Radial Drilling Applications for Enhanced O&G Production

Literature Review Project, SS2015

Radial Drilling Applications for Enhanced O&G Production Ali Erdin, Montauniversitt Leoben Advisor: Herbert Hofsttter, Univ.-Prof. Dipl.-Ing. Dr.mont. This paper was prepared for the Literature Review Project course in the Montanuniversitt Leoben

Abstract

Having to do with big variations of the oil prices, the high rate of increase in reserves due to new discoveries and the

maturity of oilfields currently being developed, companies are striving to improve the recovery factor of reserves.

Horizontal drilling and completion are opening up reserves in fields that were not previously economically profitable.

These methods are not limited to previously undeveloped fields or by lithology. It is possible to gain higher recovery

factors from old fields where production has declined over time.

Being a low investment alternative, radial drilling seems to be a good choice when it comes to improving production

of existing wells. The main objective of radial drilling is to extend the wellbore radius in order to bypass the areas that

have been damaged during drilling and completion.

Radial drilling tehnology supposes drilling small diameter horizontal bores in different directions and levels, up to one

hundred meters around the existing wellbore. These bores are made in the following three steps: At first, the casing

is milled using a small diameter bit. After this process is done, the lateral holes are extended using fluid jetting at high

pressure. The last step is the formation wash out. This tehnology can be applied to new wells and also to old wells.

Radial drilling technique can be applied usually in the following scenarios:

Formations with low production (below the economic limit)

Layers close to Oil/Water contact

Low permeability formations

Water injection (increasing secondary recovery by improving sweeping)

Viscous/low mobilty oils

The choices concerning the number, length and the radial array are usually made based on the reservoir properties

and the hydrocarbon characteristics.

This paper presents a theoretical approach on radial drilling applications and optimal parameters that are needed to

be taken into consideration for succesfully applying this technique.

Introduction

The early shut-In of wells and marginal fields is an increasing problem in the oil and gas production industry. Once a

field has been shut-in, usually the remaining resources will be abandoned due to the increasingly high cost of re-

establishing production of the field.

Increasing production and raising usable reserves from known horizons are extremely important in the O&G industry.

In order to acheive better results concerning these problems, the search for new tehnologies and methods started.

Radial drilling technology has been applied since the 1990s. It consists in creating several small diameter drains

perpendicular to the mother well by jetting. Each one can have a bending radius of maximum 30 cm. Every drain is

realized in three steps:

1. The casing is perforated by a milling bit which is connected to a flexible shaft. These two are rotated by a

conventional mud motor.

2. The horizontal extension is made by using high pressure water, diesel or acid.

3. By pulling out the hose while still pumping, the formation is being washed out.

2 Radial Drilling Applications for Enhanced O&G Production

Applying this technique combines the following factors:

Fast process; The average time spent is four to five days

Low cost; No new wells required

Low geological uncertainty

No HSE issues

Creating these type of small diameter holes with long depths of penetration should result in the increase in the

productivity index, eliminating permeability barriers and a high increase in the recovery factor.

System Technology Summary

In the first days of the system development a 10 to 12 inch radius turn was applied from vertical to horizontal direction

in order to penetrate shallow or thin formations and to minimize underreaming. An erectable whipstock equipped with

slides and rollers was used to effect the bending and straightening process. A pressure of 8000 10000 psi was used for water jet drilling. The propulsion was realised by a push-pull process: the drill penetrating the formation in self-

regulated motion propelled by internal hydrostatic pressure. Velocities varied from a low of 6 feet per minute to a high

of 120 feet per minute. The system was tested in unconsolidated reservoirs, such as Bakersfield (Kern River), Taft

and Brea (Olinda).

Surface equipment

The surface equipment is basically a simple coiled tubing including a high pressure pump (up to 10000 psi), goose

nick, monitoring equipment and an injector head with hydraulic drive.

Bottom hole assembly

The bottom hole assembly consists of a deflector sub, one side centralizer and a gyro tool. The equipment is lowered

in the borehole, being connected to the work string. The purpose of this assembly is to guide the tool from vertical

to horizontal direction through the formation, maintaining the deflector shoe on the side of the casing.

Figure 1. Down hole equipment


Figure 2. Overall drilling system

Figure 3. Deflector shoe with cardan shaft and milling bit


Process and field operation

A workover rig is set up at the surface and the well is conditioned:

The production string is pulled out Calibrate to the bottom of the well or below the layer where perforations will be made Check if any leaks in the casing occured The layer is tested in order to determine the type of fluid and flow rates The bottom hole assembly connected to the work string is lowered with the baffle anchor to the planned depth,

verifying with wireline records.

Before starting the radial drilling process, the deflector sub is used with the one side centralizer to guide the tool to

horizontal direction, along with the gyro tool on the drill pipes.

The three main steps are then performed:

Milling the casing

This operation is performed using a milling bit with a specific size connected to a flexible shaft and rotated by a

conventional mud motor. The mud motor is connected to the coiled tubing unit at the surface.

Jetting the formation

This action is performed using nozzles (three oriented forward and three oriented backward). The pressure used must

be higher than the formation fracture pressure. After it enters the formation, the hose moves horizontally due to the

force generated by the nozzle distribution.

Foce S in the driving direction:

Sjetting = u02A0 - =1 ui2cosiAi (1)

Figure 4. Jetting out the formation

Formation Wash-Out

This step is done by pulling out the hose while pumping continues through the entire operation.

Once these steps are acomplished, the coiled tubing is pulled out. The workover string is turned clockwise and the

baffle anchor is placed in the next position for performing again the three steps described.

To avoid accidents and environmental contingencies the program includes risk evaluation tests during the operation.


After the radial drilling is finished, a test tool is lowered to check and evaluate the production of the perforated area.

The following figure shows the three main weights that accrue during the jetting. The lifting weight is the only one to

reduce the overall weight and thus lowers the pulling force.

Figure 5. Diagram for hose, liftin and jetting fluid weight

The next diagram depicts the ration of the overall weight, working on the hose, and the pulling force which is alleviated

by the frictional coefficient. The nozzle ought to have a net pulling force under full load of equalling 2 kg or 20 N.

Figure 6. Diagram for pulling weight and sum of weights.

Mechanism of the penetration

According to reports in the literature (Bruni et al., 2007; Buset et al., 2001), four main penetration mechanisms take

place due to the penetroation effects of the radial drilling operation :

1. Surface erosion The jetting force exerts shear and compression forces on the rock surface. Because of this, rock

fragments are removed from this surface.

2. Hydraulic fracturing If the hydraulic pressure is higher than the stresses set by formation, cracks ar fails may

occur.


3. Poroelastic tensile failure If a quick fluid pressure drop occurs at the rock surface, effective tensile stresses equal

to the pressure decrease will appear. This may lead to rock fail in tension. Any equilibrum disturbance between

the rock grains and pore fluid will generate thee kind of tensions. Any equilibrum disturbance has to be restored

by fluid flowing through the pore space. Due to the finite permeability of the rock, the flow takes time and gives

rise to the transient poroelastic effect.

4. Cavitation The pressure increase above the vapor pressure is causing vapor bubbles to collapse or implode.

Because of the shock waves errosion and tension effects may occur.

Figure 7. Jet nozzle and its effect on a core sample

The pull effect consists of three main mechanisms:

1. The under pressure force - the nozzle is driven forward by means of a vacuum which is created at the tip of the nozzle. The vacuum is created by the fluids high speed in the distance between the nozzle and the heading. The smaller this distance is, the fluid velocity increases and the lower the static pressure decreases.

2. The jetting force is the result of the propulsion forces of the rear jets which work on the lateral heading face. The propulsion force that a rear nozzle creates is equal to the product of the mass flow and the exit velocity. The mass flow is proportional to the flow rate of a nozzle and that the exit velocity decreases significantly according to the distance of the nozzle exit when exposed to air or in the ambient medium.

3. The ejector force is created by the rear nozzles and its effect is sucking away the water from the front of the nozzle. This effect causes low pressure in front of the nozzle head and thus supports the forward thrust and the under pressure force.

Figure 8. Mechanism of penetration


Figure 9. Representative diagram of perforated laterals

Constitutive technologies

The following methods can be considered:

1. Local acidizing - a chemical treatment can be done depending on the formation, after drilling each lateral by using a modified nozzle head or by using a dual packer system to isolate the radials and inject chemicals under pressure.

The main task of the acidizing is to bring the reactive fluid to the formation in order to impair material from the

formation. Acid is pumped into the borehole to allow the removal of close formation or skin damage. Thus the

drainage radius as well as the permeability of the well will be increased.

2. Hot steam injection is used mainly in the case of high viscosity oils. The heat input reduces the oils viscosity which allows it to diffuse more easily through the pores of the formation. By means of this technology it is possible to produce heavy crude oil in wells.

3. Water or CO2 injection. Usually a water leading layer resides below an oil or gas horizon. As the reservoir is a self-contained system and oil is less heavy than water and thus floats on it, it is possible to maintain the pressure of the reservoir by flooding it with water.

4. Crack direction initiation - it is possible to predetermine the direction and depth of a crack as the lateral or laterals in itself mark a pre initiated crack and the frac forms in the areas where the resistance is the lowest.

5. Swabbing consists in maintaining a pressure working inside the laterals by using a swabbing tool, a moving pipe, a wire line tool or a rubber-cupped seals up the wellbore. When the pressure inside the lateral is reduced enough , it is easier to extract the hydrocarbon from the reservoir. Cleaning of the used drilling fluids by swabbing is recommended to avoid damaging reactions with the formation. Swabbing will also start the formation fluids to flow after stagnation during the Radial Drilling operations. During swabbing it is crucial to be caucious as there are high chances of kicks or wellbore stability problems to occur.


Figure 10. Steam injection after Radial Drilling

Substitutive technologies

The fact that the Radial Drilling Technology is versatilely applicable is demonstrated by the vast variety of its substitutional technologies. The following technologies can be counted as such: 1. Side Tracking

Side tracking is a method where a drilling tool is installed at the tip of a Coil Tubing which is then inserted into a already existing borehole. The coil is deflected inside the casing by means of a whip stock thus enabling the drilling of horizontal side tracks. Side tracking allows for significantly narrower radiuses than conventional directional drilling by means of a drilling rig and a Downhole motor. The drilling tool on the Coil Tubing can either be a jetting nozzle which jets forward and sideways or a Downhole motor with direction indicating equipment. The advantages of the Radial Drilling Technology concern the working time and the lower effort needed for a job in comparison to side tracking. However the main advantage of the side tracking is that the borehole pattern can be exactly determined by means of logging. 2. Post perforation

For the post perforation a jet gun is inserted into the casing and ignited at the pre-perforated position. The horizontal depth of the post perforation usually exceeds the depth of the first perforation significantly. The depth can come up to 1 m. Post perforations are often used in order to overcome skin damage areas in ultimate proximity to the borehole and in order to increase the permeability of the formation. With the Radial Drilling Technology it is not possible to create that many short laterals in massive formations. On the other hand it is possible to create several long side tracks in formations of moderate thickness which allow for a greater drainage radius than the short perforated tracks.

3. Hydraulic Fracturing

Hydraulic Fracturing is a technology suitable for both oil and gas production. The technology is known as a stimulation method which allows for the increase of permeability of the rock by creating fractures. The basic idea of this technology is to connect many pre-existent little fracs in the reservoir via a big fracture emerging from the casing. The length of this fracture can come up to 500 m. The big frac is created by long-term injection of fluid (e.g. water with some high viscosity fluid additives) into the reservoir. In order to keep the frac open after the stimulation, propping agents like sand or gravel are added to the high pressure fluid which prevents a closing of the fractures. An advantage of the Radial Drilling Technology is that significantly fewer equipment and time is needed and that it is more cost-efficient that a hydraulic Fracturing job. Another disadvantage of the Fracturing is that especially at wells with dangerously high water cut a frac could end right in a watercourse. The frac direction can be only to some extent predetermined as the frac forms usually where the resistance of the rock is lowest. The Radial Drilling Technology allows the predetermination of the direction but it is not possible that the borehole pattern deviates significantly from the horizontal or the azimuth as this would cause major friction and prevent the jetting hose from further penetrating the formation.


4. Matrix Acidizing

Acidizing means that an acid solution is injected into an existing well at a pre-perforated section. The acid solution undergoes a reaction with the rock formation and thus removes the skin damage in ultimate borehole proximity to a large extent and by doing so enhances the permeability for oil and gas. The Radial Drilling Technology allows for a more precise and intense injection of the acid solution into the rock formation during the jetting-out. Technical limits of radial Drilling Technology

Applying the Radial Drilling Technology must be done by taking in consideration the physical and technical limiting factors. Some of the most important factors are the following:

Casing minimum diameter of 5 Space is needed for the Radial Drilling equipment, especially for the deflector shoe and its canal radius. If the radius is too small, problems may occur in milling and jetting operations.

Maximum well inclination of 30o If the well inclination exceeds this limit, inserting the flexible shaft and hose in the deflextor shoe will become impossible.

Well depth limitation due to length, wall thickness and resistance of the Coil Tubing.

Resistance of the casing The resistance of the casing must be allowable for the milling operation.

Cementation and geological conditions of the reservoir. In case of highly compacted reservoirs or big caverns, fractures and hollows, the jetting operation cannot be performed.

The hose usually lasts up to three operations and the bit is worn out after two drillings, therefore the Radial Drilling operation cannot be performed as often as desired.

A maximum number of four deflections are allowed at one horizontal level. If otherwise, the cement or the formation close to the casing would be washed out. Thi would lead to a loss of feeding rate of the jetting.

The WOB is limited by the stiffness of the hose. If it is too high, the bit can get stuck. If it is too low, the bit will not penetrate the casing.

A GR-CCL correlation must be carried out before the milling operation in order to detect the casing couplings.

Radial Drilling can be performed in formations with a maximum portion of 50 % clay.

The pressure inside the jetting hose during the jetting works limits the flexibility of the hose and thus causes a significant increase of the friction losses (Newtonian friction by contact).

The diameter of the Coil Tubing is downwardly bounded by the increasing fall of pressure as well as by the required depth and resistance. Furthermore it is upwardly bounded by the inertia of the controllability of the Coil Tubing equipment as well as its complexity and ponderosity.

Figure 11. Left: Milling bit; Right: Chippings


Worldwide field applications

The Radial Drilling Technology is available worldwide. Most contractors providing Radial Drilling are located in the North America. This is because RDT was invented and patented in the United States. After concluding that this technology is cost-efficient, other tests were undertaken in Russia and Southern America. Nowadays, RDT is used with great succes in different parts of the World: Argentina, Brazil, Chile, Ecuador, Egypt, Indonesia, Russia, Uzbekistan and many others. The results are very effective: up to 200% increase in production, even 400% in some cases and somehow, significant decreases in the water cut.

Figure 12. Comparison between static reservoir pressure before and after Radial Drilling in an Egyptian oilfield

Conclusions

The main advantage of Radial Drilling Technology is the increase of the drainage radius and the flow profile of the well. This results in the increase of the well productivity, while being cost effective at the same time. Comparing to other stimulation methods, less time is needed for it. Ragarding the narrow laterals, it is possible to jet very small horizons or areas with high skin damage or bad facies. There is a low risk of environmental impact. The technology can be applied for oil and also gas, for storage, exploration and even injection wells. Although there are a lot of benefits, it must be considered that Radial Drilling Technology is not suitable for some cases. We can use it only for vertical boreholes. It cannot be applied for reservoirs that are highly compacted or containing conglomeratesor clay. Also, it is not recommendable to use it for hozrizons that present a high water production. The laterals are usually at risk of collapsing because the completion is not possible. There are a lot of limitations that must be take into consideration before applying Radial Drilling Technology. Nevertheless, RDT has been proven to be easy to apply and less expensive comparing to other stimulation methods. That is why it succeeded from the mechanical point of view and also the economic point of view. The enhancement of oil production ranges from 10% to even 200%. The real challenge is to keep the constant increase in production as long as possible after Radial Drilling where it is noticed that most of the increase is not extended for a long time.


List of abbreviations: RDT = Radial Drilling Technology HSE = Health, Safety and Environment n = number of nozzles A0 = hose inner area Ai = nozzle area u0 = velocity inside the hose ui = velocity in the nozzle = fluid density = angle of the nozzle WOB = Weight on Bit GR-CCL = Gamma RayCasing Collar Locators List of figures Figure 1. Down hole equipment Figure 2. Overall drilling system Figure 3. Deflector shoe with cardan shaft and milling bit Figure 4. Jetting out the formation Figure 5. Diagram for hose, liftin and jetting fluid weight Figure 6. Diagram for pulling weight and sum of weights Figure 7. Jet nozzle and its effect on a core sample Figure 8. Mechanism of penetration Figure 9. Representative diagram of perforated laterals Figure 10. Steam injection after Radial Drilling Figure 11. Left: Milling bit; Right: Chippings References: [1] Radial Drilling, 28. Feb. 2007. Radial Drilling System Components Presentation (online). Radtech International Inc. Available at: www.radialdrilling.com. [2] W.Dickinson, R.W. Dickinson, 1985. Horizontal Radial Drilling System. SPE 13949. [3] Adel M. Salem Ragab. 2013. Improving well productivity in an Egyptian oil field using radial drilling technique. Journal of Petroleum and Gas Engineering 4 (5): 103117. 10.5897. [4] M. Bruni, H. Biasotti, G. Salomone, 2007. Radial Drilling in Argentina. SPE 107382. [5] Buset, Riiber, Eek, 2001. Jet Drilling Tool: Cost effective lateral Drilling Technology for enhanced oil recovery. SPE 68504. [6] W.Dickinson, Dykstra, Nordlund, 1993. Coild-Tubing radials placed by water jet drilling: Field results, theory, and practice. SPE 26348.

Radial Drilling Applications for Enhanced O&G Production

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