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SPE 37131

A Review of Horizontal Drilling and Completion Techniques for Recovery of

Coalbed Methane

Samuel O. Osisanya, The University of Oklahoma, SPE and Robert F. Schaffitzel, Texaco Exploration and

Production Inc., SPE

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Abstract

In the last few years the oil and gas indusby has been turned

around with respect to gas production and availability. Clean

burning and abundant natural gas resources have caused high

demand, and because of the tax incentives, coalbed methane is

a popular gas source. Coalbed reservoirs are much different

from conventional natural gas reservoirs because drilling and

completion considerations are not the same due to the rock

properties. Horizontal wellbores are considered to be vesy

effective in reservoirs that are relatively thin, naturally

fi-actured, and anisotropic with regard to permeability.

Coalbed reservoirs have all of these features. The concern

today is how to get the gas out of the coalbeds in an economic

manner. Coalbed cleat systems are made up of natural

fractures, hence vertical wells drilled in the reservoir must be

fractured.

Horizontal well drilling and completion is an

alternative technique to overcome low production as well as

reservoir heterogeneities in coalbed reservoirs. Some authors

have shown that a properly drilled and completed horizontal

well can increase production by about seventy-five percent

over that of a fractured vertical well.

This paper discusses coalbed properties essential to the

application of horizontal drilling and completion.

Several

techniques used to drill and complete horizontal wells in

coalbed reservoirs are discussed and compared with each

other to distinguish their application.

Introduction

The US Bureau of Mines has demonstrated that methane

drainage by horizontal and directional boreholes is a safe and

effective method of removing methane in advance of mining

and of controlling methane emissions during mining-2.

Horizontal holes were found to have the advantages of

relatively low drilling costs and the ability to intersect the

coalbed cleat or tlacturc system, thus increasing permeability

to gas flow. Hydraulically stimulated vertical holes on the

other hand had the disadvantages of requiring large numbers

of surface sites, higher costs, and production and maintenance

problems. The concept of directionally drilled degasification

holes was originally considered by the Bureau of Mines as a

means of combining the best elements of the surface vertical

borehole and underground horizontal drilling techniques.

In the past few years the oil and gas industry has been

turned around to be called the gas and oil industry. The clean

burning, easily accessible, and overly abundant natural gas

resource has created a high demand. For many years,

coal

mines were

degasified and most operators thought little of the

value of the gas.

But recent demand and tax incentives have

made coalbed methane gas a popular item. It is believed there is

a large amount of gas stored in coalbeds throughout the United

States with current estimates of 400 trillion cubic feet of gas

being stored in coalbeds (Fig. 1, Table lY.

Todays concern is how to get the gas out of the coalbeds in

an economical manner. Recovesy of gas from coalbeds occurs

in the presence of the low porosity and very low permeability

inherent to these beds. The permeability is made up of natural

fractures which are am-anged in a cleat system. There are two

cleat systems: the face cleat and the butt cleat. The face cleat is

continuous throughout the reservoir and provides the largest

permeability. Vertical wells drilled in the reservoir must be

fractured to be in contact with the cleat system and maximize

recovery.

Coalbeds are much different than conventional natural gas

reservoirs in that drilling and completion considerations are not

the same due to the unique coalbed propetiies. Drilling vestical

wells in coalbeds has become a very simple process in Alabarn%

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A REVIEW OF HORIZONTAL DRILLING AND COMPLETION TECHNIQUES FOR RECOVERY OF COALBED METHANE SPE 37131

Colorado, New Mexico, and Utah. Improvements in vertical

well applications to coalbeds in the past few years have been

centered on completion techniques. It has been shown that a

vertical unli-actured well in a coalbed is difllcult to justify

economicall~. However, with the help of hydraulic fracturing a

vertical well can become economically feasible, Fig. 24.The use

of a horizontal wellbore, however, allows control of the

direction, so that the borehole can intersect the face cleat at right

angles.

The objective of this paper is to review the various

horizontal drilling and completion techniques being used for

the recovery of coalbed methane. Firs a discussion of coalbed

properties essential to drilling and completion of horizontal

wells in coalbeds is discussed. The various techniques used are

discussed and compared to one another. Finally, the advantages

and disadvantages of the various techniques are enumerated.

Characteristic Properties of Coalbeds Essential to

Horizontal Drilling and Completion

The

properties of a coalbed essential to evaluate its drilling

and completion are dual-porosity, permeability, gas

adsorption, stratigraphy, bottom-hole pressure, and water

production.

Dual-porosity: One of the most important properties of the

coalbeds is dual-porosity which is described by the macropore

and micropore structures. With this type of porosity, the gas is

held in the reservoir in three possible ways: (I) as adsorbed

methane molecules on the surface of micropores, (2) as free gas

within the fracture of the pores, and (3) as dissolved gas in the

formation wate?. The fmt of the three, the adsorbed methane, is

the primary source of the gas volume. The free gas that is

contained in the natural fractures is a very small portion of the

volume, and the ilactures themselves are known as the cleat

system. The cleat system is the network of primary flow

channels that provides the permeability of the coalbeds. This

system consists of a butt cleat and a face cleat where the butt

and face cleat systems are orthogonal to each other.

Permeability: Formation permeability is the critical parameter

that controls production. The primary concern of the

completion engineer is to devise a completion method that

will etllciently connect the coal cleat system to the wellbore.

Many coalbed reservoirs simply do not have enough

permeability to produce gas at economic flow rates. In order

to obtain the best estimate of formation permeability in a

specific coalbed reservoir, well tests, core analysis, and

production data must be conducted, gathered, and analyzed. If

the permeability is not above a certain critical value, then the

coalbed may not be an economical reservoir. Thus, in many

cases, hydraulic fracturing treatments serve to create a

pathway that will comect the coal cleat system to the

wellbore.

If the permeability in a particular coalbed is too high, the

coalbed cannot be properly de-watered, In cekin high

permeability coalbeds that are connected to strong aquifers, it

may be impractical to produce water at the high rates

necessary to effectively drawdown the reservoir so that

desorption can occur. Since the gas flow rate in coalbeds

increases in a non-linear manner as pressure decreases and in

order to obtain high gas flow rates (maximum gas desorption),

the bottom hole pressure must be minimized. In general, the

explorationist should look for coalbeds with permeabilities

between lmd and 100 md$. In that permeability range, the

pressure can be reduced enough to begin gas desorption and

still have the high permeability necessary to flow gas at

commercial flow rates.

In coalbeds, permeability is a function of the effective

stress6. The effective stress is the total stress minus the seam

fluid pressure. Reducing the fluid pressure tends to C1OSChe

coalbed cleats thus reducing permeability. The permeability

of

coalbeds

may be directionally-controlled by predominant

cleat sets. The cleat spacing varies over a wide mnge, horn a

few millimeters to tens of meters. Therefore, the coalbed is a

fractured system with anisotropic low permeability.

Permeability variations brought about by variations in fluid

pressures will be anisotropic, depending on the nature,

frequency, and direction of the cleats6. Such opening and

closing of the cleats is also likely to change the phase

permeabilities and capillary pressures within the coal.

Gas Adsorption: Another important parameter of a coalbed is

the amount of gas that is adsorbed to the surface of the coal.

The volume of adsorbed gas must be determined by cutting a

core and running gas content experiments with the core. This

information is critical for completion optimimtion of a

horizontal well in a coalbed, especially in new exploration

areas. With the dual-porosity, there must be a primary and

secondary porosity. The primary porosity is considered to be

the micropore system. It is assumed that the micropore system

openings are not accessible by water and this system contributes

the largest portion of storage for the gas. The gas in the

micropore system is stored as free gas and adsorbed gas. It has

been shown that coalbed can store as much as 2000 SCF of

methane per ton of coal by means of adscnptiorf. It is also

assumed that virgin coalbeds are filly saturated with water and

the volume of free gas in the micropore system is negligible

compared to the amount of gas adsorbed.

Stratigraphy of Coalbeds: The coalbed should be

continuous over the drilling area and be free of faults and rolls

so that horizontal holes may be more easily kept in the

coalbed. It is also desirable for the horizontal holes to be

drilled perpendicular to the face cleat to maximize gas

production. Horizontal drilling in a specific coal bed is based

on analysis of mined-out areas of the coalbed where the size,

shape, orientation, and distribution of discontinuities are

known. A more complete geologic evaluation of drill sites, to

include the drilling of a core hole at the proposed initial

coalbed intercept is essential to coalbed drilling. The core hole

would confum the thickness and exact elevation of the coal

for accumte drill path projections. In most coalbed reservoirs,

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SPE 37131

SAMUEL O. OSISANYA AND ROBERT F. SCHAFFITZEL 3

hydraulic fracturing must be used to stimulate production.

However, in certain basins, several thin coalbeds that spread

over several hundred feet may be encountered. For such

coalbeds, it may be diflicult to create long, propped fractures.

Also, under certain well conditions there may be problems

with diversion of the fracture treatments.

Bottom-hole pressure and water production: As the coalbed

reservoir is penetrated and completed, it is possible to produce

water and reduce the reservoir pressure. This pressure reduction

allows the adsorbed gas to desorb from the micropore system

and diffhse to the macropore system. With the increase in

desorption, the five gas within the fracture network increases.

Once the gas has diffused through the coal matrix, it may then

migrate through the cleat system to the wellbore. Dewatering of

a coalbed is essential for efficient and effective degasification.

Vertical dewatering is presently considered the best method to

accomplish this task in a directionally drilled degasification

system.

Horizontal Drilling Options And Strstegy

Horizontal wellbores were considered to be very effective in

reservoirs which were: (I) relatively thin; (2) naturaily

fractured; and (3) known to have anisotropic permeability.

Knowiedge of just these properties can iead to the use of

horizontal wellbores in coalbeds. Coalbeds were ve~ seldom

found that are greater than 100 ft in thickness and are cioser to

the 30 fl average. Natural ffactures are the basis of the coai

matrix and offer an ideal opportunity for a horizontal borehole.

Another consideration was the anisotropic permeability of the

thin coaibeds.

Coalbeds appear to be the perfect formation for a horizontal

wellbore,

but

there are other considerations to look at prior to

making a decision. Several core samples should be tested to

determine the minimum and maximum stress directions and

determine how weak and fkiable the coal is. This information is

needed to determine if the coal will be stable enough in the

lateral section. The core samples should also be tested to

determine the criticai range of driliing fluid weight required to

maintain horizontal weilbore stability. After the horizontal

weilbore has been drilied, it is common to use a siotted liner in

the wellbore to maintain stability for testing or production.

Current horizontal drilling techniques are used to drili

horizontal wellbores in coaibeds. The most commonly

employed technique is the medium radius using the speciaily

designed mud motors. With the medium radius technique, the

original objective of drilling perpendicular to the face cleat, or

the greatest permeability, can be controlled and monitored whiie

driihng with MWD techniques. One consideration in drilling

the coalbeds is that the curve section must be built prior to the

entry of the coalbed due to the low rock strength in the coal.

Rotary Drilling Technique: Verticai bit trajectory is

maintained by varying combinations of bit rotation, thrust, and

placement of centralizers on the driil string. Strategic

placement of centralizers on the drill string results in a force

being applied vertically on the drill big which determines the

direction the driil bit wiii cut. The most effective drili string

configuration is generaily determined by triai and error

causing a loss in chili time. The primary objective during

rotary drilling is to maintain bit trajectory within the coalbed

by keeping bit inclination to within 1 of coalbed dip. Short-

collared drilling assemblies have been used successflily to

drill the Beckley coalbed compared to the iong-colhsred

assemblies. The configurations of the short-collared

assemblies, ailow gravity to act and make these assemblies

drili either upward or downward. This predictable behavior

makes them usefhi in the controi drilling technique, where

certain short-coihred assemblies are used to make large

changes in the vertical trajectory of the hole. Figure 3 shows

various rotary driil string configurations.

IrI general, it has been obsemed that a fiiabie coalbed is

very soft and that the overbreak of the hole can cause the

trajectory to iower unpredictably. The strategy which was

adapted for a relatively thin undulating coalbed was to let the

assemblies drill to the roof or floor rock and make them

deflect or bounce along the rock contact in coal. Vertical

surveys, which are made with the downhoie surveying

instrument are used in control drilling to monitor hole

inclination so that bit trajectory can be maintained parallel to

the bedding plane. This impiies that direction and dip of the

coalbed bedding plane must be known.

Holes driiied in

coalbed or against the floor rock tend to arc to the right

because of right-hand bit rotation. Generally, the left-hand

trajectories are smaller than right-hand ones. Holes that arc to

the left apparently only occur because of deflection of the bit

off coalbed inclusions or the roof rock. Hence, because of bit

rotation, left-hand trajectories are much less frequent, and

usually iess in magnitude than right-hand trajectories.

Long coalbed methane drainage hoies have been drilled

with substituted short-collared assemblies for the iong-

collared assembly normally used to rotary driil horizontal

hoies. These assemblies drilled wideiy arcing hoies aiong the

roof or floor rock, depending on their configuration, but never

left the coaibed. Simple adjustments to the drilling parameters

made deflections of the bit that foliowed the contact easier.

Drilling along the contacts was abetted by the presence of

shaley roof and floor rock, the fliabie nature of the coalbed,

and by using a dulled drag bit. This strategy might have

application to other sofi coalbeds with similar geology.

Downhoie Motor Drilling Teehnique: Downhole motor

driliing of horizontal boreholes for methane drainage in coal

offered many advantages compared with rotary driiling.

Inherently, driliing productivity was observed to be greater

with downhoie motor drilling because it was not necessary to

pull the drill string out of the borehoie to change borehole

direction or to sidetrack. Figure 4 shows the major

components of a typical downhole motor. The downhole

assembly is used to maintain horizontal borehoie trajectory.

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A REVIEW OF HORIZONTAL DRILLING AND COMPLETION TECHNIQUES FOR RECOVERY OF COALBED METHANE SPE 37131

Drilling Program

Most horizontal drilling techniques for coaIbed methane were

developed from the mining industry based on modified

directional drilling techniques developed for crude petroleum.

The general technique used in drilling a horizontal well

through a coalbed is to first drill a vertical pilot hole to certain

depth above the coalbed. This depth depends on whether the

drilling technique is short-radius or medium-radius, The

vertical hole is then reamed to the actual hole size. Casing is

then set and cemented before drilling the horizontal hole. A

typical hole size/casing configuration for the pilot hole is 3 in

pilot hole reamed to 8-3/4 in hole in which a 5-3/4 in casing is

set. Usually the casing is set through the curved section. The

horizontal hole is then drilled using a 2-3/8 in diameter dyna-

drill downhole motor and a 3-inch diameter diamond drill bit.

The size of the horizontal wellbore is based on the casing

program rather the reverse. A carefid hole size and casing

configuration eliminates problems such as insufficient tubing

size which prevents effective dewatering the coalbed reservoir

and limits the ultimate recovery of methane. Also, insufficient

casing size limits the injection rate needed for effective fracture

treatment, and causes ineffective drilled cutting removal.

Control of the pilot hole is accomplished by the use of

3/4 and 1/2 bent housing (with or without standoff rings) on

the dyna-drill motor. Azimuth is controlled by rotation of the

tool face. The smaller diameter horizontal holes cidled in coal

tlom within mines, have consistently been found to stay open

for a long time, except when drilled in stressed zones near

mine openings. In some cases these horizontal holes were

ddled to follow the regional dip of the coalbed to prevent

caving. Drilling parameters which include pump pressure,

mud volume pumped, penetration rates, and pulldown

pressure are recorded using a geolograph.

Mud and Hydraulic Programs: Water-based mud systems

are generally used. On one occasion, two types of low-solids

polymer mud systems (Dexrid and XC-polymer) were used.

One to prevent fluid loss and the other to provide gel strength

and aid in removal of drill cuttings9. In order to slow down

fluid loss, such additives as cottonseed hulls, cellophane

flakes, and ground walnut shells are added to the mud

systems. Attempts have been made also to treat the lost-

circulation problem with cement and later with gunk

squeezes. The gunk squeezes were observed to be diflicult to

drill through by the dyna-drill motor. It was found that irr

horizontal drilling, some type of drilling fluid additives were

required for removal of cuttings and higher flow rates than can

reasonably be run through the dyna-drill were required for

cleaning the hole. It was recommended that a bypass valve

capable of delivering 25 to 30 gallons per minute to the dyna-

drill at differential pressures of 500 to 700 psi and bypassing

an additional 60 gallons per minute be used to improve

cuttings removal. Air/mist drilhtg fluids have been us in

some areas. These fluids provided required circulation

velocity and also prevented hole erosion and enlargement.

Bits: In general diamond bits were used for all of the ddling

operations irr coalbeds. In particular the 3-in diameter

diamond deep cone bit was found to be very effective for

horizontal drilling and allowed rapid sidetracking. The

diamond bits were more efficient because of their long life

and lower overall cost, due to their high salvage value. Testing

of tricone bits or hole openers for reaming work was also

found to be worthwhile since the diamond reaming bits were

observed to be inefficient and expensive. A drag bit was

recommended for drilling coalbed because of its penetration

rate and its usefulness for contact drilling. In contact drilling,

a worn stepped drag bh was recommended because it has lost

some of its ability to penetrate floor or roof rock and is still

able to drill coal.

The same success could be achieved by

dulling a drag bit in the shop before putting it into service.

Specially designed tri-con~ bits used in the oil and gas

industry were also found to be applicable for rotary drilhng of

horizontal wellbore. However, they are more susceptible to

corrosion if drilling with aidmist drilling fluid.

Borehole Trajectory

and

SuNey Techniques: In general,

the boreholes exhibited arcing in the right-hand or clockwise

direction, as have the majority of previously rotary drilled

boreholes. A single-shot survey instrument was generally used

to determine borehole inclination during rotary drilling. The

magnetic single-shot survey instrument was also used to

determine inclination, bearing, and tool face direction during

drilling with the in-hole motor.

Problems: Most of the drilling problems encountered were

usually mechanical in nature and involved directional control.

Mechanical problems included rig, dyna-drill, and mud pump

breakdowns. In some cases, problems in maintaining the

proper well path required setting plug backs and redrills prior

to the fwt intercept of the coalbed. Other specific problems

were caving of the holes drilled in shale and depth correlation

between the horizontal well bores and the vertical holes. In

the former case, if the shale formation was near the bottom of

the casing, recentering of the holes was likely to be difficult.

Correlation was diflicult because the accumcy of most well

surveying equipment is at best * 2 to 3 t? vertically at the

measured depth and many coalbeds are irr the range of 3 to 10

tt in thickness. In addition, experience with survey data fi-om

several directional holes indicated that the vertical depths

shown by the survey were usually deeper than the actual

vertical depth of the holes. This made correlation with the

vertical hole dlfflcult. Indication of coal tops obtained horn

dyna-drill drilling rate changes and horn cuttings was also

likely to be misleading, especially when drilling nearly

horizontally across a formation. This problem was solved by

drilling the well into it or even completely through it. The

new geosteering tool can now be used to eliminate this

probleml 1.

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SPE 37131

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5

Completion Options And Strategy

The

methods for completing horizontal coalbed wells have

evolved fkom completion experience with vertical coalbed wells

and conventional oil and gas wells. However, these methods

must be modified to accommodate the unique properties of

coalbed reservoirs. In order to successfidly complete a

horizontal wellbore in a coalbed reservoir there are a few

properties of coal that must be understood. Some of the major

properties are: (1) the coal cleat system must be effectively

connected to the wellbore, (2) the coalbed must be dewatered

before gas production can occur, and (3) the well should be

produced at minimum bottomhole pressure to maximize gas

desorption12. The strategy concerning which completion

method to choose for a particular well depends upon the

stratigraphy of the coal reservoirs, the depth to the coal, the

permeability in the coal, in-situ stresses, the amount of coal

frees that are expected to be produced, and the problems

associated with lifting and disposing of water that is produced

tlom the coalbeds. The three completion techniques that have

been commonly used for coalbed reservoirs are open-hole

completion method, a stable cavity completion method, and a

perforated casing completion methods, Fig. 5-.

Open-hole Completion Method: Initially vertical coalbed

wells were completed openhole during which the casings were

set and cemented above the coal formation to prevent damage to

the formation. This method allowed wellbore contact with the

cleat system of the formation. This method also was determined

to be troublesome due to the extensive cleaning required as a

consequence of coal sloughing. The open hole can further be

divided into true open-hole, slotted or pre-perforated

hner/casing or segmented uncemented liner/casing completion.

Generally, an uncemented completion was used when a

completion with little or no stimulation was anticipated.

True open-hole completions were generally used in

medium to high permeability competent coalbeds with little

water. No effective production control can be obtained with

open holes

or slotted liner completions. The advantages of true

open hole completions were low cost, no production loss, and

providing superior injection with the least flow resistance, with

no effective control of flow into the appropriate zones. Pseudo

open-hole perforated or slotted liner completions can be used to

protect the borehole from collapse. The slotted liner provided a

convenient method of maintaining a flow through the zone as

well as the ability to keep the hole open. Limited frees control

was achieved by choosing appropriate dimensions of slots and

drilled holes. However, the liners were susceptible to plugging.

Stable Cavity: In this method, a cavity was created by jetting

the well with gas over a long period of time until coal frees

were no longer being circulated from the well. This method

was generally recommended for a thick coalbed with

permeability greater than 50 md. That is, if high permeability,

geopressured, thick coalbeds were encountered, one may

select the stable cavity completion as the optimum method for

a particular well. That choice will be highly dependent on

factors involved with jetting out the well and disposing the

fluids. To complete a well properly, data must be obtained and

the completion must be properly designed.

The San Juan Basin has had the best results with the cavity

completion techniqut?3-lb. Here, casing was set above the main

coal target, then the coalbeds were jetted from the well until a

stable cavity was created. Normally, a slotted liner was run in

the well. The cavity completion technique creates a cavity with

a radius of up to five feet in some wells. San Juan wells with the

cavity completions have produced, on average, ten times more

gas than the hydraulic fractured wells~

Other operators have tried this technique outside San Juan

coalbeds and have found that they were not as successful. With

the open cavity, there was the long-term concern of stability.

There was also a problem with the production of coal frees

which carmot drop to the rathole (or sump) below the lowest

target zone. The cavity appeared to be more successfid in the

San Juan because of its localized coalbed properties. The

reservoir was over-pressured and the permeability was high.

These were likely to be the properties responsible for the

success. The reason that cavity completions appeared to be

more successful seems to be that the cavity enhanced the

permeability around the wellbore by (1) a tensile fhcture

induced during air/water irjection phases, and (2) shear failure

zones induced during blow-down phases of the cavitation

process, Fig. 6.

Perforated Casing: Due to the sloughing problem during

open-hole and stable cavity completions, operators began using

perforated cemented casing. This was the premium completion

approach. Implicit in the decision to case and cement was a

commitment to perforate and stimulate the well. This was a very

expensive completion, Early wells had problems due to low

strength casing and improper fluid and cements which created

large amounts of formation damage. Hydraulic fracturing was

required in both of the above scenarios. The openhole

completion technique did not prove acceptable for hydraulic

ti-acturing techniques. The perforated casings were too weak at

fmt to complete a successful fracture job. Operators soon

created casing strong enough to withstand the pressures

associated with the hydraulic fractures.

Today, hydraulic

fracture jobs are done in the cased holes with great success.

Problems such as the creation of a complex fracture network

near the bore-hole, the creation of coal frees that may block

portions of the fracture, high poroelastic effects, and slip at the

fi-acture tip, can cause the magnitude of the tiacture

propagation pressure to increase substantially. Table 2 shows

a summary of drilling and completion techniques used in

some coalbed basins. Table 3 shows some advantages and

disadvantages of the various completion techniques.

Hydraulic Fracturing in Coalbeds

In the San

Juan coalbeds, the cavity technique has been proven

to be the most prolific technique used, while in most other areas

hydraulic fracturing still remains the best. Hydraulic bxturing

for coalbeds has been under study for a great amount of time

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A REVIEW OF HORIZONTAL DRILLING AND COMPLETION TECHNIQUES FOR RECOVERY OF COALBED METHANE SPE 37131

and

much improvement has

been

made. Hydraulic fracturing

requires that the horizontal well be cased and cemented for best

results. The concept behind the fiacturirtg is to create a flow

path perpendicular to the greatest permeability, the face cleat.

With this objective in mind, studies are done on the coalbed to

determine the maximum and minimum stresses and other rock

properties. With the determination of the direction of the face

cleat, the casing can be perforated so that the fractures will be

propagated perpendicular to the face cleat. This approach is the

best possible scenario for horizontal coalbed wells with low and

anisotropic permeabilities. The horizontal wellbore then will

penetrate the reservoir perpendicuhrto the face cleat and can be

propagated further, Fig. 7. A horizontal well drilled normal to

the maximum horizontal stresses will benefit from the

maximum horizontal permeability. It has been shown through

simulation that a properly drilled and completed horizontal well

may increase production by 75 0/0 over that of a fkactured

vertical well, Fig,8S.

Conclusions

Based upon the review of the literature, the following

conclusions are presented.

1.

2.

3.

4.

5.

Horizontal well drilling and completion is an alternative

technique to overcome low production as well as

reservoir heterogeneities in coalbed reservoirs. Horizontal

wellbores are very effective in coalbed reservoirs which are

relatively thin, naturally ti-actured, and exhibit anisotropic

permeability.

Most horizontal drilling techniques for coalbed methane

were developed from the mining industry based on

modified directional drilling techniques developed for

crude petroleum. The general technique used in drilling

horizontal well through a coalbed is to fnt cldl a vertical

pilot hole to certain depth above the coalbed. This depth

depends on the ddlirtg technique whether short-radius or

medium-radius. Core samples are taken and tested in the

pilot hole to determine the critical range of drilling fluid

weight required to maintain horizontal wellbore stabili ty.

The most commonly used technique is the medium radius

using the specially designed mud motors.

With the

medium radius technique, the original objective of drilling

perpendicularto the face cleat, or the greatest permeability,

can be controlled and monitored while drilling with MWD

techniques.

Downhole motor drilling of horizontal boreholes for

methane drainage in coalbeds offers mmy advantages

compared with rotary drMng. Inherently,

drilling

productivity is observed to be greater with downhole

motor drilling because it is not necessary to pull the drill

string out of the borehole to change borehole direction or

to sidetrack.

The size of the horizontal wellbore must be based on casing

program rather the reverse. A caretlrl hole size and casing

configuration eliminates problems such as insufficient

tubing and casing size, and ineffective drilled cutting

removal.

6. The methods for completing horizontal coalbed wells have

evolved from completion experience with vertical coalbed

wells and conventional oil and gas wells. However, these

methods must be modified to accommodate the unique

properties of coalbed reservoirs.

7. The three completion techniques that have been

commonly used for coalbed reservoirs are open-hole

completion method, a stable cavity completion method,

and a perforated casirtg completion method.

8. The properties of a coalbed essential to its completion are

dual-porosity, permeability, gas adsorption, stratigraphy,

bottom-hole pressure, and water production. Core samples

should be taken and tested to determine the minimum and

maximum stress directions and determine how weak and

friable the coal is. This information is needed to determine

if the coal will be stable enough in the lateral section.

Acknowledgments

The

authors gratefully acknowledged the editing and

suggestions of Elise StriL the typing skills of Hyun Cho and

Kayode Aremu. Financial support from School of Petroleum

and Geological Engineering, The University of Oklahoma and

NSF through the NSF Faculty Career Award Grant made

possible the research and preparation time for this work.

References

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

Deul, M. and Cervik, J.: Methane Drainage in the Pittsburgh

Coalbed; Paper presented at the I~ International Conference of

Mining Safety Research,

Vama, Bulgaria,Oct. 3-7, 1977; pp 9-15.

Presser, L.J., Finfinger, G.L. and Cervik, J.: Methane Drainage

Study Using Underground Pipeline, Mariarma Mine 58V US

Bureau of Mines RI 8577,1981.

The United States Coalbed Methane Resourcej an article in&

@mrterlv Review of Methane From Coalbeds Technology

Volume 7, 3, March 1990, published by the Gas Resewch

Institute, Chicago, Illinois.

Deimbacher,F.X., Economides,M.J., Heinernann, Z.E., and

Brown, J.E.: Comparison of Methane Production From Coalbeds

Using Vertical or Horizontal Fractured Wells,SPE21280, 1990.

Ertekin,

T., Sung W., and Schwerer, F.C.: Production

Performance Analysis of Horizontal Drainage Wells for the

Degasification of Coatbeds, SPE 15453,1986.

Gray, I. Reservoir Engineering in Coalbeds: Part 1 - The

Physical Process of Gas Storage and Movement

in Coalbeds,

SPERE (Fe. 1987), 28-34.

Kravits , S.J., Sainato, A., and Finfinger, G.L.: Comparison of

Rotaty and In-hole Motor techniques for Drilling Horizontal

Boreholes in Coal, US Bureau of Mines, RI 8933, 1985.

Oyler, D.C. and Diamond, W.P.:

Drilling a

Horizontal Coalbed

Methane Drainage System From a Directional Surface

Borehole, US Bureau of Mines RM 8640, 1982.

Oyler, D.C., Diamond, W.P., and Jeran, P.W.: Directional

Dril ling For Coalbed Degasification, US Bureau of Mines RI

8380, 1978.

Goodman, T.W.: Rotary Dril ling Techniques Used in Beckley

Coalbed: US Bureau of Mines, RM 9238, 1989.

Maurer, W.C.: Recent Advances in Horizontal Drilling, JCPT,

NOV. 1995, pp. 25-33.

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SPE 37131

SAMUEL O. OSISANYA AND ROBERT F. SCHAFFITZEL

12. Holditch, S.A.: Completion Methods in Coalbed Reservoirs,

SPE 20670, paper presented at the 65* Annual Technicrd

Conference and Exhibition of the Society of Petroleum

Engineers held in New Orleans, LA, September 23-26, 1990.

13. Logan, T. L., Clark, W. F., and McBane, R.A: Comparing

Openhole Cavity and cased Hole Hydraulic Fracture Completion

Technique, San

Juan Basin, New Mexico; SPE 19010, paper

presented at the Rocky Mountain Regional Meeting in Denver,

CO, March 6-8, 1989.

14. Logan, T.L.: Horizontal Drainhole Drilling Techniques Used for

Coalbed Resource Exploitation,SPE 18254,1988.

15. Logan, T.L., Schwoebel, J.J., and Homer, D.M.: Application of

Horizontal Drainhole Drilling Technology for Coalbed Methane

Recovery, SPE/DOE 16409, 1987

16. Palmer, I.D., Mavor, M.J., Seidle, J.P., Spitler, J.L., and VOIZ

R.F.: Openhole Cavity Completions in Coalbed Methane Wells

in the San Juan Basin, SPE 24906, 1992.

17. Schraufiagel,R.A, SpafTord, S.

D. and Saulsberry,

J.L.: Multiple

Seam Completion and Production Experience at Rock Creekfl

(Water-Operationalprocedures).

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8

A REVIEW OF HORIZONTAL DRILLING AND COMPLETION TECHNIQUES FOR RECOVERY OF COALBED METHANE SPE 37131

Table 1- United States Coalbed Methane Resources ref. 3

COAL BASI?WU3GION STATES GAS IN PLACE

Arkoma Basin OklahomL Arkansas

2t04

Black Warrior Basin Alabama. Mississimi

2

Cahaba Coal Field Alabama 2

Central Appalachian Basin Tennessee, Kentucky, West Virgini& Virginia 5

Coosa Coal Field Alabama

1

Greater Green River Coal Wyoming, Colorado

1 to

3

Region

Illinois Basin Illinois, Indiana, Kentucky 5t021

Northern Appalachian Basin Pennsylvani~ Maryland, West Virgini% Ohio, 61

Kentuky

Pemsylvania Anthacite Fields Pemsylvania NQ

Piceance Basin

Colorado

84

Powder River Basin Montan~ Wyoming

3

Raton Basin Colorado, New Mexico 8to 18

Richmond and Deep River Virgini% North Carolina 2t03

Basins

San Juan Basin

Colorado, New Mexico

Fruitland formation

5

Menefee Formation

22 to 34

Valley Coa

Uinta Basin Utah, Colorado lto5

I Fields Virginia NQ

n Coal Washington 1 to 24

estern

Washingto]

Region

I

Wind River Basin I Wvomirw lto2

,

.W

,

TOTAL 296 to 394

NO

Not quantified I I

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SPE 37131

r

esources

San Juan Basin

Black Warrior

Basin

SAMUEL O. OSISANYA AND ROBERT F. SCHAFFITZEL

Table 2- Summary of Drilling and Completion Techniques

Drilling Techniques

and Data

Mud drilling

Air drilling;

cement casing with

light weight slurry

Air/ Mud drilling

Completion

Techniques and Data

Perforated casing

stable cavity

Perforated liner

completion, lower

portion of the

coalbed 0.6 to 0.8

psihl )

Same as for Warrior

Basin

Justification

Thick, high

permeability, high

messure coalbeds.

To prevent

sloughing of the

hole and caving.

Same as for

Warrior Basin

Associated

problem

High fracture

pressure

Difllculty in

pumping fracture

treatment

Same as for

Warrior Basin

Table 3- Advantages and disadvantages of various completion techniques

Type of Completion

I

Advantage Disadvantage

Open-hole completion I Reduce formation I Frequent clean out due to wall sloughing.

I

damage. I Require tlacturing due to low permeability, but

I

I this is diff]cult

Perforated Casing I Prevents sloughing hole and I Difllculty in pumping ffacture treatment

casing. through perforations.

Requires high fracture treatment pressure

unique to coalbeds (use strong casing; use fluid

that will cause minimum damage)

I Expensive.

Stable Cavity I Usefil in high permeability, I Not understood

I hi~ messure and thick coalbed I Mav be costlv

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10

A REVIEW OF HORIZONTAL DRILLING AND COMPLETION TECHNIQUES FOR RECOVERY OF COALBED METHANE SPE 37131

W ST RN

WASMIWGTOM

24 lCF w4yT:rWf

POWDER alvcn

~~

3* lCF

lLLlttOls

IIOR1 IE8U PEMMITIVANIA

1

21 TCF APPALACMIAM AMTlf8AClTE

f-l

I

II

MOR114.KOTA

---7 I

T

Y

1

sow OAtlQ,.

I \ [1{

Fig.1 - Principal United States coalbed basins and latest accepted estimates on in-place

coalbed resources (Ref. 3)

--- .. .--- ....

Froctured Vertical Well

1-

Un(roclured Vert cal Well ~

-. ..-

6-

L

2- .

-i

.

o

I

o

:00

500

Iwo

t (days)

Fig. 2- Increased flow rates of fractured vertical well over unfractured well

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SPE 37131

SAMUEL O. OSISANYA AND ROBERT F. SCHAFFl~EL

11

I

..8

. .....

NoI O stole

.70

osiw~6-inOD

0-in cen rolizcr, 3 IG-in OD

\

8il , 3 /2-inOD,

Fig. 3- Various rotary drill string configurations (ref, 7)

~Orienting sub

Lower bearintjhousing

--

Fig. 4 - Major components of a typical downhole motor (ref. 7)

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12 A REVIEW OF HORIZONTAL DRILLING AND COMPLETION TECHNIQUES FOR RECOVERY OF COALBED METHANE SPE 37131

11

EARLY OPENHOLE

STABLE CAVITY

PERFORATED AND

FRACTURE TREATED

11

-~~

Fll.-.....................-.

Fig. 5- Illustration of three different completion methods used in coalbed reservoirs ref.

13

1

FACE

CLEAT

\

50

GM

-1 501

fl

\

SHMAX

N

/

\

PROPPED

\

FRACTURE

\ 129

\

Y

\

\

SHEAR ZONES

TENSILE

K

\

FRACTURE

/

\

\

\ 128

\

CA~?TY

\

rc = 5

\

\

Fig. 6- Artists conception of permeability enhancement around cavity well: 1 tensile

fracture, 2 shear failure zones.

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SPE 37131

SAMUEL O. OSISANYA AND ROBERT F. SCHAFFl~EL

I 1

\

bu~ cleat

butt cleat

Fig. 7- Principal permeability directions and positioning of horizontal boreholes in a typical

coalbed plan view

1

0 ~

5 woo

t days

13

Fig. 8- Increased flow rates of a horizontal well over a fractured well ref. 5