Cementing: Lecture 1

Fundamentals

of

Cementing Operations

Functions of Cement in Wells

• Cementing is the process by which cement slurry is placed in the annulus, bonding the casing to the formation

• The conventional method of doing this is to pump cement down the casing and displace it around the casing shoe into the annulus

• A good cement job is essential to allow further drilling and production operations to proceed

Cementing Operation

Components of Cementing OperationFloat shoeA float shoe prevents cement from flowing back into the casing oncethe cement is displaced behind the casing. Shoes have either inner

parts made of aluminum or cement; both being easily drillable, with the advantage that cement is more resistant to impact.

Float CollarA float collar is a one way valve placed at one or two joints above the

shoe. The float collar provides the same functions as a float shoe by preventing fluid back flow into the casing: mud backflow during running in hole and cement slurry backflow after cement displacement. The distance between the shoe and float collar is called Shoe Track.

Wiper PlugsBoth top and bottom plugs are used during cementing operations.

They are used to separate the various fluids from one another.

Bottom PlugThe red bottom plug has a shallow top, is made of rubber, and has a hollow core. It is used ahead of the cement slurry to prevent cement/drilling fluid contamination and to clean the casing wall of filter cake. After the bottom plug comes into contact with the float valve, sufficient pressure (150 to 350 psi) causes the top diaphragm to rupture, allowing the cement slurry to flow through it.

Top PlugThe black top plug has a deep cup on its top and has a solid, molded rubber core. It is dropped after the cement slurry has been pumped, to prevent contamination with the displacement fluid. The top plug also signals the end of displacement by forming a seal on top of the bottom plug, causing a pressure increase.

The main functions of cement plugs are:• Separate mud from cement• Wipe the casing from mud before cement is pumped and then wipe casing fromth cement film after the complete volume of cement is pumped.• Prevent over-displacement of cement• Give surface indication that cement placement is complete• Allow the casing to be pressure tested

The bottom plug is first released and is followed by cement. When the bottom plug lands on the float collar a pressure increase on surface is indicated. A small increase in pressure will rupture the bottom plug and allow cement to flow through it, through float collar, shoe track, casing shoe and then around the casing.

The top plug is released from surface immediately after the total volume of cement is pumped. The top plug is displaced by the drilling fluid and it, in turn, pushes the cement slurry into the annulus.

When the top plug lands on the bottom plug a pressure increase is observed at surface. This is called bumping the plug. Bumping indicates that the total volume of cement is now displaced behind the casing. Usually, at this time, the casing is pressure tested to a precalculated design value to check its integrity. Pressure testing casing while the cement is still wet is recommended as this reduces the chances of breaking the set cement or creating micro-channels if the test is carried out a few hours later when the cement sets.

Bumping the Plug

Washes & SpacersPre- FlushIn any successful primary cementing operation the cement slurry must displace the fluid surrounding the casing. Mud and cement are often incompatible and contact between them can lead to severe channelling or the formation of an un-pumpable viscous mass. To avoid this problem an intermediate fluid is used as a pre-flush to clean the drilling mud from the annulus.The simplest form of pre-flush is a 'wash' - usually water, with the possible addition of a surfactant. Such a pre-flush is very effective in removing mud from the annulus as turbulence can be achieved around the complete annulus.

SpacersSpacers are difficult fluids to design. They must be compatible with both mud and cement and have the correct rheological properties to minimise mixing and channelling. Although weighted spacers can theoretically achieve turbulence, care must be exercised in assuming that turbulence will occur beneath eccentric casing, in the narrow section of the annulus. If laminar flow exists beneath the casing and turbulent flow above, channelling will result.

The most important functions of the initial or primary cement job are:

• To support the casing string;• To prevent the movement of fluids from one formation to

another through the annulus;• To protect the casing from corrosive fluids in the formations

The cement slurry is able to meet these requirements byproviding adequate compressive strength and low permeability when the cement hardens. The critical factor in obtaining a satisfactory cement job is to place the cement completely around the casing to prevent channelling

A secondary or squeeze cement job ..

may have to be done at a later stage to carry out some remedial work on the well (eg, sealing off certain zones, repairing casing leaks). This involves forcing cement through holes or perforations in the casing into the annulus and formation. Like this …..

Planning the cement job

Each cement job must be carefully planned to ensure that the correct cement and additives are being used, and that a suitable placement technique is being employed for that particular application:

• The cement can be placed correctly using the equipment available;

• The cement will achieve adequate compressive strength soon after it is placed;

• The cement will thereafter isolate zones and support the casing throughout the life of the well

Classification of Cement

Compounds (a)

API Class C3S

%

C2S

%

C3A

%

C4AF

%

CaSO

%

Fineness Sq cm/Gram

A 53 24 8 8 3.5 1600-1900

B 44 32 5 12 2.9 1500-1900

C 53 16 8 8 4.1 2000-2400

D & E 50 26 5 13 3.0 1200-1500

G 52 27 3 12 3.2 1400-1600

H 52 25 5 12 3.3 1400-1600

There are several classes of cement approved by the API. The differences between the cements lie in the distribution of the five basic compounds, which are used to make cement: C3S, C2S, C3A, C4AF, CaSO4.

• Classes A and B: These cements are generally cheaper than other classes of cement and can only be used at shallow depths where there are no special requirements. Note: Class B has a higher resistance to sulphate than Class A

• Class C: This cement has a high C3S content and so produces a high early strength

• Classes D, E and F: These are known as retarded cements due to a coarser grind, or the inclusion of organic retarders (lignosulphonates). Their increased cost must be justified by their ability to work satisfactorily in deep wells at higher temperatures and pressures

• Class G and H: These are general purpose cements which are compatible with most additives and can be used over a wide range of temperature and pressure. Class G is the most common type of cement used in most areas. Class H has a coarser grind than Class G and gives better retarding properties in deeper wells

Other types of cement not covered by the API specification include:

• Pozmix cement - formed by mixing Portland cement with pozzolan (ground volcanic ash) and 2% bentonite. Very durable & less expensive than most other types;

• Gypsum cement - formed by mixing Portland cement with gypsum, giving a high early strength and can be used for remedial work. They expand on setting and deteriorate in the presence of water;

• Diesel oil cement - a mixture of one of the basic cement classes (A, B, G, H) with diesel oil or kerosene with a surfactant. They have unlimited setting times and will only set in the presence of water. Consequently they are often used to seal off water producing zones where they absorb and set to form a dense, hard cement

Mixwater Requirements

API Cement Classification

API Class Mixing Water

Gals/Sk

Slurry Wt.

Lbs/Gal

Well Depth (a)

Ft

Static Temp

deg F

A (Portland) 5.2 15.6 0-6000 80-170

B (Portland) 5.2 15.6 0-6000 80-170

C (High Early) 6.3 14.8 0-6000 80-170

D (Retarded) 4.3 16.4 6-10000 170-230

E (Retarded) 4.3 16.4 6-14000 170-290

F (Retarded) 4.3 16.4 10-16000 230-320

G (Basic – Calif) 5.0 15.8 0-8000 80-200

H (Basic – Gulf Coast)

4.3 16.4 0-8000 80-200

Following tabulated figures are based on:• The need to have a slurry that is easily pumped;• A minimum amount of free water

Effects of reducing the amount of mixwater:• Slurry density, compressive strength, and viscosity will all

increase;• Pumpability will decrease;• Less volume of slurry will be obtained from each sack of

cement

Properties – Compressive Strength

To support the casing string a compressive strength of 500 psi is generally thought to be adequate (includes a generous safety factor). The casing shoe should not be drilled out until this strength has been attained - referred to as ‘waiting on cement’ (or WOC)

Development of compressive strength is a function of several variables:

• temperature• pressure• amount of mixwater• elapsed time since mixingWith proper accelerators added - the WOC time may be reduced to 3-

6 hours. Following table shows some typical compressive strengths for different cements under varying conditions

Compressive Strength

Temperature deg F

Pressure

(psi)

Typical compressive strength (psi) at 24 hours

Class A & B

Portland

High early

strength

class C

API class G

API class H

Retarded

class

D,E,F

60 0 615 780 440 325 -

80 0 1,470 1,870 1,185 1,065

95 800 2,085 2,015 2,540 2,110

110 1,600 2,925 2,705 2,915 2,525

140 3,000 5,050 3,560 4,200 3,160 3,045

170 3,000 5,920 3,710 4,830 4,485 4,150

200 3,000 - - 5,110 4,575 4,775

Compressive Strength

Temperature deg F

Pressure

(psi)

Typical compressive strength (psi) at 72 hours

60 0 2,870 2,535 - - -

80 0 4,130 3,935 - - -

95 800 4,670 4,105 - - -

110 1,600 5,840 4,780 - - -

140 3,000 6,550 4.960 - 7,125 4,000

170 3,000 6,210 4,460 5,685 7,310 5,425

200 3,000 - - 7,360 9,900 5,920

Properties of Dry

Cement & Neat

Slurries

Properties of Dry Cement

Class A and B

(Portland)

High early strength

class C

Basic

API class G

Basic

API class H

Retarded

class

D,E,F

Specific gravity (average)

3.14 3.14 3.15 3.15 3.16

Surface area (range), (sq cm /

gm)

1,500-1,900

2,000-2.800

1,400-1,700

1,400-1,700

1,200-1,600

Weight per sack (lb)

94 94 94 94 94

Bulk volume (cu ft/sack)

1 1 1 1 1

Absolute volume (gal / sk)

3.6 3.6 3.58 3.58 3.57

Properties of Neat Slurries

Water (gal/sack) (API)

5.19 6.32 4.97 4.29 4.29

Slurry weight (lb/gal)

15.6 14.8 15.8 16.5 16.5

Slurry volume (cu ft/sk)

1.18 1.33 1.14 1.05 1.05

Thickening Times (‘pumpability’)

Thickening Times

Depth (ft)

Static Temp deg F

Circu- lating Temp deg F

High Pressure Thickening Time (hours:min)

Portland High early

strength

API class

G

API class

H

Retarded

class

D,E,F

2000 110 91 4::00+ 4:00+ 3:00+ 3:57

4000 140 103 3:36 3:10 2:30 3:20 4:00+

6000 170 113 2:25 2:06 2:10 1:57 4:00+

8000 200 125 1:40* 1:37 1:44 1:40 4:00+

Properties – Slurry Density

Standard slurry densities (shown in an earlier table) may have to be altered to meet specific requirements (eg, a low strength formation may not be able to support the hydrostatic pressure of a cement whose density is around 15 ppg). The density can be altered by changing the amount of mixwater or by using certain additives. Most slurry densities vary between 11-18.5 ppg.

Properties – Water Loss

The setting process is the result of a dehydration reaction. If water is lost from the cement slurry before it reaches its intended position its ‘pumpability’ will decrease and water sensitive formations may be adversely affected. The amount of water loss that can be tolerated depends on the type of cement job, for example:

• Squeeze cementing requires a low water loss since the cement must be squeezed before the filter cake builds up and blocks the perforations;

• Primary cementing is not so critically dependent on fluid loss. The amount of fluid loss from a particular slurry should be determined from a pilot test. Under standard laboratory conditions (1000 psi filter pressure, with 325 mesh) a slurry for a squeeze job should give a fluid loss of 50-200 cc. For a primary cement job 250-400 cc is adequate

Properties – Corrosion Resistance

Formation water contains certain corrosive elements, which may cause deterioration of the cement. Two commonly found compounds are sodium sulphate and magnesium sulphate. These will react with lime and C3S to form large crystals of calcium sulphoaluminate. These crystals expand and cause cracks to develop in the cement structure. Lowering the C3A content of the cement increases the sulphate resistance. For high sulphate resistant cement the C3A content should be 0-3%

Properties - Permeability

After the cement has hardened the permeability is very low (<0.1 millidarcy). This is much lower than most producing formations. However if the cement is disturbed during setting (e.g.. gas intrusion) higher permeability may occur (5-10 darcies).

Cement Additives

Most cement slurries will contain some additives to modify the properties of the slurry to produce a better cement job to suit particular requirements. Most additives are known by certain trade names used by various cement service companies. Additives used to:

• Vary the slurry density;• Change the compressive strength;• Accelerate or retard the setting time;• Control filtration and fluid loss;• Reduce slurry viscosity

Major Cement Additives

Accelerators

These are added to shorten the time taken for the cement to set. WOC time is therefore reduced and less rig time is wasted. Accelerators are especially important in shallow wells where temperatures are low. In deeper wells the higher temperatures promote the setting process, and accelerators may not be necessary. The WOC time is usually based on the time taken for the cement to attain a compressive strength of 500 psi.Common types of accelerator used include:

• Calcium chloride (CaCI2) 1.5 - 2.0%;• Sodium chloride (NaCl) 2.0 - 2.5%;• Seawater

Retarders

In deep wells the higher temperatures will reduce the thickening time of the cement slurry and the cement becomes less pumpable. Retarders are used to prolong the thickening time and avoid the risk of the cement setting in the casing prematurely. The bottom hole temperature is the critical factor for the use of retarders. Above a static temperature of 260 - 275°F the effect of retarders should be measured in pilot tests.Common types of retarders used include:

• Calcium lignospulphanate (sometimes with organic acids) 0.1 - 1.5%;

• Saturated Salt Solutions (eg, sea water)

Lightweight Additives (extenders)

These are used to reduce slurry density for jobs where the hydrostatic head of the cement may exceed the fracture strength of certain formations. In reducing the slurry density the compressive strength is also reduced and the thickening time increases. The use of these additives allows more mixwater to be added, and hence increases the yield of the slurry. Such additives are therefore sometimes called ‘extenders’

Common types of lightweight additives used include:

• Bentonite (2% -16%) – This is by far the commonest type of additive used to lower slurry density. Bentonite absorbs water, and therefore allows more mixwater to be added. It will also however reduce compressive strength and sulphate resistance. The increased yield due to the bentonite added may be seen in cement tables

• Pozzolan – This may be used in a 50% / 50% mix with the Portland cements. The result is a slight decrease in compressive strength, and increase sulphate resistance;

• Diatomaceous earth (10% - 40%) - the large surface area allows more water absorption, and produces low density slurries (down to 11 ppg)

Heavy Additives

These are used when cementing through over-pressured zonesCommon types of additive used include:

• Barite (barium sulphate) – This can be used to attain slurry densities of up to 18 ppg. It also causes a reduction in strength and pumpability;

• Hematite (Fe203) - The high specific gravity of hematite can be used to raise slurry densities to 22 ppg. (Friction reducing additives may be required);

• Sand – graded sand (40-60 mesh) gives a 2 ppg increase in slurry density

Fluid Loss Additives

Used to prevent dehydration of the cement slurry and premature settingCommon additives used include:

• Organic polymers (cellulose) 0.5% - 1.5%;• Carboxymethyl hydroxyethyl cellulose

(CMHEC) 0.3% - 1.0% (CMHEC will also act as a retarder)

Friction Reducing Additives (dispersants)

These are added to improve the flow properties of the slurry. In particular they will lower the viscosity so that turbulence will occur at a lower circulating pressure, thereby reducing the risk of breaking down formationsCommonly used additives include:

• Polymers 0.3-0.5 lb/sx of cement;• Salt 1-16 lb/sx;• Calcium lignosulphanate 0.5-1.5 lb/sx

Mud Contaminants

As well as the compounds deliberately added to the slurry on surface to improve the slurry properties, there will also be the effect of the mud downhole which comes into contact with the cement in the casing or in the annulus. The chemicals in the mud may react with the cement to give undesirable side effects. Some of these are listed below:

Mud additive Effect on cement

Barite increases density reduces compressive strength

caustic calcium compounds acts as an accelerator

diesel oil decreases density

Thinners act as retarders

Cement Excess