 in, 40 # set at 6000 ft drillpipe is Grade E, 5 in OD.

SolutionCapacity of annulus between 5 in drillpipe and 9 5/8 in casing

Height of water column ~ 1276 H.0515“ '■M. "U* i " - » - . . . . ft

When the well is balanced, we have

Formation pressure = pressure due to mud column + pressure due to water column

= 0515 bbl/ft.

Dr. M.S. Farabat

Drilling Problems 7

75 *(8000-1276) 6:2*1276 .= ----- — ----------- - + ----------- = 4051 psi144 144 ^

(Note: Hole depth = 8000 ft, of which 1276 ft is filled with water and 6724 ft with mud.)

144 *4051Re quired mud weight = — = 72.9 = 73p c i

Practically, lost circulation is a very expensive problem to deal with. To cure most zones, lost circulation material (LCM) is added to the mud.

When a drill bit penetrates a lost circulation zone, the usual procedure is to pull the bit one foot off the bottom and reduce the pump strokes to about one - half normal operation. This will reduce the equivalent circulating density (LCD) and allow time for LCM to be mixed. Lost circulation material must be mixed sv/iftly to solve the downhole problem. A quick calculation can be made to determine how fast the LCM will hit the lost circulation zone.

Assuming there is lost circulation at the bottom of the wellbore; the Surface-to-BU time (in minutes) is calculated by the formula:

SioB (bbl/ ftDP)(depth) + (bb/ ftDQjdepth)( bbi / stroke){ strokes / min)

DP = drill pipe DC = drill collar

Example 1.2At 8,000 ft with a 4.5 in. XO drill pipe weighting 16.6 lb/ft to 7,408 ft, the capacity

can be found from the cement book to be 0.01422 bbl/ft with a BHS of 592 ft and a capacity of 0.01776 bbl/ft. calculating 0.09 barrels per stroke and 60 strokes per minute, we get:

(0.1422 bbl! ./?)(7,408 ft) + (0,1776 bbl! /?)(592 ft)(0.09 bbl! stroke){60 strokes! min)

105.34 +10.51 _ 115*5 , . .5.4 ” 54

21.45 min

StoB =

S to B -

S t o B -

As calculated, in 21.45 minutes the LCM w 11 hit bottom. So after a short circulating time, the problem should be solved. If the problem is serious, the shale shaker can be bypassed to keep from losing the LCM. Once the mud pits stop showing a loss, drilling may be resumed, but must be watched closely (Figure 1.2 ).

It is also important when fighting lost circulation to keep the pipe moving, since>the ? pipe could become stuck due to the permeability of the hole and the possible heaving of 4hey formation. The pipe should be rotated for five minutes and then the kelly lifted and reaped >■ back down. Lost circulation is dangerous and should not be taken lightly.

Many blowouts have resulted from mud going into the formation and gas coming back up the hole (see Figure 1.3). When liquid mud is available, the consultant should have it sent to location so he doesn’t run out of mud. Always remember to reduce the pump to one half normal operation range to give you more time to mix the LCM with the mud.

Dr. M.S. Faraliat

8 Chapter I

..... Sometimes thelost circulation problem takes days to solve and can cost thousands of dollars. If the zone cannot be sealed, it will tax the brains of the consultant and the engineer. Putting a cement plug downhole sometimes will there the problem. Then you can drill it out and go from there. Some wells have been abandoned because the zone could not be cured; however rare, this has happened. Sometime pumping a large slurry of LCM downhole, pulling ten stands of drill pipe, shutting off the pumps, and letting the slurry sit overnight works.

When a lost circulation zone is encountered, the mud bill increases drastically.The operator maybecome alarmed but you Figuraneed to stay calm during this problem so you can thinkclearly and keep operations running smoothly Usually the problem can be solved.

A loot circulation zone.

1.3.6 Field techniques to restore circulation corrected to severity

1.3.6.1 Seeping losses

In case of a seeping loss circulation can be restored by using one of the following methods:

1. Waiting period.2. Plugging the loss with lost circulation materials.3. High filter loss slurry squeeze.

Waiting period as a method of restoring circulation has been discussed previously.

1.3.6.2 Plugging with bridging materials in the mud.

Any granulated fibrous or flake which enters the channels in the thief formation can bridge the channels if the sizes of lost circulation material particles correspond to the sizes of

Dr. M.S. Kara hat

Drilling Problems 9

channels. The bridging material, and mud solids may form plugs in the channels and stop the flow of mud into the loss zone. Bridging or lost circulation materials can be classified into three main group:

v - 6

Figur* I ■ •> UndertwJanc*d mud in a lost circulation zone.

1.3.6.2 Plugging with bridging materials in the mud. ».

Any granulated fibrous or flake which enters the channels in the thief formation can bridge the channels if the sizes of lost circulation material particles correspond to the sizes of channels. The bridging material, and mud solids may form plugs in the channels and stop the flow of mud into the loss zone. Bridging or lost circulation materials can be classified into three main group:

1. Granulated materials: nut shells, fruits pits, rubber, perlite etc.2. Fibrous materials: cane, hay, straw, cotton, wood, leather, asbestos fibers.3. Flake material: cellophane, mica, cotton seed hul'ls, sunflower seed hulls etc.

Efficiency of lost circulation materials does not depend very much on the concentration of it in mud but rather on the size of particles and the proportion between the amounts of particles of various sizes.

High content of a coarse bridging agent results in forming a plug on the wall of the hole. Such plug cannot provide good seal as it is easily destroyed by the drill string and the bit.

A lack in coarse bridging agent can make it impossible to obtain a seal as fine particles will easily pass into the formation without any plugging effect. The proper proportion between

Dr. M.S. Farahat

10 Chapter /

lost circulation materials of different types and the proper proportion between coarse and fine particles will provide the best result. The lost circulation zone will be successfully plugged.

1.3.6.3 Application o f lost circulation materials.

Two methods can be used to cure lost circulation with bridging agents.

1. Adding bridging agents to the entire mud system,2. Adding bridging agent to a portion of mud, which will deliver the agents to the loss

zone and subsequent squeezing them into the formation.

1.3.6.4 Addition o f lost circulation materials to the entire mud system.This method can be recommended only for fine lost circulation materials. Application

of coarse bridging agents to the whole mud system is not economical by the following reasons:

^ Some materials lose strength and the ability to seal.ts. The shale shaker should be by passed and this results in solids content increase and mud

density build up,î*. Pressure losses increase due to the presence of bridging agents in the mud,

The bit may plugged, t*. The turbodrill may be plugged,"a. It is difficult to get tools to bottom,a . Large amounts of lost circulation materials are required.

a1.3.6.5 Application o f bridging material in spotting slurries

It is more economical to mix lost circulation materials to some volume pf the mud, which was in use at the time of circulation loss. Usually the volume of the mud and bridging agent mixture rringes ffbm 15 to 80 m \ The mixture may contain about 40 - 50 kg/m5 of coarse granulated material, about 15 kg/m3 of coarse to medium fibrous materials, about 15 kg/m3 of medium to fine fibers, and about 15 kg/m3 of a flake material.

The plug should be placed through the open - ended drill pipe opposite the thief zone. The mixture is pumped slowly, at the rate of 2.5 - 3 L/sec until the materials stop the loss. When the level of mud reached the top of the well, preventers should be closed and the plugging material squeezed carefully into the loss zone at the pressure 3.5 kg/m2 on the annular space. Such pressure should be maintained for 30 minutés.

If the hole does not fill the procedure should be repeated once again. In case of a failure to restore circulation with this technique the high water loss slurry squeeze should be used.1.3.6.6 High water loss slurry squeeze.

This method can be used to combat not seeping losses but partial and not very severe complete losses.

This method uses high water loss slurries containing an appropriate amount of properly selected mixture of various lost circulation materials. Bridging material which is present in the slurry forms a screen in the channels of the rock. Fast filtration of the liquid phase of the sluny through such screen results in quick depositing of the filter cake inside the channels of the thief zone, plugging the channels and restoration of circulation.

There are slight differences in application of the jnethod to lost circulation zones of different severity. However the main distinction is an increase is an increase of the size of bridging agent as losses become more sever.

One of possible composition of a high water loss slurry for combating seeping losses is as follows:

Dr. M.S. Farahat

Drilling Probl ems 11

Attapulhite 2 8 -5 7 kg/m3. Diatomaseous earth 140 kg/m3. Fine nut shell 14 kg/m3. Shredded leather 3 kg/m3.

Lime 1.5 kg/m3.Fine mica 14 kg/m3.Medium to fine fiber 12 kg/m3. Water up to 16 m3.

The slurry should be pumped to the thief zone through the open end drill string, and then carefully displaced and squeezed into the loss zone at closed preventer and opened mud gun. .

1.3.6.7 Partial circulation losses.Method to combat partial losses are similar to those applied in case of seeping losses:

plugging with bridging material in mud and squeezing with high water loss slurries. As partial losses occur in formation which have larger openings bridging material o f larger" size is required.

1.3,6.8 Complete losses with the static mud level at 6 0 - ISO m. _Several methods of restoring circulation c an secure good results: plugging with

bridging material in mud, squeezing with high water loss slurries, containing granulated flake and fibrous lost circulation material of large size ( V* - V i'). If the channels in the formation are so large as they cannot be plugging with lost circulation material the lost circulation zone should be isolated with one of the cement containing mixtures. - , •

1.3.6.9 Sealing o ff lost circulation zones with cement.Cement is a-material which, being mixed with water, is able to solidify and turn into a

high strength stone.If cement enters the channels in the formation, fills them and gets solid it will plug the

channels and stop circulation loss.Cement can be used alone (neat cement) or in combination with some other materials

which improve the sealing ability of cement.Neat cement is often used for plugging lost circulation zones because this material is

usually available at any drilling rig. However neat cement cannot be considered as a good sealing material because: is has a long setting time, cement slurry has poor thyxotropic properties and a small angle of repose. For these reasons slurry mixed of neat cement Cab flow deep into the thief zone without plugging it. Such slurry, while it remains liquid,- can be diluted and washed away with water present in the los.s zone.

Sealing ability of cement can be considembly improved by adding bentonite or gilsonite to cement

1.3.6.10 Bentonite or gel cementBentonite cement is formed by adding cement to a suspension of pie-hydrated

bentonite. Concentration of bentonite constitutes usually 4 - 8 % by weight of dry cement. Where strength of the plug is not important the amount of bentonite maybe increased up to 25 %. When cement is added to a suspension of pre-hydrated bentonite the resulting slurry has lower density, higher gel strength and higher set strength compare to a slurry obtained by mixing dry cement and bentonite with water.

A mixture of cement with bentonite has an ability to develop the gel structure rapidly. Very high gel strength is responsible for a high angle of repose of the slurry and a good sealing ability of it.

Dr. M.S. Farahut

12 Chapter l

As the setting time of bentonite - cement slurries is rather long, this mixture can be successfully used if there is no water flow in the loss zone. Water, flowing through the thief zone, can easily wash the slurry away before it sets.

1.3.6.11 Gilsonite - cement slurryGilsonite is a light density material which resembles asphalt in colour. Being added to

a cement slurry gilsonite reduces its density and acts as a bridging agent. Both of these functions help to keep the slurry in the vicinity of the well bore. The concentration of gilsonite in cement slurry usually ranges from 25 to 100 % by weight of dry cement.

. Not only gel cement or gilsonite cement slurries can be used for sealing off lost circulation zones. Any other mixture prepared on the basis of cement can be used.

A cement slurry is brought to a lost circulation zones through the drill string equipped with a cementing sub. The slurry should be pumped and displaced out of the drill string carefully so the pressure balance between formation and hydrostatic pressures was maintained tti avoid a break of the mud or formation fluid through the cement plug squeezed into the loss zone.

, r.fit: 'ol :>!

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• *»f« H'

1.36.12 Diesel oil bentonite cement slurry | ;I* a mixture of cement bentonite and diesel oil there is no interaction between the

components. Such mixtures can remain liquid for indefinitely long time. However, as soon as such slurry comes to a contact with water or mud it forms a plug of stiff consistency. If the contact of the slurry takes place in the well near the thief zone and in the thief zone itself the plug squeezed into the formation channels will seal them and restore circulation.

' Solid components of the slurry are usually added to diesel oil in the folloVing proportion: 300 kg of cement and 300 kg of bentonite per 1 m ^of diesel oil. The slurry is brought to a loss zone through the open end drill string. 1-1.5 m3 of water free diesel oil should be pumped ahead and behind the slurry to prevent1 anycontact of the slurry with mud inside the drill string. When the first diesel oil cushiorl teaches the end of the drill string preventors should be closed and mud should'be pumped into the annular spaces. The ratio of the slurry and mud volumes should 'be maintained at the level of 2:1. The plugging paste which isi formed near the end of the drill string is squeezed into the thief zone and left there for setting. It seals the fractures and caverns and helps to restore circulation.

... i.1.3.6.13 Partial or complete losses to deep induced fractures

If induced fractures are the reason of lost circulation soft plugs may be the best remedy to restore circulation. Soft plugs arc formed of materials, which being mixed with water or special aetivators, turn into strong gels. Such gels fill the channels in formations, seal a thief zone off and help to restore normal circulation.

Two types of soft plugs arc distinguished: f . Surface mixed soft plugs, n2. Down hole mixed soft plugs.

' ' • t’-\r1.3.6.14 Surface mixed soft plug

Surface mixed soft plugs have been differentiated form those mixed down hole because on many occasions surface mixed “Form A Plug” has outperformed diesel oil bentonite plugs mixed down hole. '

“Form A Plug” is a trade name of a small a special'time setting clay cement. It consists of a specially selected clay, a small amount of alumina silicate and a retarder. The clay in this mixture has a unique tendency to time harden when hydrated. Alumina silicate is used to

Dr. M.S. Farahat

Drilling Frobiems 13

increase the strength of the set clay cement, and a retarder is used to increase the pumping time of the slurry during placement.

Initial slurry properties, thickening time, final set strength are primarily functions of concentration of clay cement solids and the temperaiure encountered.

“Form A Plug” must penetrate the loss zone a sufficient distance to offer resistance to movement after the strength of its gel has developed.

The method of applying closely resembles a cementing operation. “Form A Plug” can be mixed with fresh water using the jet hopper. After mixing it is pumped directly into the drill string and then displaced to the loss zone with drilling mud pumped simultaneously into the drill pipe and the annular space. When almost all the slurry is displaced out of the drill string the preventor is closed and the slurry squeeze d into the thief zone, by careful pumping the mud into the annulus. The well is then shut in and held under squeeze pressure for 8 hours. The slurry set in the well and inside he loss zone will seal it and circulation may be restored. If the first operation is a failure the procedure should be repeated with larger amounts of ”Fotm A Plug” ...... * r :i

1.3.6.15 Down hole mixed soft plugA common feature of all soft plugs mixed down hole is that mixing and interaction of

components which form the plug takes place near or inside the loss zone. Close attention should be paid to bridging the right amounts of components and mixing them near the thief zone, as the proportion of the components determines the success of the plugging operation. There are several compositions used for soft plugs mixed down hole. v

1.3.6.16 Diesel oil-bentonite soft plugThe solids to liquid ratio in the slurry is similar to that of the diesel oil bentonite

cement slurry. The diesel oil bentonite slurry is prepared by mixing 600 kgs of bentonite with lm 3 of diesel oil. The slurry is pumped and displaced to the loss zone through the drill string, white mud is simultaneously pumped to the thief zone through the annular space. Coming into contact with mud the mixture of bentonite and diesel oil the latter turns into a phste which being squeezed into the loss zone plugs the channels in the formation and restores circulation.

1.3.6.17 Bengum Bentonite Diesel oil plugBengum No. 1 is a natural gum mixed with a preservative and a complexing agent.

Bengum is used with bentonite in proportion 1:9. The mixture of dry materials is mixed with diesel oil. (1 m3 diesel oil - 800,925 kg bengum - bentonite). ; '

Because of organic compounds it contains, this mixture sets harder than the dieseT oil bentonite combination particularly when it is mixed in salt water or mud. The used for the diesel oil bentonite of the plugging material into the loss zone is similar to that used for the diesel oil bentonite plug.

1.3.6.18 Mil-Squeeze soft plugMilchem’s mil -Squeeze is a two compound system consisting of copolymer slurry

and 15 volume % of an activator or setting fluid. Ifre activator is placed in the drill with a spaces of crude or diesel oil. The activator is separated from the mud and slurry with a spacer crude or diesel oil. The materials are pumped to the loss zone through the drill string. As they leave the pipe they mix in the well bore and in the thief zone and flow a highly galled mass. When the well fills the preventor is closed and the plugging material; is carefully squeezd into the loss zone. The plugging mixture will fill the fractures and stop the flow of mud>into>flie formation.

Dr. M.S. Farahat

14 Chapter I

1.3.6.19. Severe complete losses(Long interval o f honeycomb fractured or cavernous formation, Mud static level 150 - 300 m down, Water moving within or into the loss zone)

Circulation losses of this type can be combated by some of the methods previously discussed. High water loss slurries containing large size bridging material or large amounts of Diesel oil bentonite cement slurry may be helpful in restoring circulation. However if the channels in the loss zone are very large or there is an intensive flow of water in the thief zone these methods will not be of any help, and other methods to restore circulation should be used.

1.3.6.20 Application o f AC-set cement tool

The standard AC-set cement tool consists of aluminum pipe covered with a nylon bag. The bag can expand up to 1.27 m in diameter when it is filled with cement slurry.

* The tool is run down the hole at the lower end of the drill string and set opposite the loss zone. Cement slurry is then pumped into the nylon sack which expands into the caverns. A part of missing water filtrates from the slurry, but cement can not leave the bag which holds it in the vicinity of the well bore.

Then the cement slurry required to fill the bag and to compensate the filtrate loss is displaced to the bag, the tool is disconnected from the drill string and left in the hole for setting the cement slurry. The latter sets and plugs the caverns, helping to restore circulation.

The standard AC-set cement tool is about 9 m long. Therefore it can handle up to 6 m of caverns. In case an interval of cavernous or honeycomb formation is long “ blind” drilling or drilling with aerated fluid should be used with subsequent setting a casing string.

Blind drilling is drilling without circulation return. Cuttings are washed into the loss zone partly and partly may remain in the hole As soon as all the thief zone is drilled through, casing string should be set because blind drilling requires very big volumes of drilling fluid and is not absolutely safe against pipe sticking.

If the thief zone does not accept all the cuttings and the danger of pipe sticking is acute it will be necessary to remove cuttings by maintaining circulation of an aerated drilling fluid. In this method drilling fluid air are brought into the well. Air present in the drilling fluid considerably decrease of the fluid column exerted on the loss zone. Low hydrostatic pressure makes possible to restore and maintain circulation while drilling the loss zone. After the thief zone is drilled a casing string must be set.

1.4 (¡Controlling Hole Deviation Problems

.' F,.

Controlling hole deviation will not be a major problem if some simple rules are followed. First, you should take survey every 500 to 700 ft while drilling. In the shallow part of the well a wireline survey can be run, but at the well gets deeper a “drop survey” will need to be run at the end of each bit run. (Most bit runs will be under 500 ft) This will give a good picture of the hole while drilling. A deviation chart (see Figure 1.5) should be kept on the hole for a clear picture and for locating dog legs and keyseats.

The following is an example of what steps might be taken as the survey information is gathered. '■ ' !

Dr. M.S. Farahat

Drilling Problems 15

1000 I.

Example 1.3

For the first 6000 ft the deviation is between 0.5°, and 1° which is accepted.Then the bit deviates 1.5°, and must be brought back on the right track. The first step is to take weight off the bit by reducing the weight to about 25000 lb. pump strokes are increased eight strokes per minute, raising the pressure about 200 psi. this in turn increases the RPM about ten turns - say, from 90 to 98 RPM.

The next survey at 6800 ft shows a deviation of 2° - the deviation has slowed down but the hole is still going off course.'Fhe weight on bit is decreased to 20000 lb while the RPM is kept the same, in an attempt to use the weight of the collars to create a pendulum effect on the string and thus bring it back to center. The rate of penetration will decrease, but this approach is necessary. The time to stop the hole from kioking off is while you are drilling.

The next survey is at 7300 ft and shows a 1.5° deviation. So the reduction of weight hole caused the string to bend back towards a straight hole. Weight on the bit should be increased to 25000 lb and the RPM maintained about the same. As a result of the increased weight, the rate of penetration will improve. The next survey at 7800 ft shows a 1° deviation, which

. means deviation is under control.Next, a fault line is encountered, which in many areas will cause a rapid hole deviation,

because the bit will follow the path of least resistance. The bit will follow the fault line as long as the fault goes undetected. The next survey at 8250 ft shows 3° deviation, and immediate action is needed.

We have a good stable string, so we must rely on weight reduction again. We go back to 20000 lb and the unavoidable decrease in penetration rate. The next survey is at 8570 ft, and the deviation is 4°, so weight reduction has not done much to stop the bit walking in die faulted zone. The only thing to do is further reduce weight and start reaming each drilled joint down twice. This will widen the hole toward center (see Figure 1.6). The weight is reduced to 15000 lb, with the resulting penetration rate being very slow.

He next survey is at 9100 ft and shows a 2.5° deviation. The hole is coming back in but a little too fast. A dog leg can be formed if the string comes back to center at too rapid a rate. The weight is increased to 20000 lb to stabilize the deviation and to allow you to bripg it in slower.

A rule of thumb to use to determine the footage to control deviation is 2.5 times BHA length. If you kick off, say 3° or 4° and BHA is 575 ft should be enough footage'to keep a dogleg out of -si;,:, -arr

Hgura > Hoi» davtettor] chart.

Dr. M.S. Farahat

16 Chapter /

the hole. If you bring the string back slowly, stay calm and take your time, you will stay out of trouble.

When a well kicks off, it kicks off in a spiral path, not a vertical one. The hole, if it could be seen, looks like a corkscrew. This is why it can be brought back to center easily. By running a gyro or multishot survey, the hole can be accurately shown on a graph and the direction of the hole recorded.

Always keep the operator informed about what is going on and what is being done to correct the problems you have encountered. When in trouble keep the company engineer update on successes and failures. Sometimes he may want to change and make decisions from his office, but this does not happen often unless he is new on, the job. 'Once he makes a few decisions, he will generally allow the consultant to handle things from then on. If the operator’s engineer radically changes you program, you should call your boss first and discuss what is going on. If your boss sees a problem, he should call the company engineer and discuss the idea with him before letting you use it. Most of the time when the consultant has reached that depth on the well, the operator trusts his judgment.

Figure l Reaming the hote to bring ft bee* to canter.

The main thing to remember in hole deviation is that reducing the weight and increasing the RPM will solve the problem in most cases.

• 111 ■ ----------- ---— ------------------------------------------------ " --------■ '------------------------- — ------------------------------------------------- ------------------ --------- !----------------------

J*’ts

1.5 Problems of sticking and Torauine Pine t ^Drill pipe sticking is the phenomenon, when due to one or another reason the drill

string cannot be pulled out of the bore hole at an overpull equal to the tensile strength of the drill pipe.

Stuck pipe is one of common problems encountered in drilling. It results in loss of time due to necessity to free the drill string. If attempts to free the drill string are failure stuck pipe requires fishing which may take long time and be insuccess fill. In the latter case a part of the drill string is lost and it should be sidetracked: that is a part of the bore hole should be drilled again. 1 . '

Drill pipe sticking may occur due to several various reasons. The following factors may cause sticking:

•— ur. r o a m r r a H T------ - - ~ -----------


1. Pressure differential.2. Thick filter cake and narrowing the bore hole.3. Balling the bit, tool joints, and drill collars.4. Shale sloughing.5. Accumulation o f cavings and cuttings in the annular space.6. Key seats. i- .7. Mud thickening. v8. Carelessness o f personnel. - *

Also, some definition for condition that causes sticking pipe those a re ;. . .

th e hole sloughing in and around the bit or drill collars - Most hole sloughing can be prevented by adding gel and be weighting-up some. If shale is sloughing in the hole, the mud can be treated with an asphalt-base chemical that will prevent water in the hole from getting behind the shale and pushing it into the wellbore or causing the shale to swell up and push out the wellbore »

tak The m ud not cleaning the hole properly - In most cases gel will improve this problem by adding viscosity and bridging up the cuttings.

'ss. A dog leg - .th is may cause dragging or torquing problems that could lead, to the

. formation tearing up and sticking at the collars or around the bit. (See Figure 1 7).

Keyseating - this is caused by the pipe wearing into the side of the wellbore so that the string gets stuck as it is pulled through the keyseat. Normally a three - to six - point roller reamer will solve the problem by wiping the hole every trip. Also the stabilizer will help greatly in wiping the hole.

Ssk Drill collars - these can stick if there is a washout above the bit. I f there is a washout, the pump pressure will decrease slightly until the washout gets bigger. The bit should be pulled off bottom and the system checked for leaks. If none

Dr. M.S, Farahat

18 Chapter /

are found, then the decrease in pump pressure is probably due to a washout. The consultant must always watch for this problem, because sometimes a new driller will kick up the pump strokes to maintain the pressure That should never be allowed to happen since a twist-off could result or the string could become stuck below the washout because the hole is not cleaning properly due to less fluid reaching the bit (See Figure 1.9).

/. 5.1 Differential pipe stickingr

Differential pipe sticking arises when the differential pressure (the differential between hydrostatic pressure of mud and formation pore pressure) becomes excessively large across a porous and permeable formation such as sandstone or limestone (Figure 1.10). Other conditions must be conductive to differential sticking include a thick filter cake and when a drill string is left motionless for some time inside the open hole. Differential pipe sticking can normally be recognized when pipe movement in the upward or downward direction is impossible but free circulation is easily established, since obstruction exists on only one side o f the pipe. In a complete stuck pipe situation, neither circulation nor pipe movement are possible Figure 1 10 gives a schematic drawing of a differentially stuck pipe.

From Figure 1.10:

•s*

WASHOUT OM PIPS TH « HOLLINO FLUID IS SSCAFWO

FILL COVSMNO THS W T, M C A U S I OF «A D MVMUWUCS

Figura i Problems caused by pip« washout.

Flo- ' Differential pipe sticking.

Differential force = (Hs - Pj) * area o f contact * frictionfactor. ( 1 - 1 )Where

H, = hydrostatic pressure of mud; and

----- wr Dr. M.S. Farahat


Pf = formation pressure.Area o f contact — thickness ofpermeable ¿one * thickness o f filter cake

- h * t. ( 1 ~ 2 )

The friction factor (which will be denoted by f) is used to allow for variation m the magnitude of contact between steel and filter cakes o f different composition.

Substituting Equation 2 in Equation 1 yields:

Differential force DF = (Hs - Pjj * (h * t) * f ( 1 - 3 )

In imperial units, Equation (3) becomes:DF = (Hs - P f psi *h(ft *]2in/ft) * t (in) * f JDF - 12 (Hs ~Pj) * h * t *f. ;( 1 - 4 )The magnitude of the differential force is very sensitive to changes in the values of the

contact area and the friction factor. Which are both ti me-dependent. As the time in which the pipe is left motionless increases, the thickness of the filter cake increases. Also, the friction factor increases by virtue of more water being filtered out of the filter cake.

The differential force is also extremely sensitive to changes in differentijal pressure (H, - Pf). In normal drilling operations an overbalance of between 100 and 200 psi is maintained. Excessive overbalance may arise as a result of the following situations: (a) sudden increases in mud density resulting in an increase in the hydrostatic pressure of the mud and* in turn, in the value of the overbalance; )b) drilling through depleted reservoirs and pressure regressions.

Pressure regression is encountered in deep drilling when the formation pressure gradient is recorded.

Figure 1.11 gives a possible picture of the situation at the start ofdiftfcrential sticking and after several hours.

Rfl., l it Development of differential sticking with time: (a) initial; (b) after several hours.

Example 1.4

Determine the magnitude of the differential sticking force across a permeable zone of 30 ft in thickness using the following data: differential pressure ~ 1000 psi; thickness of filter cake = Vi in; friction factor = 0.1.

From Equation (4):Solution

DF = 12 (Hs - Pf) * h * t * f.= 12*1000 psi * 30 ft *'A in *0.1 = 18 0001b.

Far ah ai Y ¡l/Si: ■ .

20 ‘ 1 Chapter /.......... • .... ■ — — * 1 ' 1 11,11 l — 11 " '

Example 1.5

A drill string consists o f 15000 ft of 19 5-lbm/ft drillpipe and 500 ft of 150 Ibm/ft drill collars. It is established that the drill string is differentially stuck at the first drill collar below the drillpipe

Given that:Mud weight =* 13 ppg.Differential force = 1080001b

. Grade o f drillpipe available are E, X, G and S.

Determine:(a) the buoyant weight of the drillpipe; (b) the total hook load when pulling on the differentially stuck pipe; (c) the magnitude of the overpull (MOP) margin for the four grades, if the pipe body yield is 394,600 lbs, assuming the conditions of pipes to be premium ,■>

Solution r ■ ...• , o fit; Kfit.': '-I V ’ "(a) Note that only the weight of drillpipe will be considered, as the weight o f drill collars is

take« up by the stuck point ■. jBuoyant weight o f drillpipe <«= 1$ 000* 19.5 * BF

= 15000 * 19.5 * f l -v, v . 489.5 J= 234.394 lb

(b) Hook load including differential force = 234.394 + 108000 = 342.394,

(«) This part can best be illustrated in tabular form, as follows:

...,l“ ' * ib ,- ' ’ ■ MOP

Grade WeightLbm/ft

Pipe body yield* Lb

Yield strength - hook toadLb

E 19.5 * 311540 -30854X 19.5 394600 52206G 19.5 436150 93756S 19.5 560760 218366

Thus, for the existing conditions, Grade E gives a negative MOP, implying that the pipe will part if the required force o f 342 394 lb is applied to free the pipe. Only grades X, G, and S can be used in this type o f well, where the magnitude of the differential force is 108 000 lb

1.5.1.1 Prevention o f differential sticking

Observation of Equation (4) shows that the differential sticking force can be reducedby:

1. Reducing the differential pressure, H, - Pf. This means drilling with a minimumoverbalance necessary to sustain formation pressure and to allow for surging and swabbing effects. Mud density increase can be monitored by controlling the rate o f penetration, especially in large holes where large quantities of drill cuttings are

Dr. M.S. Farahat

Drilling Problems 21 >■ ; 4 £'

produced, resulting in an excessive increase in mud density and, an excessive increase in the value of the differential pressure, Hs - Pf.

2. Reducing the contact area, h * t. Since the thickness, h, of the porous formation cannot be physically changed, the contact area can only be decreased by reducing the thicknessof the filter cake, t. “

This, in effect, means reducing the solids in mud to a minimum and using a mud o f low water loss. ''T

The friction factor, f, is directly related to the rate of water loss, and its value should be kept to a minimum muds appear to be ideal for drilling formations susceptible to* differential sticking, if conditions allow. r: ■ 'Vrjr.yM

The contact area is also related to the area of pipe steel in contact with the permeable formation. Most pipe sticking problems are associated with drill collars, and the ideal solution is to use drill collars with minimum surface area. A spirally grooved drill collar has 50 % less area than a smooth drill collar and consequently, produces half as much differential sticking force. The reduction in surface area of drill collars reduces the weight by only 4 - 7 % and, if extra weight is required, additional drill collars can be used.

The contact area can also be reduced by using stabilizers, which centralize the drill collars within the hole. '?:>>"

3. Since both contact area and friction factor increase with time, a reduction in the time during which the drill string is kept stationary directly results in less chance of severe differential sticking.^

4. Oil and walnut hulls can be used to reduce the friction factor, f, when drilling formations with potential differential sticking problems.

/. 5.1.2 Freeing differentially stuck pipe ^

If, despite the above precautions, the pipe does become stuck, a number of methods can be used to free the stuck pipe.

The most commonly used methods include: (a) hydrostatic reduction; (b)r spotting fluids; (c) back-off operations; (d) DST (for recovering the fish); and (e) fishing. Only methods a - c will be discussed here.

1.5.1.2.1 Hydrostatic reductionThe normal method used to reduce the

hydrostatic pressure of mud is the U-tube method. The drill string and annulus can be thought of as a U-tube, with the drill bit connecting the two limbs, as shown in Figure 1.12.

Two situations of differential sticking can be recognized: (1) when the formation pressure is known (e.g. in development wells); and (2) when the formation pressure is unknown, as in exploration drilling.

When the formation pressure is known, the overbalance, H s - Pf, can be gradually reduced to a safe level such that the hydrostatic pressure is always greater than the formation pressure.

Surfact

Dr. M.S. Farahat

22

Hydrostatic pressure is reduced either by pumping a new' triiici of lower density, or by pumping small volume of a fluid of low specific gravity; Diesel oil is normally used because of its low specific gravity, but fresh or saline water can also be used for hydrostatic pressure reduction. The volume o f fluid of low specific gravity is determined by calculating the required reduction in hydrostatic pressure and then converting this value to height and volume of diesel oil (or water). As shown in Example 1.6. Diesel oil is then pumped down the drill string until the complete volume is used. Since diesel has a lower pressure gradient than mud, the total pressure in the drillpipe will be less than that in the annulus and a back-pressure will be exerted on the drillpipe. The excess pressure is contained by closing a kelly cock on top of the drillpipe. A safe tension which is equal to the original hook load plus an extra overpull is then applied to the drill string.

The drillpipe is then allowed to back-flow at equal intervals until the entire volume of diesel oil is reversed out. At this point, the annulus level has dropped such that the hydrostatic pressure is equal to or slightly higher than the formation pressure.

■ During the back-flow operation, the drill string should be worked continuously until the pipe is free. If a drilling jar is used in the drill string, it should be activated to provide additional force for freeing the pipe. The jar is only useful if it happens to be above the stuck zone.

Also, during back-flow operations, the drillpipe and annulus pressures should be monitored continuously. When the well is static (or dead), the drillpipe pressure declines slowly with back-flow and there is no movement of fluid up the annulus. If the well is kicking, a gradual rise in the annulus level will be obserevd, and the drillpipe pressure increases slowly with back-flow. When this situation is encountered, the operation of freeing of dkill string should be stopped, and the well killing operation should commence.

Chapter I ...... - ....— '

Example 1.6r . : I ’ ' '

Calculate the volume of Diesel oil required to reduce the hydrostatic pressure in a well by 500 psi using the following data;

Mud weight Hole depth Drillpipe Hole size

Speeific gravity of oil

- lOppg.= 9843 ft.= OD/ID « Sin / 4.276 in. »12.25 in.= 0 .8.

SolutionInitial hydrostatic pressure = (10 * 7.48 * 9843 ) / 144 = 5113 psi.Required hydrostatic pressure = 5113 500 = 4613 psi.

Thus,blew hydrostatic pressure = pressure due to (mud and oil) in drillpipe

= [10 * 7.48 * Y/144]mud + [0.8 * 62.3 (9843-Y)/144]oihWhere

Y * height of mud in drillpipe. Therefore, (Y = 6959 ft).Hence,

Height of oil = 9843 - 6959 = 2884 ftVolume of oil = capacity of drillpipe * height r = (n/4) (4.276)2 * (1/144) * 2884

»287.61ft3.• • • !" ! = 51.2 bbl.

Hr. !V(.S> Farahat


Note that when the required volume of diesel oil is pumped inside the drillpipe, the hydrostatic pressure at the drillpipe shoe becomes 4613 psi. while the hydrostatic pressure in the annulus is still 5113 psi. This difference in the pressure of the two limbs o f the well causes a back-pressure on the drillpipe which is the driving force for removing the diesel oil from the drillpipe and reducing the level of mud in the annulus. It is only when the annulus level decreases that the hydrostatic pressure against the formation is reduced.

When formation pressure is unknown, it is customary to reduce the hydrostatic pressure of mud in small increments by the U - tube technique until the pipe is free.

A variation of the U-tube method is to pump water into both the annulus and the drillpipe to reduce hydrostatic pressure to a value equal to or just greater than die formation pressure. This method is best illustrated by an example.

E xam ple 1.7

The following data refer to a differentially stuck pipe at 11 400 ft:Formation pressure =5840 psi. ,Intermediate casing = 95/8 in, 40# at 10 600 ft.Drillpipe = OD 5 in/ID 4.276 inMud density = 92 pcf.

It is required to reduce the hydrostatic pressure in the drillpipe and the annulus so that both are equal to the formation pressure.

Calculate the volume of water requir^ tijffj annulus and drillpipe*,assumingthat the density of saltwater = 65 pcf.

Solution .. .

Assume the height of water in the annulus to be Y. required hydrostatic pressure at stuck point = 5840 psi, or 'mo

92(11400 - y)5840

( Yx 65\ “I 144 J 144

K= 7696 ft.

Required volume of water in annulus= Annulus capacity between drillpipe and 9 5/8 in casing * height of water.= 0.0515 (bbl/ft)* 7696 ft = 396.3 bbl.

Hence, pump 396.3 bbl of water in the annulus to reduce the hydrostatic pressure in the annulus to 5840 psi at the stuck point. When 396.3 bbl of water is pumped *nto the annulus, the drillpipe is still filled with the original mud of 92 pcf having a hydrostatic pressure at the stuck point (92 * 51 400) / 144 = 7283 psi. Thus, a back pressu^ equivalent to 7283 -5840 = 1443 psi will be acting on the annulus and will be attempting to equalize pressure by back-flowing water from the annulus.

In order to contain the 396.3 bbl of water in the annulus, th* drillpipe must contain a column of water equal in height to that in the annulus. ■

Thus,Volume of water required in drillpipe to prevent back flow from annulus

= Capacity of drillpipe * height of water v =0.0178 *7696= 137 bbl.

-v. . !>«• Dr. M.S. Farahat

24 Chapter /

Balancing of the columns of water in the driHpipe and the annulus can be achieved as follows: (a) circulate 396.3 bbl o f water down the annulus; (b) displace 137 bbl of water down the annulus; (c) circulate 137 bbl of water in the drillpipe to remove 137 bbl of water from the annulus and to reduce the hydrostatic pressure in the drillpipe to 5840 psi.

If the well should kick during the operation, reverse-circulate down the annulus using the 92 pcf, mud to recover all the water from the drillpipe. Then circulate in the normal way through the drillpipe, using 92 pcf mud until all the water is removed from the annulus.

/. 5. L2.2 Çpottine organic fluids

Organic fluids are normally spotted across the stuck zone to reduce the filter cake thickness and the friction factor. A mixture of surfactant and diesel oil is by far the most widely used fluid, owing to its ability to wet the circumference of the pipe, thereby creating a thin layer between pipe and mud cake. This action decreases the value of the coefficient of friction, thereby increasing the effectiveness of mechanical attempts to pull free. The normal procedure is to pump the organic fluid into the drillpipe and gradually pump small volumes into the annulus until the entire stuck zone is covered. The pipe should be worked continuously during the spotting of organic fluid. The success of this operation is dependent on the volume of organic fluid used, the characteristics of the mud cake, the magnitude of the differential force and spotting the fluid against the correct zone. For effective freeing of stuck pipe, a minimum volume of 150 bbl for organic fluid is suggested. The fluid should be left for a minimum of 8 hrs to work through the filter cake properly. An organic solution may also be added to the mud used to drill formations, which are amenable to differential sticking. [The use of oil will produce a reduction in the hydrostatic pressure of mud, and weighting materials can be used to compensate for the loss of pressure gradient. This is most important in wells potentials kick problems.

f. 5.1.2.3 Back - o ff operations

if none of the above mentioned methods are successful in freeing thé pipe, back- off operations are a final solution.

Back-off operations involve the removal of the free portion of drill string from the hole. This effectively means parting the drill string at or above the stuck zone and removing the free portion from the hole. The remaining portion of the drill string (the fish) can then be removed by using either DST tools or washover tools. Alternatively, the hole may be plugged backed and side tracked. (

Before a back off operation can be attempted, the position of the stuck pipe should be determined as accurately as possible Two methods are normally used: (1) the pipe stretch method using surface observations, (2) the pipe stretch method using specialized strain tools,

. commonly known as free point indicators.Surface measurement of pipe stretch Brouse details the method for estimating the

position of the stuck zone from surface measurements as follows:

1. Pull to normal hook load and mark the position above the rotary table a stick, say XI.2. Pull an additional 20 000 lb and release slowly until the weight indicator reads the

hook load again. Mark the new position as X2.3. Note the average distance between XI and X2 as : Y1 = (XI * X2)/2.4. Increase tension load to 40 000 lb and mark position X3 above the rotary table.5. Increase tension to 60 000 lb above hook load and release until the weight indicator

reads HL + 40 000 lb. Mark the new position as X4.6. Note the average distance between X3 and X4 as : Y2 = (X3 * X4)/2.7. The pipe stretch is then measured as the difference between Y2 and Y1.

Dr. M.S. Farahat


Using Hook’s law:

imbjr- E = F I A e ! L

Where L is the free length of drill string. Therefore,_ AEe

is “

’«8

Wheree = Yz- Y |F = (HL + 40 0 0 0 )-HL

Therefore,L A 5 ( y - y )

or F = 40 000 lb.

( 1 - 5 )

Equation (5) can be simplified by replacing the cross-sectional area by the weight per unit length, using the relation

HiA =

3.4

Where Wdp is the weight of drillpipe. Thus,

L = 3.4( . , I » )a in*----- xv 12 in)

if» V.'1 =

F

WL x ax E( 1 - 6 )

40.8 F

Using E = 30 * 106 psi, Equation (6) becomes: 735294 x ax

d - 7 )

The drillpipe stretch measurements do not account for drill collars or heavy-wall drillpipe stretch. Pipe stretch will also be influenced by hole conditions such as dog-legs, hole angle, drag force, etc.

The above procedure can be normally applied in the field in the following simplified version: c...1. Pull drill string to normal hook load and mark position XI.2. Pull additional 40 000 - 60 000 lb and mark a new position X2.3. The difference between X2 and XI is the stretch due to additional pull.

Hence, "£U ~ , AEe AE(X2 - Xt)ync a-- ■ L = ------= ----- ------------

F F

Where F is the additional pull, or73S294(A^~ A j)x %,

Dr. M.S. Kara hat

[

2S Chapter I

Free point indicators:Two types of free point indicator are in use: ( |) strain gauge; and (2) sub-surfaee

probe.Strain gauge method: Strain

gauge methods rely on measurement of axial strain and angular deformation of the drill string at selected positions. The strain gauge toll (shown in Figure 1.13) measures the pipe stretch or angular deflection between the length of two prespaced belly- type springs. The strain gauge

' toll is run on a wire line containing electricalconnections to a surface display! unit which translates extension into a percentage of free pipe. - 1 <•

The tool is run to hole bottom and the driller applies a pull equal to the buoyant weight of the entire string in the hole. The strain gauge tool is then positioned against the drill string and an additional pull AP above the buoyant weight is applied. The strain measured,by the toolis compared with the predicted strain due to the differential pull, to determine whether the pipe is completely free or partially or completely stuck. The theoretical or predicted strain is calculated by direct application of Hook’s law. The tool is slowly moved up the hole and the tensioning procedure is repeated until the first 100 % point is located.

The flee point indicator is also designed to measure the angle of twist between the two measuring springs for a given amount of torque as applied at the surface. The angle1 of twist, 0, for a given torque, T, can be determined from:

e~~fl , ( 1- 8 )¿isr

Where( ’ "1; = length (ft);J = polar moment of inertia (in4) - (ti/32) (OD4 -ID4);Es - modulus of elasticity in shear (psi).

The free point indicator is designed to measure the angular strain, 0/L, in revolutions / 1000 ft. Using Equation (8). it can be shown that the angular strain at any section of drill string can be calculated from:

6_=____9, lL ~ L.

■V•n

FIg. ( 13 Stuck point indicator toot Schiumberger)

(CourtMy ot

E* Ax103rv/100p

( 1 —9)

■i TA

■■VJi

Dr M S. Farahat


Where subscript 1 refers to first section of drill string, subscript 2 refers to second section o f drill string; Esx = modulus of elasticity in shear for section under consideration; Jx = polar moment of inertia for section under applied at the surface.

Once again, by comparing the measured angular strain with the calculated angular strain from Equation (9), the percentage o f free pipe in tension can be determined (Figure 1.14).

The pipe stretch and pipe torsion data are used to construct a graph o f percentage of free pipe (in both tension and torsion) against depth.

Sub-surface probe: A sub-surface probe is, like the strain gauge tool, run on wire line and positioned against the drill string while tension is being applied. The instrument consists of an oscillator, which sends a high-frequency current, and a receiver.

The principle of operation of this tool is that, during tensioning, the molecular structure o f the pipe changes, which alters the high-frequency signal. The change in the signal is proportional to the degree of pipe distortion. The frequency change of the signal is picked up by the receiver and transmitted to a surface display unit. The frequency change is then converted to strain reading by use of calibration charts.

This instrument is not capable of producing readings unless it is positioned against a free portion o f pipe which can stretch under tension. The tool is normally run to hole bottom and gradually pulled up until a reading is obtained.

Back - o ff procedure: A back-off shot is positioned against a drillpipe tool joint that is found to be free in both tension and torsion (point A in Figure 1.14) point A is described as the back-off'point A left-hand torque and a slight positive tension above the back - off weight (pre-stuck hook load minus stuck pipe weight are applied at the back-off point, and the back-off shot is detonated The pipe should come free, which wilt be indicated by a sudden decrease in hook load. The pipe is rotated to the left and picked up to confirm back-off

The portion o f stuck drillpipe, drill collars and bit that are left in the hole are described as “fish”. Fishing operations attempt to remove the equipment from the open hole.

teay of Schlumberger)

a

1.5.2 Mechanically Stuck Pipe

A pipe can become mechanically stuck when: (a) drill cuttings or sloughing formations pack off the annular space around the drill strings, (b) a drill string is run too fast, such that it hits a bridge or tight spot or the bottom of the hole; or (c) pulling into a key seat.

Tight spot can result from drilling undersized (under gauged) holes due to use of worn drill bits or undersized diamond coring bits. Tight spots can normally be recognized during

Dr M.S. Farahat

' : • ■. V:V= \tripping out as extra overpull (i.e. load in excess of the buoyant weight of the string). To prevent mechanical sticking, tight spots should be reamed prior to drilling new sections o f hole.

The usual method used to free a mechanically stuck pipe is to work the drill string either by rotating and pulling it or by activating a drilling jar, if the latter is used. If this method is unsuccessful, an organic fluid should be spotted and the above procedure repeated.

If everything else fails, then drill string should be freed by use of back-off operations as previously discussed

1.5.3' Kev - seatine

In a dog-legged hole containing soft formations, a drillpipe tool joint can drill an extra hole or a key-seat in addition to the major hole created by the bit, as shown in Figure 1.15.

During drilling, the drillpipe is always kept in tension and as it passes through a dos-leg, it tries to straighten, thereby creating a lateral force as depicted in Figure 115 This lateral force causes the drillpipe joint to dig into the formation at the dogleg bow, creating a new hole as the drill string is rotated The new hole is described as a “keyseat” .

A key seat can only be formed if the formation drilled is soft and the hanging weight below the dog-leg is large enough to create a substantial lateral force.

The problem o f key seating can be diagnosed when the drill string can be moved downwards but not upwards. Other symptoms include increased drag, increased noise at the rotary table and the ability to have full circulation.

To remove a keyseat, the hole should be reamed, and if a jar is used, an upward jarring actio* should be applied. Organic fluids can be spotted to reduce friction round the key seat, thus facilitating the working o f the pipe

Key seating can be prevented by drilling straight hole or avoiding sudden changes in hole inclination and/or direction in deviated wells

The width of a key seat usually corresponds to the size of tool joints used. Therefore elements of the drill string which have considerably larger diameter compare to the tool joints pass by easily the key seats occurring in the well. However if in the drill string there is an element which diameter is only slightly bigger than the width of the key seat, such element can be stuck in the key seat. For instance, 5 !/2 “ drill pipe will be very likely stuck in a key seat made by 4 Vi” drill pipes. < '

If a key seat pipe sticking happens the drill string can easily go down and rotates freely. Circulation is restored without any difficulties. </;.

Preventive measures against this type o f pipe sticking can be easily formulated, prevent dog-legs and don’t include in the drill string such elements which can be stuck in key seats. If key seats appear they should be reamed.

28 Chapter I

Twvtion

------Rafort tohotaclMttopod by tool joint

Fig. (• ^Development ot a key-seat* (After Wilson,'

, rri

Dr M.S. Farahat


1.5.4 Drill Pipe Sticking due to Thick Filter Cake and Narrowing the Bore Hole

Excessive buildup o f fitter cake on permeable formations may be the season o f drill'pipe sticking. Hole diameter decreases due to thick filter cake. The hole may become still more narrow if at low velocity o f mud in the annular space cuttings stick to the cake. Hydration of shale cuttings and high adhesion properties o f filter cake promote such sticking.

The thick filter cake with cuttings deposited is scrapped off the wall o f the hole by the bit, drill collars, and tool joints The sticky material accumulated above the bit, drill collars or tool joints may jam the drill string in the hole. *'<-

Usually if this type o f pipe sticking happens circulation cannot be restored? The principal preventive measures against such type o f sticking are as follows:

1. Good filtration properties of mud should be maintained.2. Inhibitive muds should be used to prevent hydration o f shale cuttings.3. The turbulent flow o f mud should be maintained in the annular space. This can be

achieved by both increase o f the circulation rate and decrease o f mud rheological parameters.

4. Oil and graphite should be added to decrease adhesive properties o f the filter cake.

Troublesome sections o f the bpre hole should be periodically calibrated by running the bit and reaming.

1.5.5 Pipe Sticking due to Balling the Bit, Drill Collars. and Tool Joints

This type o f pipe sticking happens while drilling plastic shales at inadequate circulation rate. When the drill string is pulled up the balled elements o f the drilling string scrape the filter cake off the wall of the bore hole Gummy material accumulated in the annulus causes drill string jamming.

Preventive measures against pipe sticking should be directed towards eliminating of minimizing the reason o f sticking the balling up.

It is advisable to use drag bits to drill plastic shales. Bailing up can be minimized by application o f oil emulsion muds. Minimum solids content and low water loss should be maintained to keep should be provided If it is not possible it is advisable to limit the penetration rate.

1.5.6 Pipe Sticking due to Shale Sloughing or Heaving

If a large volume o f shale falls down the bore hole in which the drill string is placed, the latter may be jammed by the fallen shales. Heaving of plastic shales may also cause drill pipe sticking.

To minimize chances of this pipe sticking type all preventive measures against sh^le sloughing and heaving should be taken

Falling down of a large mass o f the o f the shale is marked by sharp and considerable increase o f the pump pressure followed sometimes by lost circulation. - ‘

1.5.7 Pipe sticking due to solids in Ike annular space

Cuttings and cavings, accumulated in the annular space can easily cause a drill pipe sticking if they are allowed to settle down. Several factors can promote accumulation of cuttings and cavings in the annulus: ~

L Caverns and local hole enlargements due to shale cavings. Large cuttings ;ar%:;;-rc accumulated in the enlarged sections o f the bore hole due to abrupt decreases o f the velocity o f mud in caves and caverns o f the annulus. When circulation is stopped cuttings and cavings may fall out o f the cave, form a plug and cause drill pipe sticking.

Dr. M,S. Farahat

° 30 Chapter I

This can be avoided by preventing shale sloughing with the appropriate measures. If large caves develop, cutting accumulation can be prevented by cementing the caves.

H. Accumulation o f cuttings in the annulus may result from poor solids control It is obvious that better solids control may prevent pipe sticking

IIL Drilling without circulation return may result in accumulation of cuttings in the annulus if the loss zone does not take all the formations Pipe sticking due to setting of cuttings remaining I the bore hole can be prevented by careful drilling, with subsequent isolation of the thief zone.

IV. Tool joint washouts result in a decrease of mud velocity in the portion of the annulus below the defected tool joints Due to low mud velocity in the annular space cuttings can be accumulated in he lower portion of the bore hole and cause pipe sticking.

This problem can be easily avoided if tool joints are checked carefully during each round trip and all damaged tool joints are immediately replaced by new ones or repaired on the spot.

I . 5.8 Drill Pipe Sticking due Mud Thickening and Solidification

Some calcium treated muds are able to severely thicken and even solidify if they stay quit for a substantial time exposed to the effect of high bottom hole temperature. If the drill string is left in the bore hole it may be stuck by the thickened mud.

This type o f pipe sticking can be prevented by application o f thermostable mud systems at high bottom hole temperature.

1.5.9 P /ill pipe sticking due to carelessness o f personnel

It is known that in hard abrasive formation bits,become undergauge. Therefore, the lower portion of the bore hole is undergauge too and should be calibrated carefully by a new bit. If a driller pushes a new bit carelessly into the lower undergauge part of the hole, the new bit gets jammed in the hole, and the drill string is stuck.

The drill string may be jammed in the bore hole by a foreign object fallen in the hole due to carelessness o f the drilling crew members

Such types of pipe sticking can be prevented easily by careful fulfilling all the operations and careful handling the hand tools

1.5.10 Method o f determination o f the free portion o f a stuck drill strineTo free a drill string which is stuck in the bore hole, it is necessary first of all to

determine the upper point o f the stuck part of the string or the lower point o f the free part of the string.

Electromagnetic instruments

An electromagnetic instrument consists essentially of two electromagnets, connected with a telescopic joint, and a sensitive electronic strain gauge placed between the electromagnets

The instrument is run into the drill string on an electric cable. Starting from some depth electric current is switched on and magnets attach themselves to the wall of the drill string.

If the instrument is placed above the struck point, the strain gauge will register an elongation of the pipe Below the stuck point no elongation will be registered By moving the instrument up and down the drill string and taking measurements the stuck point can be precisely defined.

Dr. M.S. Farahat


1.5.11 Determination o f the Stuck Point by Drill Pipe Elongation

Elongation o f the drill string under a tensile load applied is used successfully fordetermination of the stuck point. The procedure is as follows:

A. A tensile load equaled to the total weight o f the drill string, plus extra 5 tons is applied. The mark (a) is made on the kelly at the level o f the rotary table or the preventer stack bell nipple, which is preferable due to higher accuracy.

B. An additional load of 5 tons is applied and immediately released. This is necessary to exclude the influence o f friction in the hoisting system. The mark (b) is made on thekelly

C. The interval between the two marks is divided into half and the first main mark (1) is made on the kelly.

D. The load is increased by 20 - 25 tons. However, the total load should not exceed the tensile strength of the drill string The third mark © is made on the kelly.

E. The overpull is increased by 5 ton and immediately released to the previous magnitude. The fourth auxiliary mark (d) is made

F. The second major mark is made just in the midway between the marks (c) & (d). Thedistance between the two major marks represented the elongation o f the free portion of the drill string corresponding to the applied additional tensile Igjld. . ■. *

According to the H ook’s. law:A, ANL , ATAL - ------ ,orL = ----- EA,

EA AN

\ 1

Where: i, VbK ¿O.-.silitsTjiftib eidm hrvyL = length o f the free portion o f the drill string.

A L = drill string elongationAN = additional load which caused the elongation, sE = Young’s modulusA = drill pipe cross sectional area

Expressing A in terms of weight per one meter of the drill pipe and the density of drill pipe material:

« ' = A y «And substituting:

w in kg/m; E in kg/cm2; A N in tons; y* in g/cm3; and A L in m.

And substituting, we obtain:

¿ = 0 .2 7 * 1 0 4w ^ , mA N

This formula is valid only if the drill string consists of the only one size o f drill pipes, with the same wall thickness (or weight per unit o f length).

If the drill string consists of two or more sections of different diameter or different weight per meter another formula should be derived.

In case a drill string consists of two different sections each section will have its own elongation depending upon the length and cross section area of the pipe.

Elongation of section I: ;kV?ANl '

AL = = ^ - ! EAX

Elongation o f section II:

Dr M.S. Farahat

5 32 Chapter /

AA, = * * *EA,

Total elongation o f the free o f the drill string:

A, , r i A r . AN (L I L 2 \AA — AA1 + AA2 — — ■—| ------1------ I£ Vyll

-■**** 1

.4U

Let us assume that the stuck point is situated somewhere within section II. Since the length o f section I is known the position of the stuck point can be found by solving the equation for L2. Expressing the cross section area in terms of the weight of unit length we obtain:

L2 = w2 E AAL \W- 1

wly ANor using practical units as before

• A2 = 0.27* 104^2 — -A l — ,mAN wl

If the result o f calculations by this formula is a positive one the length o f the free portion of the drill string will be

A - A 7 < L2

The negative results o f calculations shows that the drill string is stuck in hp region is section I. Therefore, the position of the stuck point should be recalculated by thp previous formula for a uniform drill string. Similar fotmulas can be obtained for drill strings consisting of three and more different sections

1.5.12 Freeing the stuck drill string

If circulation can be restored after the drill pipe was stuck the most popular technique of freeing the pipe is spotting oil or oil base mud around the stuck portion of the drill string

1 5.12.1 Spotting

Oil (diesel or crude) or an oil base mud is pumped into the annulus and left there for some time. It wets the surface of drill collars and drill pipes, reduces friction and adhesion forces between the drill string and the filter cake, reduces clay hydration and makes it easier to pull the drill string out o f the bore hole.

Oil alone is not as effective as in combination with oil soluble surfactants like “pipe lax” , “spot-free”, “Imco-free pipe” etc. Oil soluble surfactants promote wetting the pipes with oil thus improving o f the filtration cake This phenomenon in its turn decreases the contact area between the cake and the drill string and creates conditions for releasing pressure differential through the cracks in the cake.

The amount of the spotted fluid should be sufficient to cover the stuck portion of the drill string. It is determined by the capacity of the annulus and the length o f the stuck part of the drill string. Some extra volume is required to cover hole enlargements and to make possible to displace from time to time fresh portions o f the oil solution into the annular space, to keep the stuek pipes covered with it completely.

The necessary volume o f oil can be calculated by the following formula:

Voii = a[Vadc Ldc + Vap(E , A* -5 0 )] + Vt, m3where

V«fc: capacity o f one meter o f the annulus in the region o f the drill collars, m3/3V.p: capacity o f one meter of the annulus in the region of the drill pipe, m3/m,

Dr. M.S. Farahat

b? • 10 ■ wiv

Drilling Problems___________ - • ' -i.- ~ ' 33

U,: length of the stuck portion ofthe drill string, m. - ^L* : length of the drill collars assembly, m.a : coefficient which takes into account hole enlargements.VI = 1-1.5 m3: additional volume should be pumped to displace the necessary amount

of the spotted fluid to the annular space.

ÏV.

This can be determined from the following formula:

V = (VPLP - V * U ) Vt, m-In this formula: ' “

. Vp : capacity of one meter o f the drill pipe, nrVm.Vdc: capacity of one meter o f the drill collars, m3/m. 'Lp : length o f the drill pipes, m.

Spotting operation is usually accomplished with a cementing truck which is able to develop higher circulating pressure and is provided with a calibrated tank for measuring the volume of pumped fluid. !

After displacing the spotting fluid into the annular space the pump is shut down and the hole left quiet for a while. Then the drill string should be worked: ¿ ‘ -

Working the drill string is fulfilled in the following way. The pipe should be put in compression, by releasing the hook load by about 5 tons. Then, while still in compression the

Dr. M.S. Farahat

34 Chapter f

drill string should be turned clockwise by approximately 1 Vi revolution per every thousand meters. Rotation can be accomplished by either the rotary table or tongs. The next step is to release the torque and to puli up to the original hook load. The cycle should be repeated every five minutes. Every half hour about 0.1 m3 o f oil should be displaced from the drill string into the annulus. After several hours o f spotting and working the pipe may be releases. If the operation does not result in freeing the drill string spotting should be repeated, but in case the repeated spotting fails to free the string the free portion of the string extracting the stuck part of the drill string. :

If a necessary to place the spotted fluid in the middle o f the bore hole appears, spotting can be accomplished in the following wayA. The necessary volume o f oil is calculated taking into account the capacity o f the annulus

and the length o f the interval to be filledB. , Mud and oil densities should be measured and the difference in pressure gradients o f both

fluid should be found.C. The whole volume o f the spotting fluid is pumped into the drill string. Stand pipe pressure

is measured after pumps are shut and the length o f the oil column in the drill string is calculated by the formula.

L0 = (JOPsp. y - y j, m mwhere: ; 'H

: stand pipe pressure, kg/cm2 , >y : density o f m ud, g/cm3. jj!y0 . density o f the spotted fluid, g/cm3. ! 'f

This step is done to check the volume in the drill pipes against the volume measured in the suction tank because sometimes the pump is not able to take all the fluid, or sometimes improper allowance for line fill is taken. T ^ checking is possible only if the volume o f oil does not exceed the total drill string capacity. If the reverse case is true one has to rely upon the suction tank measurements o f the oil volume.

D.

E.

F.

G.

H.

!.

J.

The level o f mud in all mud pits should be checked and marked before displacement is started.The oil is displaced from the drill string by pumping the amount o f drilling fluid, equaled to the total capacity o f the drill string. After displacement is completed the pump should be stopped and preventors closedPressure on the annular space should be measured and the length o f the oil column in the annular space is calculated by the formula: ■

L„ = 10 Pa / ( y - y0), mwhere:

P« : pressure on the closed annular space, kg/cm2 ...

Oil now occupies the lower portion o f the annular space from the bottom up.The volume o f mud in the pits should (l?e checked and any decreased in mud volume must be registered.Oil should be moved upward in the annulus by pumping the volume o f mud equaled to the original volume o f oil less any losses in the pits, fly this action the situated after the first displacement/ .... /,••. > s rPreventer should be closed again and the annular pressure measured. The length of oil column in the annular space is calculated again and the position o f oil determined.The level o f mud in the pits is checked and any loss o f mud is registered. In case there is a loss it is assumed that it was oil which was lost in the well. Consequently, to move the Qfl,- durrng the next step of displacement from position II to position III it is necessary to

Dr. M.S, Farahat

D rifting Problem s 35

pump the original volume o f mud less all the losses registered by measuring the mud volume in the pits.

Displacement of the spotted fluid is contained as described until the fluid reaches the desired section o f the annular space. Then the drill string should be worked.

Not only oil or oil base mud are used as spotting fluids. Water can sometimes be useful in case of a differential pipe sticking as it decreases hydrostatic pressure and pressure differential. -

Spotting technique can help to free stuck drill string in case o f differential pipe stucking, sticking due to sloughing shales, thick filter cake and accumulation. o f cuttings provided that circulation can be restored. . • «, ,

It is clear that spotting technique is o f no use for freeing a drill string atuck in akey seat or due to jamming the bit in the undergauge part of the bore hole.

‘i'L.'ifc.v.'q1 5 12.2 Application o f a drill stem tester fo r freeing a drill string stuck '

due to pressure differential

Application of a drill stem tester may be effective if a drill string is stuck due to pressure differential. The main aim of using this method is to release pressure differential which keeps the drill stuck.

The free portion of the drill string should be disconnected from the stuck part of the drill column. A drill stem test tool with an open ended drill pipe below is ruii into the hole. The drill pipe is screwed into the fish, drill stem tester packers are set and the tool is»opened.

The drill string above the tester is usually run empty. Therefore after the valves of the tester are opened the part o f the bore hole below the packers communicates to tbbktmosphere through the drill string. Hydrostatic pressure which was pressing the pipes agaiilst the wail does not act any longer. Pressure differential disappears and the stuck part o f the string may be set free. '*■*- ■

1.5.12.3 Application ofjars and vibrators fo r freeing stuck drill pipes

If vibration or shocks are applied to a stuck drill pipe they may cause an effect which helps to free the stuck fish. For this reason special jars and downhole vibrators are provided

The free part o f the drill column is disconnected from the stuck part. A jar or vibrator is run down the hole on drill pipes It is connected then to the stuck part o f the string and jarring or vibration is commenced Shocks experienced by the fish may help to set it free.

1.6 Hole Stability ProblemsInstability o f formations which form the wall of a bore hole is a very serious and

common problem encountered in drilling practice. Hole instability is not as dangerous as abnormal pressures, but nevertheless it slows down drilling process and increases drilling cost.

Formation o f two types cause hole stability problems: -a. unconsolidated formations, b. shales.

When a formation losses its stability it enters the well. The consequences o f tins phenomenon are as follows:1. Accumulation of cavings and cuttings in the hole, increase of solids content iff the

mud, followed by related shortcomings.2. Bridging and filling up the bore hole, which necessitate drilling the plugs o f cavings to

reach the bottom. In its turn drilling the cavings results in bit waer, shorter footage per bit, slower drilling rate, longer rig time.

3. Drill pipe sticking.

Dr. M.S. Farahat

36 Chapter /

4. Hole enlargement, which results in an increase of amounts of cement, mud making ; iI materials, poor cementing, and poor removal o f cutting fom the bore hole.

1.6.1 Fundamentals o f hole stability

A rock in the earth crust experiences three dimensional compression due to the effect of the overburden pressure. Besides the compression stresses caused by the overburden pressure a formation may experience some tectonical stresses resulting from diastrophic movements of the earth crust.

In any point of the earth crust an element o f rock is in the State of equilibrium. Such state o f equilibrium exists until the rock is penetrated by a bore hole. An element o f rock oh the wall of the wall of the bore does not have any longer the support of the rock massive - the side pressure, which existed before and provided stability Inst a id of the formation side pressure the formation on the wall of the hole experiences hydrostatic pressure o f drilling fluid. This pressure is lower than the original side pressure of the rock ma ssive Due to this, stresses appear on the wall of the bore hole. • .

Consolidated formations of high strength experiences some elastic deformations, which do not affect hole stability. Unconsolidated formations as well as shales which have many microscopic fractures and bedding planes are not strong enough to resist stresses. They start falling into the bore hole. Some natural and induced fractures usually contribute to the loss o f .„ formation stability.

Gravity is the contributing factor to instability of unconsolidated formations Fortunately, instability of unconsolidated formations is not a very series problem as it can be r rather easily overcome if a good filter cake is formed on the wall o f the bore hole and care is taken to avoid any mechanical damage of such formation in the process o f drilling.

1 .6.2 §loughin£ shale

Shale is a sedimentary Yock formed by the deposition and compaction of sediments over periods o f geological time. It is primarily composed of clays, silt, water and small quantities of quartz and feldspar depending on water content, shale may be a highly compacted rock or a soft, unconsolidated rock, normally described as mud or clay shale Shale may also exist in a metamorphic form such as slate, phyllite and mica schist.

In oil well drilling, two types of sedimentary shale are normally encountered: unconsolidated shale and compacted shale. Drilling of both types results in sloughing or caving of the shale section. Drillers normally refer to the type o f hole instability resulting from drilling shale sections as sloughing shale.

Research has shown that the severity of shale sloughing is related to the percentage content of montmorillonite (or active clay content) and the age of the rock.

Darley used a scale for characterizing the degree o f shale dispersion based on the percentage of 50 pm particles produced when shale is brought into contact with water. It was found that the degree o f dispersion is 100% when a sample of shale is composed o f 100% sodium montmorillonite. A 60% dispersion is produced with pure calcium montmorillonite. The degree o f dispersion was also found to be dependent upon the age o f shale, older shales containing a high percentage o f montmorillonite dispersion less than younger shales with a lower content of montmorillonite

Factors influencing shale sloughing can be conveniently divided into three groups( 1) mechanical factors; (2) hydration factors; and(3) miscellaneous factors.

Dr M.S. Farahat

D rilling Problem s 37

\ .6.2. \ M echanical factors

Mechanical factors affecting shale sloughing are attribute largely to the.erosion effects caused by the annular flow of mud. Erosion of shale is directly related to the degree of turbulence in the annulus, and mud viscosity. Most hydraulic programmes are designed with the object o f providing laminar annular flow.

Other mechanical effects include the breakage of shale due to impaction by the drill string and caving due to horizontal movement of the shale section. The latter effect is due to the fact that creating a hole in the earth disturbs the local stress system, which leads to dynamic movement within the shale section. This movement leads to breakage of the shale bed adjacent to the well into small fragments which fall into the hole

1 .6.2.2 Hydration factors

A number o f factors are involved in the hydration of shale. For practical purposes, shale hydration force and osmotic hydration are recognized and are quantifiable. Shale hydration force is related to the relief o f compaction on the shale section. Osmotic hydration is related to the difference in salinity between the drilling mud and formation water of the shale.........

During sedimentation, the shale section is progressively compacted by the weight of the overburden The force of compaction squeezed out large percentage o f the adsorbed water and water from the pores o f the shale The compaction force is equal to the matrix stress. The drilling o f a shale section relieves the compaction force on the borehole face and, as a result, a shale hydration force is developed. The shale hydration force is a approximately equal to the matrix stress.

Osmotic hydration occurs when the salinity o f the formation water o f shale, ¡¿.greater than that of the drilling mud In water base muds, the shale surface acts as a semi permeable membrane, across which osmotic hydration takes place In oil base muds, the semi-permeable membrane is the oil film and the layer of emulsifier around the water droplet.

Since osmotic hydration is dependent on the difference in salinity between the formation water in shale and drilling mud, the process can result in either an adsorption or desorption force. An adsorption force is developed when the formation water of sfi&te is more saline than the drilling mud. A desorption force is produced when the salinity of drilling mud is greater than that o f the formation water of the shale. v . ...

Adsorption of water by shale usually leads to dispersion and swelling. Dispersion occurs when the shale subdivides into small particles and enters the drilling mud as drill solids. Swelling occurs due to increase in size o f the silicate minerals making up the clay structure, and if the developed seYwelling pressure increases the hoop stress around the borehole above the yield strength o f the shale, hole destabilization takes place. Hole destabilization manifests itself by means o f caving or sloughing shale. '

1.6 2 3 M iscellaneous factors

Shale sloughing has been correlated with a number of factors which were found accelerated the rate of shale heaving into the well bore.

Dipping shales were found to slough more than horizontally laid shales. ■This is because during the adsorption o f water, shale expansion takes place in a direction perpendicular to its bedding planes, which results in a greater shale heaving when the section is highly dipping.

The process o f heaving in brittle shales containing no active clays is explained by the penetration of water between the bedding planes and microfissures o f the break the cohesive forces between the fracture surfaces causing the shale to fall apart.

Dr. M,S. Farahat

38 Chapter l

In abnormally - or - geo-pressured shales-the water content o f the rock is much higher compared with that o f normally pressured shales. In addition, the plasticity o f the shale is abnormally high relative to the overburden load. Hence, when a hole is drilled through a shale section containing abnormal pressure, the shale will be squeezed into the hole owing to the difference between formation pore pressure and mud hydrostatic pressure. It follows that if such abnormally high pressures were catered for prior to drilling the shale section, sloughing o f shale could be reduced.

1 6 2 4 Prevention o f shale sloughing

The problem of shale sloughing is directly related to adsorption o f water from the drilling mud. Hence, a change in the type or chemical composition o f mud will provide possible solutions to shale sloughing.

The use o f oil base mud has been successful in reducing shale sloughing. The success is related to the fact that the oil phase provides a membrane around the hole which prevents water contacting the shale.

The water phase o f an oil base mud may also be prepared in such a way that its salt concentration matches that o f the shale section. In this case, the osmotic (or hydration) force is equal to the shale hydration force and the pressure causing water to flow between mud and shale is zero. ■

Potassium chloride polymer muds have also been successful in preventing shale sloughing. These muds reduce the swelling o f shale duo to the replacement o f sodium ions Na* (by cationic exchange) by potassium ion K' which allow the clay sheets to be strongly bonded. Dispersion is also reduced due to encapsulation o f the broken edges o f shaleg by the polymer. Other types o f mud’ which have been successful^ reducing sloughing problems include lime mud, gyp mud, calcium chloride and silicate muds, surfactant mud, polymer muds, lignosulphonate mud, etc.

Other preventive measures include minimization o f the time for which an open hole containing a shale section is left uncased.

Hole deviation should be kept to a minimum, and swabbing and surging effects should be reduced, to avoid fracturing o f the open hole sections.

High annular velocities are also be avoided, to limit hole erosion and shale sloughing by mechanical action.

1.7 Kicks. Blowouts o f Fluids o r Gases Problem sFormation containing gas, oil or water may appear to be the source o f troubles if

reservoir fluid intrusion occurs. Blowouts, uncontrolled flows break normal drilling operations and make and operator spend additional time and money to bring the well under control.

No problems appear when formations with normal pressure gradients are encountered. Normal reservoir pressures are easily controlled by the hydrostatic pressure o f un-weighted drilling fluids. Serious problems occur when formations with abnormally high pressures are encountered.

Abnormally high pressures are usually encountered in porous and permeable formationsenclosed in thick massive o f shales- ■' ......

\ tIn the course o f sediments accumulation such shales contained water. Due to great

thickness and low permeability o f shales water could not be squeezed out o f them. This residual water prevents shale compaction and bears a part o f the weight o f overlying formations In other words, water in such shales bears a portion of the overburden pressure.

Dr. IVf.S. Farahat


Such shales are not dangerous themselves because of their very low permeability, but V any permeable formations enclosed in such massive shales transmit this high pressure and due W- to this they become very dangerous, as fluids tend to flow into the well from such horizons, v~; .

In the early days of drilling wells, a gusher was a welcome sight. Earthen dams were, build around the well so the oil could be reclaimed and sold This was before deeper holes and high pressure gas, which increase the risk of fire. The drillers drilled into a formation with the rig until there was either a blowout or a gusher. Today a gusher is a blowout and is definitely ,,not a welcome slight. An offshore blowout is worse, since it also pollutes the water and s: ,attracts considerable adverse publicity. as-

Well control and blowout prevention have been developed over the years in the oilfield. 5,. Yet, no one has all the right answer, because blowouts and oilfield fires still occur. ^

An oil well fire is the most dangerous aspect of drilling a well - and it happens all the *,» time. An oil well fire is simply a blowout that catches on fire. It can means death, heavy financial losses, and bad publicity.

Someday, the drilling business will be safer through the elimination of blowouts. But for now, our present technology will have to suffice.

The lack o f skilled crews is the main cause o f blowouts worldwide. The petroleum industry is growing so fast that many companies do not have the time to properly train their personnel on how to control kicks, which, if not properly handled, can result in arblowout. If you ask hands on a rig how to figure kill mud, they will shake their heads They simply have not been taught. As hard as that statement is to believe, it is true! This is like driving a diesel truck down the road without knowing how to drive a car Sometime is going to get hurt.

Since blowouts are so common in the oilfield, the US government has assigned the Minerals Management Service the responsibility of setting standards for blowout control schools around the country. These schools have helped tremendously in making the drilling business safer.

To simplify the business of well control, let us first examine the world kick. A kick is the entering into the wellbore o f water, gas, oil, or other fluids associated with the formation.This occurs when the column o f drilling fluid is lighter than the formation pressure and fluid or gas enters the wellbore (see Figure 1.20).

If a kick is not controlled, a blowout may occur so it is necessary to notice the signs of a kick. They are:

■?s Rate o f penetration increases."a. Change in shape and size of cuttings.

Increase in rotary torque. 9"a. Increase in drag.is Sloughing shale D"a. Increase in gas contentis Variation from normal “d” exponentis Increase in flow line temperature.is. Decrease in shale density.is Increase in chloride content. '-

A drilling break usually indicates entry into a higher formation pressure. When higher formation pressure is hit, the mud weight becomes under-balanced and the drilling rate increases, sometimes dramatically, sometimes slightly. The mud logger will be calculated the rate of penetration and should notice the difference and report it.

Always keep the mud loggers and drilling shack rigged up with an intercom tareport any changes in the rate of penetration (ROP). Quick response with everyone notified cah give*“

D *

Dr M.S. Farahat

40 Chapter /

time to check for a kick. Normally, you will notice a slowing o f the ROP while you are drilling on the top o f high pressure, which in some areas is called the cap rock. After you drill through the cap rock, a faster ROP will be noticed, because the mud becomes under balanced due to the higher formation pressures, allowing gas or oil to enter the wellbore.

SURFACE PIPE

m m raoM arrmNa. b u t m o t MKftW ENOUGH TO KEEP OUT THEGAS INVASION

----- ZONE ABOVEGAS ZONE

O M H M M K f o r m a tio n p n e h u h e t h a n HYWNMTXnC rHESSUHE OF WUOCOLUMN

«M IW iW H rttU N O C lM M LA N C W -J ANO GAB EKTER# THE WELLBORN

Fig. 1.19 Figure 1 - t o Drilling Into a gas zoos and gstong a "Wcto”

*:■ ■ ■ ' 1 ■ '4 BA gain in the mud pits always indicates a kick is on the way or has entered the

wellbore The pit gain indicator will signal the driller on the floor when a pit gain is encountered. Have the derrickman record the barrels gained on the worksheet. The sooner the driller catches a pit gain, the quicker the kick can be controlled. After a pit gain is noted, pull the kelly up and pull out the bushing until drill pipe is in the annular preventor and then shut the annular preventor. Shut the pump off and check for flow. A flow indicates formation fluid or gas entering the wellbore and up the annulus, pushing the mud up the hole. Gas expands as it rises up the bore, and at the flow line it looks like the pump is on (see Figure 1.21). Shut the well and start kill operations.

To shut the well in, do the following;

1, Raise the kelly to clear the drill pipe safety valve abbve the rotary table.2, Shut off the mud pump.3, Check for flow.4« Open the choke manifold or hydraulic closing ram (HCR). This will avoid shocking the

well., 5» Close the annular preventor. If the pipe rams are to be used, check that the tool joint is out

o f the rams.

Dr. M.S. Farahat

D rilling Problem s P

m m f a c i camnq

' r<y ba«o¿'(tic

6. Close the adjacent choke and watch the casing pressure.

7. Read and record shut in drill pipe pressure (SIDPP), shut in casing pressure (SICP), and pit volume increase (in barrels).

8. Watch and record the increase in pressure until the well stabilizes.

'9. Check for leaks around the rig and the choke manifold and the BOPs

10. Check the mud weights in the pits.

11. Make sure everyone is at the kickstations.

1.7.1 Shows o f eas. o il or salt water

A show wilt first be seen in the shale shaker area. It may be a small column o f oil, which is easily recognized, or gas cut mud. Salt water can be recognized by an increase in the chlorides in the system. When drilling in sands, make sure to hold back on the weight o f the bit so you can watch for kicks and can handle the drilling better.

If gas enters the wellbore, the circulating pressure will decrease because o f the loss o f pressure balance in the annulus. Normally,the pump speed increases because of the difference 1 pressure and is easily recognized

As a consultant on location, it is up to you to train the drillers to handle the paperwork when you are not on location Always display and leave the information on the driller’s wall, since in times o f trouble people panic and do not think clearly. Train the driller and his hands

’on the procedures so everyone will be confident when trouble arises.Ensure that all hands know their stations when fighting a kick. First, the derrickman

and the youngest floorhand need to work at the mud hopper. The driller will work the pipe and handle the blowout preventors. The toolpusher needs to be available to work the superchoke and check for leaks around the rig. The motorman needs to be on the ground to check for teaks on the stack. In the dark, he needs to have a flashlight ready at all times. The floor hand needs to be on the mud pits to record loss or gain in the pits. The consultant needs a high intensity beam flashlight with fresh batteries ready at all times. The rig also needs five water proof flashlights. Make sure the pusher has these and shows them to you every two or three days in case they are needed.

If the hands know their places, fighting a kick becomes less confusing and difficult. Most people have been killed or injured because they did know what to do or panicked-in the face o f unknown danger. A successful consultant will train the n ^ op ^ d i^ r e w every week on what to do and explain the nature o f a kick to all personnel. ~ (<tr ' "

OAIZONB, OVtMMANCKC

?9ivsj»fj;riffgunMLt. A Kick.

> r

Dr. M.S. Farahat

42 Chapter 1

The pressures have been recorded and everyone notified; the-next step is to determine which method to use to kill the kick. There are three methods in the.oil patch.

1. Driller’s method. 2. Wait and weight method3; Circulate and weight method

1 7 11 D riller’s method |

The driller’s method is normally used on land rigs. It is an old method of controlling a well. It is not recommended by all U K blowout schools and is not taught, but material on the method is included in the handbook at blowout schools. The method is widely used and taught in Canada.

The method involved circulating the kick out of the hole, then a second and third circulation o f kill weight mud It is used on drilling rigs where crews are shorthanded and mixing facilities are slow. The only problem is the higher casing pressures The method is simple, and it is easy for one or two men to do.

turn

The procedures are as follows:< Shut the well in after a

kick is recognized.< Record the shut in drill

pipe arid shut in casing pressures.

'V Circulate the kick outo f the hole.

< Shut the well in asecond time to build the mud weight.

< Circulate the well thesecond time with the heavier mud

1 7 1 2 Wait and Weight method

The wait and weight is widely used in hand rock areas and overseas. On the Gulf Coast and in some sandy areas, it can in some cases, get you in trouble by causing lost circulation and formation breakdown This will be explained later Wait and weight requires only one circulation to control the kick, which saves time and money In hard rock areas, it is the best method to use where pore pressures and formation breakdown pressures are greater (see Figure 1.22).

N«W MUD K H P S (MS OUT o r HOC*

»/>.•»* -

Figura * 1 2rhe wait-and-woxjht method lot oootrofflng kicks.

Dr. M.S. Fa rah at


After you decide what kill weight is needed, the barite is added to the existing mud until the kill weight is achieved. Then the mud is pumped downhole to control the formation pressure. After determining the following information, weight up the mud and pump it down the hole. First, record all data on the well kilting worksheet (see Figure 1.23).

PRERECORDED DATA

O rig in a l Mud Weight Measured Depth Piagj #1 SPP Rate Puwp »2 SPP Rote Annulus Voluavs D r i l l S tr in g V a lu e P u p Output

D r i l l S t r in g S tro k es

KILL SHEET DATA

. Wg feet

’ p s i a t ________’ p s i a t ________

b a r r e l sb a r r e l sb a i r o l s / s t r o k e s

D r i l l S tr in g Vol f » ( 1 b a r r e lsP m p O u tp u t? ) Barra la /S tro k e_______________ strofcen

*P*si*

KICK DATA

SIDPPSICPP i t GainTrue V e r tic a l Depth

p s ipsib a r r e l s

Example 1.8

KILL MID DATA

Kill (Aid Weight T m r r t O S r S ë ï T * PP* original sari

( ) ppg k i n « id w i g h t

On the chart (Figure 1 23) we see the following:.Well depth = 9000 ft.Bit size = 9 7/8 in.Mud weight =10.5 ppg.Drill casing = 10 V* in.

40.5 Ib/ft 3000 ft. Drill pipe = 4.5 in XO

16.6 Ib/ft at 8450 ft Pump pressure = 2600psi

PUXP PRESSURE

I n i t i a l D r i l l P ipe P ressu re

Final D rill Pipe Pressure

SIDPP p s i ♦ SPPpsi

p s i

l“11 " t s s l is i j s y■J.*

p s i

) P*À

P ressu re C hartS tro k es P ressu re

InitialDrillP ipe P ressu re

P ro ssu r• O titrtS tro k es F m w o

V

at 64 strokes/min Figure *■-¿Svm MHing «wtoi**»».Shut in casing press. = 450 psi.Shut in drill pipe press. = 300 psi. ,Pit gain = 20 bbl (gas invasion)Reduced circulating pressure = 1000 psi at 30 strokes per minute.Pump displacement = 233 gpm or 5.55 bbi per minute at 30 strokes per minute.Barrels per strokes = 0.185 bbl per stroke

The mud loggers normally keep a record of surface to bit time, simply called a “round trip” by dropping a carbide bomb This is pumped down the hole and when it returns to the surface the number o f strokes recorded to “round trip it,” will yield washout factors. When a kick occurs, this will enable you to more accurately calculate round trip time at the reduced stroke count (A good consultant employs a chart as in Figure 1.24).

Annular capacity of drill pipes in 10-3/4 in casing = 0.0784 bbl/ft * 3000 ft — 235.2 bbl'Length o f drill collars in open hole = 550 ftLength o f drill pipes open hole = 9000 - 3000 - 550 = 5 450 ftAnnular capacity o f drill collars in 9-7/8-in hole = 0.0471 bbl/ft * 550 ft = 25.90 bbl.Annular capacity o f drill pipes in 9-7/8-in hole = 0.0751 bbl/ft * 5450 ft = 409.29 bbl.Total Annular Capacity = 409.29 + 25 90 + 235 2 = 607.39 bbl.Capacity DP =0.01422 bbl/ft * 8450 ft. = 120.15 bbl (120 bbl rounded off).Capacity DC =0.0108 bbl/ft * 550 ft = 5.94 bbl (6 barrels rounded off)Total capacity = 120+6 = 126 bbl.Pump strokes = 126/0.185 = 681 strokes.

Dr. M.S. Farahat

44 Çhapter /

S to B time = 681 / 30 spm = 22.7 min This also is recorded on the chart

Next, calculate the killing mud weight, fV2, as follows:

W2 = [SIDPP / (0.052*depth)] + W,where

Wi = old mud weight.S1DPP - shut in drill pipe pressure psi.W2 = new mud weight.

W2 * [300 / (.052*9000)] + 10 5 = 1114 (round off to 11.2 ppg)

The initial circulating pressure (1CP) will be the shut in drill pressure plus the kill rate pump pressure

ICP = SIDPP + KRPP

When the new mud is added, it is heavy enough to replace the shut in pressure

The FCP is the kill rate pump pressure (KRPP) times the kill mud weight (KMW). divided by the present mud weight (PMW):

FCP = KRPP * KMW +PMW

In the field, most consultants use the kill formula and start kill operations without using the worksheet. But until you can kill a well through experience, use theworksheet if one is available. A consultant must carry a worksheet with him since no worksheets are kept at therig

Fig. 1.24: The daily volume chart used to keep track o f how much'

mud volume is in the hole

17.1.3 Circulate and weight method■ i f

This method is basically the same as the wait and weight method except you start mixing and pumping at the same time, instead of building mud weight, then pumping it down.

This method is very good in highly porous zones, since too much weight can cause lost circulation, which leads to more problems. By bringing up the weight two or three points at a time, you can feel your way to the right mud weight. Many times you will find the “kick” is controlled without reaching the kill weight This is important because the less mud weight in the hole, the better the drilling rate will be and also the less pressure on the surface casing shoe. ,

Anytime there is a kick, the casing shoe is in danger. For example: if the casing shoe is at 3000 ft and you tested to a 13.5 EMW, then the maximum psi you can put on the shoe is determined by the following: ^

j , 13 5 * 0 052 * 3000 ft =-2 106 psi

So if the mud weight is at 13 ppg. the maximum pressure you can hold on the casing while handling the kick is; i' ; ; ’

f it. [MS. Farahat


13.5 EMW for the well is 2 106 psi '

13 ppg * 0 52 * 3000 ft = 2028 psi (pressure on the shoe).

To calculate the difference simply subtract the following:2 106 psi (maximum tested pressure) - 2028 psi (pressure on shoe now)

= 78 psi (difference)

As can be seen, there is trouble, so keep the superchoke system open all the way. This situation demands full attention from all hands, as the casing seat is about to be lost. (Whenever this kind occurs, do not get shaken up or at least do not show it in front of the men, as they are depending on you) Get on the superchoke yourself and make sure that the casing pressure is bled off If it goes up, open the choke until it goes back to 10 psi. After the kick is circulated out, the pressure will fall to zero when the pumps are turned off

If you have maxed out the mud weight and still are having trouble controlling pressure, one way to heal the high pressure zone is to pull one stand o f pipe and rehook up the Kelly, circulate, and slowly rotate the pipe. This allows the formation to bridge over below the bit and stop the kicking of the formation. This sometimes works, but after you pull down. This may take 24 hours. (Figure 1 25)

Another thing you can do is send a plug o f barite or cement downhole, pull two or three stands, reconnect the kelly, and circulate for 24 hours. This sometimes heals the zone If that does not heals the zone, an intermediate string must be run if drilling is going to continue

On proven fields, kicks are not really a problem because a correlation between the nearest welltells where the pressure will come up, so it’s easy to keep the mud weight right when a high pressure zone is encountered, since the mud can be weighted up before drilling through that zone. On wildcats, keep an eye peeled all the time. Trouble is usually around the comer.

Controlling pressure in a delicate situation requires the teamwork o f many men and a consultant who knows his business.

Flgur* • PuHing one stand of pipe allows tarnation to bridge and stops the kick.

Dr. M,S, Farahat

46 Chapter I

1.7,2 Predicting and locating abnormal pressure •formations ______

Due to increase o f the overburden pressure there is a general trend for shales to increase their compaction and density with depth, if water was squeezed away from them. But if water was not squeezed away it prevents shale compaction aftd causes a decrease break the normal shale density. Therefore, such high pressure, under compaction and water containing shales break thenormal shale density increase trend. This change o f the conventional trend is used for locating shale pressure zones.

Presence o f formation with abnormally high reservoir pressures in the geological column can be predicted from data obtained in the process o f seismograph survey. The method is based on the phenomenon o f decreasing the velocity o f compression waves in under compacted formations.

Normally, the velocity o f compression waves increase with depth as formation at larger depths are more compacted due to greater overburden pressure. Therefore, travel time, which is reciprocal o f velocity, normally decreases with depth. However, in under compacted shales, which have abnormal pressure, travel time increases. Such inversions define zones o f abnormal pressure. The degree o f departure from the normal travel time depth line is directly proportional to the abnormal pressure.

Other sources formation; paleontological data, log plots o f nearly wells, regional geology and drilling experience in the area can be useful for predicting high pressures.

More precisely, formations with abnormally high reservoir pressures can be located in the process o f drilling.

One o f the indications o f approaching a high pressure zone is slow, consistent increase in penetration rate that will occur as formation pressure increases and reduces the differential between formation and hydrostatic pressure

Usually, there is a transition zone between the normal and the ultrahigh pressures so that a trouble can be avoided if drilling is stopped in the transition zone.

When the trend toward increased penetration rate is noticed an electric resistivity log and a sonic log should be run. Interpretation o f these logs also can indicate whether the well reached the top o f a high pressure zone Shale density log is a good tool to locate abnormal pressure too.

1.7.2.1 Electric log

High pressure under compacted shales show lower resistivity compare to normal shales because they contain more water Therefore, an abrupt decrease o f shale electric resistivity on the log indicates the presence o f under compacted shales wit abnormally high pressures.

1.7.2.2 Shale density log

Location o f high pressure zones can be determined by measuring the density o f shale cuttings coming out o f the well in the process o f drilling For this purpose shale cuttings are obtained from the mud at 3 - 5 m intervals o f drilling. Cuttings are classified w ith* series o f screens to separate large ones which may have fallen from the walls o f the hole For analysis cuttings caught on a 0.84 mm openings screen are taken. They are washed from the mud blotted on paper towels and dried with warm air until they reach dark dull appearance.

sh * la d en * ity

f

■ 5GFig. 126 • v

Dr M.S. Farahat


A sample o f about 25 grams is takenrand weighed. Then the bulk volume o f the sample is measured with the help of a mercury volumetric pump.

The density of shale cuttings is obtained by division of the known weight over the known bulk volume. -

Shale density is plotted versus the depth. The deviation o f the shale density from the normal trend indicates the presence of under compacted abnormally high pressure formations and the depth where abnormally high pressure formations are located.

* .

'Compaction oorraaponding F lu id under a o r t a l r a a a r r o i r pre»*ura, to th e overburden p re ssa - Greeted by th e w eight a ty « in e r a l l ie d " re e t H] e n te r of 1,07 g/on? d e n s ity .

Compaction corresponding F lu id under normal ra a c rv o U rU ifa y u ra . ■/ to o rerburden p res su re a t o rea te d by th e w»iKMvaO»nSi*rtT±z edH, d ep thnA de r nortoal coo- w ate r o f 1,07 g/qw? deeuiity . d i t io o e . P a r t of f lu id aqueeied away from p o res .

CoKg>aotion a t the depth Hg, F lu id under abnormally h igh rese rv o ir The sane as a t the depth p ressure crea ted b y i^ tfi + .S 'S j. FluisJ could no t be l ) th e weight of^m ineraliz ed w ater i tsqueesed Trow pores and *up- 1,07 g / v d en s ity ’ ports^a p o rtio n of the overl 2) the wolght of fo jraa tiow i r th e biirdea preaaure. i a t e r r a l

Fig. 1.27a, b, ande

1.7.3 Calculation o f the Necessary M ud Density oh (he Basis o f S hah D erm frlo e ,

Hydrostatic pressure of the mud column is the principle measure for formation pressure control. Mud hydrostatic head depends upon the height o f the mud column and the mud density Therefore, proper selection of the mud density is vital for high formation pressure control.

Shale density and shale resistivity logs can be used to determine the density of mud necessary to balance formation pressure encountered.

Let us assume that shale density deviated from the normal compaction trend line and decreased at a depth H2. At the depth H2 shales have the same compaction as those at the depth H I.

Normally at the depth H2 shales should be more compacted and have lower porosity. But due to low permeability of shales fluid was not squeezed away and still remains in their pores. Under normal compaction conditions fluid in formation pores at the depth H2 would bear only the weight of salt water column, extending from H2 to the surface.

Actually, fluid in the undercompacted shales supports both the weight o f salt water and the weight o f formations laying in the interval H I, H2 while the skeleton o f the shales supports only the weight o f rocks laying in the interval from the surface to H I .

Assuming that 2 34 g/cnr’ is the average bulk density o f formation (porosity is taken into account) we may write the equation which expresses the pressure experienced by formation fluid in the undercompacted shales at the depth H2

0.1 yH 2 = P>bn = 0.1 [ 1.07 H2 + 2.34 (H2-H1)], kg/cra2.

Mud density which is necessary to balance this pressure can be found from the equation:

yB = 10 P«bo/H2 = 3.41 - 2.34 (H1/H2), g/cc.

D r. M-S. Farahat

48 Chapter /

1.7.4 Preventive Measures A eainst Blowouts •

Maintenance o f sufficient pressure overbalance is the principle preventive measure against blowouts. Such pressure overbalance is achieved by maintaining the density o f drilling fluid at a level necessary to keep the hydrostatic pressure o f mud column higher than formation pressures encountered.

However even at sufficient mud density the total pressure applied to a high pressure zone may, due to various reasons drop below the reservoir pressure and intrusion o f formation fluids into the well may start. m js>1

Several reasons may lead to a decrease of counterbalancing pressure in the well.

/. Loss o f pump pressure s"'-sC;

In the process o f drilling fluid circulation the total pressure, applied to fo rm a tio n ^ ^ consists o f two components: hydrostatic pressure o f mud column and pressure losses in the annular space along the interval from the high pressure zone up to the top o f the well.

Pm = P + APa

Pressure losses in the annular space can be expressed by the following formula:

1t V

Æ^ S B 2 6 K (Db - D ) \ D b - D f

H,

where:X, : friction factor for the annulus; Q : circulation rate, l/sec;Db bit djajneter, cm ;. D : OD o f he drill pipe, cm.

Additional, circulating pressure applied to formations may be considered as equivalent increase o f drilling fluid density. The equivalent circulating mud density, can be found from the followiag equation:

0.1 H y«~ 0.1 H y + APa

Solving for ye we obtain;

Ye = y + (10 APH ),g/cc

ye : equivalent mud density is an apparent density o f a mud which could create 5 hydrostatic pressure equal to the total pressure exerted on formations in the process o f mud circulation.

The total pressure applied in the process o f drilling may be sufficient to overbalance a newly drilled high pressure zone. But as soon as the pumps are shut down and a trip begins the additional circulation pressure disappears and the well may start flowing if the hydrostatic pressure alone is not able to overbalance the formation pressure.

For this reason the well should be observed carefully and if it floWs the density o f mud should be increased before lifting the drill string. • i i .; -

2. Swabbing effect

When the drill string is pulled out o f the well it acts somewhat like a piston and causes a temporary reduction in pressure applied to formations while the pipe is moving.

The magnitude o f the pressure drop caused by pipe movement is greater if: . is Mud viscosity is high, is. Wall cake is thick, is Clearance is small, i s The bit is balled.is There is a back pressure valve on the string.

Dr. M.S. Farahat


Therefore, swabbing effect can be minimized by maintaining good rheological and filtration properties o f drilling fluid. Swabbing effect can be compensated by a corresponding increase o f mud density. There is a recommendation to maintain the density necessary to balance the formation pressure plus the value equivalent to two times o f the annulus pressure losses, that is: . . >

// = Yb + (20 APJH) , g/cc

The swabbing tendency is greatest at the bit and least at the surface. It occurs proportionality alongside the whole pipe length Therefore, the greatest amount of swabbing tendency occurs just as the bit leaves the bottom It is at this time that most careful checks need to be made to determine if formation fluids are being swabbed into the hole. *

One method o f checking is a shod trip, i .e. pulling the drill string partly out o f the hole, returning to bottom and circulating out to observe if mud is out with gas oil or water.

The other method is based on checking the volume of mud needed to replace the volume o f steel removed from the hole. If 1 m3 of steel is pulled from the hole the same volume of mud is necessary to fill the well. If the well takes only 0.8 m3 o f mud this means that 0.2 m3 o f gas, oil or salt water has entered the hole

Measurements o f mud used to fill the hole can be checked from changes in the pit mud level. It can be done with greater accuracy if the volume o f mud is measured from a small calibrated tank intended specially for filling the hole during round trips.

The easiest accurate check is to convert the volume o f drill pipes pulled out of the well to pump strokes and count the pump strokes until the mud is seen to flow out o f the flow line.

3. Effect o f displacement

This effect is observed in the process o f pulling the drill string out o f the well. As drill pipes and particularly drill collars are lifted the level of mud drops down. This results in a decrease of the hydrostatic pressure, which may be followed by a flow of formation fluids into the well

Pressure drops caused by caused by pulling steel out o f the well should be prevented by regular filling the well in the process o f a round trip. The well can be filled either by pumping the mud with pumps or by flowing the mud from the calibrated filling tank installed above the level o f the rig floor. Mud flows from this tank into the well due to the gravity. .....

4. Lost circulation

Lost circulation may also be a reason of blowouts. Circulation can be lost in the process of running the drill string into the hole, if pressure surges caused by pipe movement are high enough to induce fractures An abrupt drop of the mud level in the well results in decrease o f hydrostatic pressure followed by the flow o f formation fluids into the well.

Therefore, all appropriate measures should be taken to prevent lost circulation and avoid subsequent blowouts which may results from it.

r- ■ . *

1.7.5 Blowout Preventors

In case an operator fails to apply preventive measures he must have a possibility to put the well under control. For this purpose the wells drilled are equipped'with blowout preventors.

A blowout preventor is a unit which is attached to the casing string or to some unit of well head equipment installed on the casing string. The function o f a preventor is control o f pressure in the annular space between the casing and some inner string o f pipe during shilling and completion operations. -

Dr. M.S. Farahat

50’ )

A set of blowout preventors installed on the casing string allows to shut the well both „ when the drill string is in and when it is out A preventor stack should allow to move the drill string in and out, to rotate it while keeping the well closed with the pressure on the annulus.

All these functions can not be fulfilled by the only types of preventor, therefore, several types of blowout preventors are used Common practice is equip a well with a combination of different preventors.

The following types of preventors are known: Oi1. Ram type preventors [Blind (blank) ram preventors, Pipe ram preventors]. -2. Annular type preventors.3. Rotating preventors. 4. Pack off or stripper type preventors. <5. Inside preventors. 6, Wire line preventors.

• • ‘U1 .7 5 1 Ram type preventors i uh

Ram type preventors close the space between their inside walls and the pipe in the hole “ by moving the ram from the retracted position into the position where they close around the pipe A ram is a part of a blowout preventor which moves in and out of the bore of the preventor Ram type preventors are supplied with pairs o f rams, which seal the space below them when they are closed.

Two kinds o f ram type preventors are available:

Blind ram preventors are equipped with rams, which can close the bore hole when the drftf - i string is removed from it. ■ • ,-: i

Pipe ram preventors ■. „ have a pair ,of rams, providing a circular opening for the drill pipe, so ,,- that such preventors can seal a well while the drill string is in it.

Very often blind rams and pipes rams are mounted in two corresponding compartments of the same body, so that both kinds o f ram type preventors are combined in one unit. Ram ' type preventors have a string steel body with the vertical cylindrical opening for passing the drill string or casing strings. Horizontal compartments on both sides o f the body provide space for rams, which can be moved out to steel the vertical opening and taken back into the compartments to provide free passage for drilling fluid. Modem ram type preventors are hydraulically operated, though in case of emergency they can be closed by hand

Ram type preventors arc popular because of simple design, and high degree of confinement These preventors can hold hot chemically active fluids at maximum operating pressures for infinite time. Most of them can support pipes against pressure, which tends to blow them out o f the hole.

Along with the above mentioned advantages ram type preventors have certain drawbacks inherent to their structure. Ram type preventors do not allow to move tool joints in or out of the well. If excessive pressure is applied to the front packing elements of the rams they can be easily warn away if the operator moves the pipe through the rams.

1.7.5.2 Annular type Preventors

Annular type preventors have a ring shaped packing element made of rubber. In the relaxed state the packing element has the opening which is of the same inside diameter as tlje preventor stack. Being compressed in the vertical direction the packing, element is substantially expanded in the horizontal direction due to its elasticity, and seal the annular space between the drill string and the body o f the preventor.

Since the rubber packer can alter its shape considerably these preventors can be closed practically around any element of the drill string except the bit, reamers, and stabilizers. The shape o f the drill string element does not make any difference for such preventors. The packing

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4N.Dr. M.S. Farahat

D rilling Problem s $1

element can seal the space around square or hexagonal Kellies, round pipes, and even around a wire line.

Annular type preventors are not intended for sealing open holes, but in case Of emergency the operator can close the preventors on an open hole too.

These preventors are designed for hydraulic operation and can ot be closed manually. Most of annular type preventors are supplied with pressure regulators, which allow the operator to alter the pressure applied by the packer. These regulators used properly let the operator move tool joints in and out of the hole.

1.7.5.3 Rotating preventors and strippers

Rotating heads are used to seal off a well while the drill string is rotated and moved in or out o f the hole. They can keep the well under pressure control in the process o f drilling. These preventors are not used as primary blowout prevention equipment. Rotating heads are used for some special purpose like air and gas drilling, where they prevent a fog of dust and fluids from forming around the rotary table. Rotating preventors are necessary for reverse circulation in which drilling fluid is pumped down through the annulus and returns to the surface through the drill string.

Rotating preventors have a rotating packing assembly mounted on thrust and radial bearings in the sleeve a special seal prevents drilling fluid from leaking between the packing assembly and the sleeve.

The rubber packer is designed to close the well while the Kelley is in it. Rotating preventors are used in combination with ram type preventors. They work without any outside power and are kept closed by the pressure o f the drilling fluid, that pushes the rubber packing element upward and against the Kelley

1.7.5.4 Pack-off or stripper type preventors

These preventors are used in combination with the ram type preventors to secure the possibility to move the drill string with the pressure on the annulus. Utilization o f pack off preventors helps to secure rams against undesirable wear in case of the necessity to move the drill string in a sealed well.

There are two types o f pack-off assemblies, strippers and warp around. A stripper allows passage o f tool joints and collars. A warp around pack off permits the drill string to be rotated or raised and lowered between tool joints to prevent sticking. With a pack-off preventor an operator can run the drill string down to bottom without putting ram type preventors into operations.

1.7.5.5 Inside preventors

Inside preventors or internal preventors are modifications o f check valves which are placed between two adjacent drill pipes These preventors block the flow of any fluid upward through the inside of the drill string Inside preventors give little resistance to normal circulation.

v -

1.7 5.6 Wire line preventors

Wire line preventors are intended for sealing off the well in the presence o f perforators and wire tools like core barrels, stuck pipe indicators etc. in the well.

1.7.6 Blowout Preventor Stack Installation

A blowout preventor stack is an assembly of control equipment connected to the top of the casing head. It consists of some or all o f the following items an annular type preventor, ram

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type preventors, a drilling spool, a choke and flow manifold, a kill line and a bed nipple with a fill line. The stack is used to control any flow o f drilling or formation fluids which can not be controlled by the drilling fluid hydrostatic pressure.

The number of blowout preventors, the blow out preveritor stack differ from one well to another the following element listed from top to bottom, the bell nipple, the annual preventor, the ram type preventor with blind rams, the drilling spool, the ram type preventer with drill pipe rams, and the casing head The kill line and the choke and flow manifold are attached to the drilling spool The fill line enters the dell nipple

■ With the items mentioned above some variations in the position of the blind ram preventor and the location o f the choke and flow manifold and the kilfline are possible. The blind ram preventor can be placed either between the annular type preventor and the pipe ram preventor or at the bottom o f the stack. Correspondingly the drilling spool with the kill line and choke manifold attached may be positioned either between the two ram type preventors or below the blind ram preventor. '-.r

This preventor stack allows closing the well with the drill string in the hole and without the pipe being in the hole. Circulation with back pressure can be maintained through the choke and flow line. Formation pressure can be subdued by pumping the mud into the well through the kill line.

1.7,7 Kick Control Technique * - ■■.?.■■■■

Several techniques can be used to control high pressure if a kick occurs. The choice o f a technique depends on the equipment available and personnel experience

A method o f kick control which can be used with adjacent chokes is described herebelow

I v b e l l n i p p l eVf 2 , annular preventer

3, M in d ram p re v e n te r4, pip* ram preventer5, d r i l l in g «pool6, caeing bead

7;. choice * flow manifold 8Tr k i l l lin e 9. f i l l lin e

10. adjustable ohoka '11 . fixed choke 12. pressure gauge

Fig. 1.28: When the thread o f a blowout has realized it is necessary:

I. To shut down pumps,II. To close preventors, after these steps have been taken an operator should.III. Take the reading of the pressure gauge on the stand pipe and register it, P^, kg/cm3.

Dr. M,S. Farahat

D rilling Problem s S3

IV. Take the reading o f the pressure gauge on the well annular space, and register it, Pc, kg/cm3.

V. Register the increase o f mud volume in the pit, AV, m3.

On the basis of these data it a possible to determine the magnitude o f the encountered reservoir pressure, the density of mud necessary to keep thf. well under control and estimate the nature o f the formation fluid intruding the well.

In a closed well the formation pressure is balanced pressure supported by sure of the , mud inside the drill string and some additional pressure supported by the valves of the pumps. This additional pressure can be read on the stand pipe pressure gauge.

I Therefore, the formation pressure may be calculated from the following equation:

P / = 0.1 H y + Psp, kg/cm2

where, Psp: pressure read on the stand pipe pressure gauge

ns

The mud density which is necessary to just balance the formation pressure encountered can be found from the equation:

Q.l H yh= P /= ^ 0.1 H y + Psp, kg/cm2 . . A

Solving for Yb we obtain: 4Yb= Y + (10 P ,/H ). g/cc [ ■ -

As it was discussed previously a safety allowance for sufficient, overbalance should be added:

Yi = Yb + (20 APa/H) - y 4 (10 Psp/H) + (20APa/H),

or, Yt = r + (iO/H) (Psp + 2APa)

The nature o f a fluid flowing into the well from high pressure formations can be estimated from the increase of the mud volume and the pressure on the stand pipe and on the casing gauge while preventors are dosed.

A formation fluid enters the well and occupies some volume in the annular space (see Figure 1 29). It can be assumed that the fluid in concentrates at the top o f the well where it fills L meters o f the annulus. The corresponding volume of drilling fluid is moved out of the well into the pit where the increase o f mud volume is registered.

When the pumps are shut down and preventors are closed the following pressure balance equation is valid.

Pa + 0.1 L f t* 0.1 (H-L) y ^ P»p * 0.1 H y In this equation:

Pc : pressure read on the casing string gauge.P* ; pressure on the stand pipe gauge.Yf : density o f a formation fluid flowing into the well.

Solving ithe equation for yf we obtain

Yf ~ Y~ [ I0(Pcs-P,p)/LJ

The supposed length of the formation fluid column in the annulus can be expressed in terms o f the mud volume increase AV and the capacity o f the annulus space Va:

Dr. M.S. Farahat

'L * AV Va, m

The formula for calculating the density o f the intruding fluid is obtained by substituting k for L in the previous formula:

Yf = Y- 110(PCS -"Psp) Va/AV]

If Yf ■ 1 .08 - 1.20 g/cc, there is a flow o f salt water into the well,If Yf 0 . 12 - 0.36 g/cc the fluid intruding the well is gas.

To put the well under control it is necessary first o f all to remove the formation fluid, which entered the annular space out o f the well

For this the adjustable choke should be opened and pumping resumed, to prevent any additional flow of formation fluid into the well a sufficient back pressure should be applied to the annular space and consequently to the high pressure zone itself. Therefore the choke should be adjusted so that the pressure gauge on the annular space slowed the pressure by approximately 15 kg/cm2 higher than the initial closed annular pressure:

Pete = Pcs + IS kg/cm2After the choke was initially adjusted the volume o f mud in the pit should be watched closely. If the volume does not increase the applied back pressure is sufficient. If mud volume increases the back pressure is not adequate and should be increased by a corresponding adjustment of the choke size

When the necessary back pressure is established the number o f pump strokes per minute and the circulating pressure on the stand pipe gauge - should reregistered The established pump rate should be maintained until the formation fluid is circulated out the well. The back pressure should also be maintained.

During circulation the mud cut with formation fluid will go out o f the well and the pressure on the annulus, as well as pressure applied to the high pressure zone, may tend to change. No change o f .the back pressure should be allowed. The choke should be readjusted immediately when any change o f the stand pipe circulating pressure, P«p is noticed.

Circulation should be continued for the period necessary for removing all the mud which was originally in the annular space,

The circulating time from bottom can be calculated by the following formula:

Tb = 16.7 (Va/Q), minwhere

V, : volume d f the annular space, m3. Q : pump output, L/sec.After all he cut mud is removed from the well it is necessary to increase the density of

mud up to the value sufficient to keep the well under control.The density o f mud is increased gradually by adding weighting material and chemicals

in the process o f mud circulation In the process o f mud weighting the pumping rate established originally should be maintained The stand pipe circulating pressure will change due to increase o f the density o f the mud which fills the well. The back pressure applied to the high pressure zone will also change. However care should be taken to maintain sufficient back pressure to keep the well under control and in the same time not to run it too high to avoid lost circulation

Stand pipe circulating pressure read in the process o f circulating out the mud cut with formation fluid can be expressed as the sum of two components:

jP = P 4- P 1 spe 1 sp 1 cr

where Per : pressure losses in the circulating system, which can be found as thedifference between stand pipe circulating and stand pipe static pressure.

. ; Stand pipe circulating pressure in the process o f mud weighting will be:P ’ ~ P ’ + P ’* spc * sp 1 1 cr

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Dr. M.S, Farahat

D ri lling Problem s

It is known that pressure losses in the circulating system is proportional f6 the mud density, consequently: '

P 'c r - P c r M ,where y: anerage density o f the mud being weighted on some stage of the weighting process. Static stand pipe pressure can be expressed in terms of the mud densities and reservoir pressure:

P \p c ~ P \p - 0.1 H yThe reservoir pressure in its turn can be expressed in terms o f the mud density which is

necessary just to balance the formation pressure:Pf - 0.1 H n

Substituting for Pf we obtain:P ’,p - O .lH fo - y d . ,

And finally:P\pc = 0. ! H (Yb-Yi) + Per (Y2- Y).

This formula is valid at y2 < yb. when the density of the weighted mud exceeds yb the circulating stand pipe pressure should be determined by the following formula:

P ’spc-P erfrMThe density o f mud is to be increased fro y2 to y l . It is necessary to calculate the

magnitudes of the circulating stand pipe pressure for some intermediate values o f mud density: y < y2 < yl . In the process of mud weighting the choke should be adjusted maintain the calculated magnitudes o f the stand pipe circulating pressure.

When the density of mud reaches yb, and the mud of such density starts coming out of the annular space the annular pressure will drop down to near zero because the formation pressure will be entirely balanced by the hydrostatic pressure o f the column.

After the casing string pressure drops the well can be opened and the process of weighting containing till the desirable density yr is achieved.

The time of total mud cycle (from surface to total depth and back to the surface) is calculated by the formula:

Tc = 16.7 (Vp + Va/Q), min

Vp : the capacity of the drill string, m \

1.8 Equipm ent failures: Bits. Tools, Rig. PumpsDrilling bits are designed to function for a given period of time without failure;

however, sometimes when the bit is placed on bottom it will not operate properly The penetration rate may be slow, or torquing may occur after only a short time. It will be obvious that something is wrong.

To find the problem check all possible sources. For example, if the torque gauge shows that torque has been released, it may be that there is a key seat or that the hole is not cleaning properly If the pressure does not drop, it is not likely that a washout is causing the problem.

If the penetration rate slows considerably, then the bit is bad. It should be pulled and checked Defective bits are rare, but they can be real tVouble markers. Refuse to allow the operator to pay for a defective bit, since it has caused lost rig time, headaches, and great loss of money. Advise the bit company that if they charge for the bit, their bits will not be used in the future. As in any commercial business, including the oil business, a customer is entitled to a replacement o f a refund for defective products.

Equipment failure is usually caused by poor maintenance or by using worn out equipment that should have been retired. When the oil business is booming, it is harder to find

Dr. M.S. Farahat

56 ' ‘/C hapter /

good shape When the consultant gets on location, he should look for problems and ask the tool pusher to have them fixed before the well is drilled too deep. Most problems involve engines, pumps, draw works, brakes, old lines, etc., and most of these things can be repaired quickly. However, if the rig goes down it is the contractor’s representative records all downtime. Most rigs are allowed a given amount of downtime per month, and an unrecorded hour here and there could cost the operator money Some tool pushers Some tool pushers try to cover downtime if the consultant is in town or asleep. The geolograph will give the first indication that the rig was down, but some older drillers cover up the geolograph so the consultant must watch it closely

Downhole tool failure is not difficult to prevent All that is necessary is a record of the hours on the tool. By checking with the suppliers on the standard hours o f operation, you can keep an accurate rotating hour chart on any tool in the hole, including drilling jars, shock tools, and stabilizers, in operating condition Remind the pusher to check tools and replace them if necessary. Always check the elevators and slips If their conditions are borderline, demand they be changed or repaired. It is better to wait on replacements than to have to account for downtime

1.9 BridgingBridging, a common phenomenon, is the

sloughing in of the hole due to some of the following:■a. Improper viscosity.^ Under balanced mud.■?sl Swabbing the hole when tripping out

In shale areas bridging is common, since water get behind the shale and push it out onto the wellbore or cause the shale to swell and fall into the wellbore (see Figure 1.30). Bridging is what causes most fires and blowouts to put themselves out).

If there is a reduction in weight on the weight indicator when tripping into the hole, there is a bridge.When that happens the string can be pulled back up through the area to provide a clearing action If the string becomes stuck, the kelly can be hooked up and the bit washed down to the bridge When the bit hits the bridge, simple ream or drill through it slowly Then you can break the kelly and continue tripping in the hole

Sometimes when you start reaming through a bridge a new hole starts and it becomes impossible to find the old hole Occasionally a consultant has had to call i* a report saying that the rig has lost 4 000 ft of hole. So when you ream through a bridge, go slowly and let the bit gradually wash its way down (see Figure 131).

1.10 Going Back to BottomWhen returning to the hole with a new bit, it is always a good idea to ream back to the

bottom two or three joints to further widen and clean the bore. In shale areas doing this will give you some margin of safety against bridging or getting stuck. When tripping opt of the hole, some fill will fall to the bottom, but reaming will keep fill from being a problem.

.slum :

Figure t - f t t ¿ridging.

Dr. M.S. Farahat


U 1 The Twist-OffWhen drill pipe separates in the hole, it is called a twist- off. Check the geolograph to see if the driller is to blame. Most twist-offs are caused by one o f the following:

❖ Washout in the drill pipe or in the drill collars. Too much weight on the bit, which causes torque and parting of the pipe.

❖ Encountering hydrogen sulfide gas. Hydrogen sulfide gas may come up in a fault from deep down even at shallow depths when it is not expected.

Once the problem is located, action must be taken quickly. The longer the pipe sits in the hole the more fill will fall in around the pipe, which makes the pipe stick more Weighing the string will help determine where twist-off occurred.

Exam ple 1.9If the string and the block weighed 222,000 lb to begin with, but after the twist-off only

150,000 lb are indicated, the length of string in the hole can be calculated. First subtract the weight of the blocks (45,000 lb subtracted from 150,000 lb leaves 105,000 lb - the weight of the remaining string), See Figure 1 32.

If the drill pipe is 4.5 in XO pipe at 16 60 lb/ft, divide 105,000 / 16.6 - 6,325 o f string.

If you were at a depth o f 8572 ft when the problem occurred, then the pipe parted at 6325 ft leaving 2247 ft in the hole to be fished out. Remember that if you will draw a picture of what is in the hole when you have a problem, the problem will look easier to solve. In this particular case the blocks weigh 45,000 lb and the drill collars weigh 45,000 lb, so the string must weigh 132,000 lb. Write in the weights on your drawings so you will have an accurate picture of what is happening.

The next step is for the driller to trip out of the hole, while strapping the pipe. Then you call out a fisherman. Tell the fisherman that there is about 2247 ft o f pipe in the hole at the TD o f 8572 ft. The fisherman will bring out the necessary tools.

Fishing is relatively simple, but is best left to the fisherman. Many consultant make the mistake of trying to fish the hole themselves. This is like a lawyer trying to defined^himself. Fishermen are paid to fish and solve the problem. .,. .

Dr. M.S. Farahat

If you turn the job over to the fisherman, then his reputation is at stake and it relieves you of responsibility if something goes worn.Let the fisherman explain his plan to you, so you can explain it to your boss and the operator. In most cases, by the time string has been tripped out of the hole, the fisherman will be on location and ready to take over.

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1.12 Pipe W ashoutPipe washout is a common

problem because o f the many times pipe is broken out and made up Some of the causes of pipe washout are:

❖ Banging the pipe facing when making up the pipe

❖ Not using enough pipe dope.

A driller should take enough time making up joints to ensure no damage occurs, sincedittle nicks and pings can result in pipe washout downhole under pressure. Make a driller who is trying to set an in and out record slow down

Pipe dope is designed to withstand heat and pressure - it has kept the oil industry functioning for years. The proper application o f a good API dope will save many problems.

Redope drill pipe using the crossing method. In simple terms, this means breaking the stands at different joints each trip so that older connections are broken and redoped. The driller should lay down one or two joints at the beginning o f the trip then bFeak the remainder of the string into normal stands He should break the bottom hole assembly every third or fourth trip, check for washout in the facings, lay down bad drill collars, then redope. It is very important to keep the BHA in good shape It is also important to check the XO (changeover) sub and the first joint o f drill pipe, since they take a lot o f torquing Being cautious will keep the consultant on the job - and on the next one comes up

It is almost certain that every three or four runs a washout will be found in one or more drill collars and they will have to be laid down Everyone will disagree with the order, but when the first collar is laid down, that will quit people down and add to the oil company’s confidence in the consultant. In a string of drill collars; one or two collars are bound to go bad every three or four bit runs. (You can count on it happening! In the past fifteen years out o f twenty as a drilling engineer I have not tost one collar personally or while in charge o f other personnel, because I inspect them faithfully and have my crew do the same.)

Hire an inspecting company to test the threads with Magnaflux or electronic inspection to check for cracks in the tool joints This will keep you out of trouble at all times.

Dr. M.S. Farahat

Drilling Problems

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