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IADC/SPE 128712

Utilizing an Engineered Particle Drilling Fluid to Overcome Coal DrillingChallengesSabine Zeilinger, Fred Dupriest, ExxonMobil Development Company, Ryan Turton, Esso Australia Pty Ltd.,Hayden Butler, Hong (Max) Wang, Halliburton

Copyright 2010, IADC/SPE Drilling Conference and Exhibition

This paper was prepared for presentation at the 2010 IADC/SPE Drilling Conference and Exhibition held in New Orleans, Louisiana, USA, 2–4 February 2010.

This paper was selected for presentation by an IADC/SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have notbeen reviewed by the International Association of Drilling Contractors or the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflectany position of the International Association of Drilling Contractors or the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of thispaper without the written consent of the International Association of Drilling Contractors or the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to anabstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of IADC/SPE copyright.

AbstractBorehole instability and stuck pipe while drilling coal can lead to significant non-productive time. This paper describes a

drilling fluid developed to stabilize coals in a program that was previously unable to achieve all extended reach objectives.The fluid design principles are believed to apply broadly in stabilizing coals, fractured shale, and other cleated formations.The rock mechanics that govern instability in coals is identical to shale. However, coal instability often does not respond tothe same remediation used in shale, which is to simply raise the mud weight to reduce the compression hoop stress to belowthe strength of the rock. Despite using an optimum mud weight, hole breakout or borehole collapse may still occur when thecoal cleats and natural fractures of the coal allow the drilling fluid filtrate to invade. This leads to a pressurization of the near-

wellbore region and loss of the effectiveness of mud weight support for coal stability.A fluid was developed based on the belief that the wellbore is stabilized with increased mud weight if the additional pressure

acts specifically on the face of the borehole. This stabilization effect can be achieved by preventing pressure penetration into

the near-wellbore region through coal cleats or natural fractures. The fluid design developed included bridging particles with asize distribution based on analysis of the coal cleat apertures, as well as filtration control material to effectively reduce the permeability of the bridge. Sealing alone does not provide stability and an analysis of the coal strength and in-situ stresses

were conducted to select a mud weight that would stabilize the very weak coals.The paper discusses the rock mechanics concepts, fluid design criteria for determining the allowed leakage rate when

designing the bridging process, and the operational learnings from implementation. The use of the coal stabilization fluid andstability mud weight allowed the objectives to be achieved and contributed to record performance in this a narrow-margindrilling environment in Australia.

IntroductionWhen drilling for hydrocarbons a number of formations have to be drilled before reaching the objective. These formations are

as varied as shale, sandstone and carbonate, and sometimes coal seams. The recent years, however, have seen an increase incoal drilling due to an increased exploration into coal bed methane. Instability in coal is common, both in the overburden and

within the productive interval. The high angles at which many extended reach wells are drilled contribute to both the incidentrate and the consequence of instability. Several papers have been written on the experience in coal drilling and strategies toavoid or overcome these challenges. The practices used to drill coals vary, and many appear to be contradictory. One designdecision that is often debated is whether it is generally more effective to increase or decrease the fluid density. Various

authors have taken differing positions. The same concerns have also been debated by those drilling fracture shales, withsimilar logic and recommended practices. In both coals and fractured shale, field success is claimed with both higher and

lower fluid densities. A comprehensive physics-based understanding of the failure process is required to explain thecontradictory observations and develop design and field practices that are universally correct, rather than those that have beenderived empirically and succeed only under specific local conditions.

Coal Cleats

Coals are deposits of organic material. Overburden and time convert these sediments into carbon-rich coal seams.

Depending on time, overburden and age, the maturity of coals leads to a differentiation of the quality of the coal, such as

carbon content, hardness and morphology. One of the main characteristics of coal is a system of cleats, regular gaps and

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fractures in the coal that vary with coal maturity (Figure 1). Different theories exist on the genesis of cleats, and some pointtowards a combination of burial, tectonics, dehydration and shrinkage of the organic material. In a review Laubach et al (1998) describe coal cleats, their appearance and origins. The fractures occur most commonly perpendicular to each other, andtypically perpendicular to the bedding plane. In microscopy studies ( Laubach et al 1998) the observed cleat widths rangedfrom 0.001 to 20 mm, although these studies were conducted without any confining pressure. Modelling suggests that under

pressure coal cleats may be 3-40 μm wide. These cleats form an interconnected network of fractures that contributes to

Figure 1. - Coal with cleats and fractures.

methane production. In the case of coal bed methane, some methane adsorbed on the surfaces of the cleats may contribute tothe total methane production. Drilling fluid invasion while drilling can lead to significant formation and production damage(Palmer et al , 2005, Gentzis et al , 2009). Concepts that have been developed for stabilization of uniform, low permeabilityshales may not yield the expected results due to the unique physical characteristics of coals, particularly the existence of a cleatsystem.

Published Concepts on the Eff ect of Coal Cleats in Dr il li ng

It is this network of cleats that is also thought to be at the origin of many drilling-related coal stability problems (Palmer et

al , 2005; Zhang, 2005; Baltoiu et al , 2006; Barr, 2009). Many publications describe the mechanism for the problems as one offluid invasion that pressurizes the coal cleats and leads to failure of the rock. However, the published solutions to this problemare varied and some may only address part of the cause for the coal instability. It is suggested that the key to managing coal

stability is avoiding fluid invasion along the cleats:Zhang (2005) suggests that the invasion can be avoided by keeping the mud hydrostatic pressure below the calculated

fracture propagation pressure to avoid pressurizing the existing fractures. However, in the authors’ experience coals are seento destabilize without the occurrence of lost returns, which would be the observed behavior if the cleats were forced openedand extended.

Santarelli et al (1992) investigated the mechanisms of borehole instability in naturally fractured rock media and concludethat “increasing mud weight to solve borehole instability can have disastrous consequences, particularly when the formation

is naturally fractured .” This view is also echoed by Baltoiu et al (2006), who describe the mechanism of coal failure as aneffect of pore pressure penetration. It appears that the prevalent view is that the drilling fluids overbalance increases the mudor filtrate flow into the cleats, leading to pressure equalization of the wellbore with the near wellbore region. The overbalance

holding the rock back is lost and blocky coal pieces fall into the wellbore as a consequence. A suggested remediation is proposed by using a low fluid density to minimize differential pressure and also formulating a fluid that inhibits filtration orflow of the drilling fluid into the cleats. This optimized drilling fluid is achieved with polymers and fibrous additives thatform low-permeability bridges across at the mouth of the cleat.

However, in some applications lower borehole pressure appears to increase instability and success has been reported withhigher mud weights. Last et al (1995) reported success with increased mud weight, but they recognized that in order for theincrease to be effective, it is necessary to seal the fractures. This is an important observation. It is generally known that filtercakes do not achieve complete sealing and that some fluid and pressure penetration always occurs. The relationship betweenfactors such as fluid penetration rate, far field leak-off, coal strength, and borehole geometry must be further understood inorder to consistently design successful drilling operations. This paper explores these relationships further and seeks to provide

general design criteria that incorporate the various critical issues. Finally, a case is presented in which problematic coal seamswere drilled successfully off the coast of Australia.

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Mechanisms of Borehole InstabilityA formation in the earth’s crust is exposed to horizontal and overburden stresses. When a hole is drilled, the borehole wallmust carry the load that was previously supported by the removed rock. This causes the hole to attempt to contract in theradial direction, leading to a stress concentration in the immediate vicinity of the wellbore, the so-called tangential or hoopstress. If the resulting effective stress in the near-wellbore region exceeds the strength of the rock, the formation fails. The

drilling fluid pressure resists the effective stress, which limits the strain and stress that can develop in the tangential direction.Fluid density has to be raised to a level which reduces the effective stress below the strength of the rock. In this manner, a

sufficiently high mud weight can alleviate the stress concentration around the wellbore and provide enough support on thewall. The basic failure concept is no different in coals than shales and other formations.

Methods for calculating both effective stress and tangential stress are well established. However, when considering uniquesituations, like the cleats in coal, it is important to understand the actual physical deformation that the equations describe and

the manner in which the effective stress is affected in the near-wellbore region.While models exist to calculate the mud weight required to achieve mechanical borehole stability based on a set of rock-

specific input parameters, these models are typically built for uniform shale and assume little fluid invasion occurs in the near-wellbore region. In the model, the increased mud weight acts on the face of the borehole to reduce the tangential stress.However, if fluid pressure is allowed to invade the pore space, the differential pressure across the face of the borehole isreduced, resulting in higher effective stress. As a result, an even higher mud weight may be required, which may not be possible to apply if the integrity of other zones is inadequate. Over time the pressure across the face of the borehole may

continue to equalize regardless of the level of mud weight chosen.The mechanism of this so-called pore pressure penetration in shales is described by Detourney and Cheng (1988); van Oort

et al (1996); Chen and Ewy, (2002). In the field one may observe that an initially stable borehole transitions into boreholeinstability when the drilling fluid is allowed to invade into the formation. However, in fractured shale the pressure invasion isnot uniform and a region with two permeabilities must be modeled, one of the matrix and another along the path of the cleat.The pressure drop along a fracture will depend largely on its geometry and the degree of connection to other fractures so thatthe fluid is able to leak to the far field. If the fracture is not well connected, back pressure may build in the fracture system

over time so that the pressure in the fracture equalizes with that in the wellbore. The failure that eventually occurs is the resultof the change in effective stress and shear strength that the pressurization creates along the face of the fracture. If the mudweight used is inadequate to reduce the stress to below the shear strength along the face of the fracture, borehole breakoutoccurs (Ottesen, 2010).

A similar process creates the borehole enlargements that are commonly observed in unconsolidated sands, though the stressand shear strength changes created by pressure penetration occur throughout the matrix, rather than along fracture planes.

These enlargements in unconsolidated sands are often believed to be due to fluid erosion. However, the laminar and eventurbulent flow regimes do not usually apply sufficient fluid shear force to cause failure of the rock due to erosion, given the

confined strength in these sands. While extremely high bit nozzle velocities may create some enlargement in very softformation (< 1000 psi confined strength), the enlargement is usually less than 1-2 centimeters due to the rate at which thevelocity falls as the fluid travels away from the nozzle. The enlargement is more properly explained using rock mechanics principles. Unconsolidated sands are typically encountered in shallow drilling, where low mud weights are employed.Drilling fluids with low mud weight lack a sufficient concentration of solids (weighting agents or other solids) to form

effective filter cakes to prevent the influx of filtrate or even whole mud into the formation. As the fluid invades, pressureequalization across the face of the sand allows the effective stress to rise in the face of the borehole so that the low-strengthsand fails in shear. The enlargement is easily prevented by establishing a more effective filter cake on the unconsolidated sandso that the pressure drop occurs at the face of the borehole to limit the inward radial strain. This can be achieved by addingsolids to the fluid system that effectively block the pore throats and pore bodies to the fluid penetration. Depending on thelocal stress field and confined strength of the sand, a small increase in mud weight may also be required and a gauge hole can be drilled.

Design Concept for Borehole Instability in CoalsIt appears logical to extend the analogy of naturally fractured shale and unconsolidated sands to a coal cleat system. However,there are significant differences between the stress distribution that develops around the borehole if the pressure penetrationoccurs only along the cleates, rather than uniformly across the pore space. If the penetration is uniform, the resulting reductionin effective stress and strength is uniform. In coals, this occurs largely within the face of the cleats, and not the adjacent intact

rock. As was observed by previous authors (Baltoiu et al , 2006; Barr, K., 2009, Palmer, 2005), the pressure drop into thecleats will occur over some distance, rather than at the immediate face and this will reduce the effective stress between thefaces of the cleat (Figure 2). As the effective stress declines, the shear resistance between the faces declines more rapidly,and continuing pressurization leads to failure.The design strategy to maintain coal stability is to ensure two objectives are achieved: (1) control the rate of fluid entering thecleat system so that the leakage to the far-field cleat system is greater than the leak rate through the face of the wellbore, and(2) utilize sufficient mud weight to reduce the effective stress to below the shear resistance along the cleat faces, given that

some pressure penetration is certain to occur. The first requirement limits the backpressure built over time that causes thedifferential pressure to decline across the face.

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hydrostatic mudpressure

pore pressure

pore

pressure


pore

pressurepore

pressure

Radial distance

from wellbore

Radial distance

from wellbore

pressure pressure

Initial exposure of cleats to drilling fluid:

Overbalance must be maintained to

retain borehole stability.

Fluid driven into cleats if no barrier at the

cleat mouth:

Overbalance cannot be maintained.


pore pressure

pore

pressure


pore

pressurepore

pressure

Radial distance

from wellbore

Radial distance

from wellbore

pressure pressure

Initial exposure of cleats to drilling fluid:

Overbalance must be maintained to

retain borehole stability.

Fluid driven into cleats if no barrier at the

cleat mouth:

Overbalance cannot be maintained.

Figure 2. – Mechanism for pore pressure penetration in coals.

Modeli ng and M odel Resul ts

An existing wellbore stability model has been extended to model borehole stability in fractured formations, as well as thetime effect of the pressurization of the fractures in the near well region. The model accounts for dual permeability: an almost

impermeable coal matrix, and some permeability through the coal cleats (Table 1). It is important to note that this boreholestability model is based on continuum-mechanics, and may provide only approximate quantitative results. However, themodel captures the global conclusions of a fractured/cleated coal system. This model is applied to an Australia-type well.

Input parameters for the wellbore stability model are shown in Table 1; some assumptions have been made for the coalformations. Not all input parameters were available for this case; however, for understanding the concept, reasonableassumptions for the coal formations would provide reasonable results (Carmichael, 1982).

Table 1. Model Input Parameters

MW [ppg] 11.0

Wellbore deviation [°] 35

Azimuth [°] 223

Direction of max. horizontal stress [°] 139

Ratio of maximum horizontal stress/minimum horizontal stress 1.05

Coal matrix permeability [nD] 0.1

Coal cleat permeability [mD] 0.1

The simulation assumes that fluid has no sealing capability; fluid flows into the cleats without any barrier. The pore pressure distribution in the cleats is shown in Figure 3 for several time intervals. In the absence of fracture sealing, the pressure in the cleats in the near-wellbore region rapidly increases to the wellbore fluid pressure. This simulation assumes

there is far field leakoff. If this near-wellbore pressure cannot dissipate as rapidly into the far-field as the fluid is invading the pressure will rise even further than assumed in this simulation, and the required MW will be higher.

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Figure 3. – Pore pressure variation inside cleat with time and distance from wellbore.

Figure 4 shows critical mud weights and corresponding fracture effective tangential stress around the wellbore as time progresses. While Figure 4 depicts a smooth curve, cleats are obviously not present in all directions. The smooth curveindicates the stress that would exist should a cleat be present at the given point on the circumference. If the pore permeability within the coal is very low, which is common, the effective stress within the intact coal will be quite differentthan that within the pressurized cleat. The model accommodates this dual permeability behavior and shows that the failure

plane will be within the cleats rather than the intact coal.As pressure rises within the cleat, the effective stress declines. While the stress is falling, the shear strength along the

cleat declines more rapidly and the stress may exceed the shear strength. When the mud weight is increased, the effectivestress is increased, but the shear strength decreases faster. In the ideal case pressure penetration is prevented completelyand the original effective stress and strength are maintained. The calculation of the stability mud weight is then no differentthan in intact shale or other homogeneous material.

Without cleat sealing, wellbore instability would be observed during normal drilling soon after the coal formation isexposed. It is thus important to prevent fluid invasion into the cleats, or slow the flow significantly so that pressure build-upin the near wellbore region does not occur.

Figure 4. – Change in effective tangential stress around the circumference of the wellbore over time as pressure invades the cleatsystem. Annotation shows the resultant change in stability mud weight.

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Drilling Fluid Design for Coal DrillingThis theory was put to the test in the Snapper / Moonfish campaign, offshore Australia in 2008 (Kilroy and Dupriest, 2009).Offset analysis of the wells drilled during the previous Snapper / Moonfish drilling campaigns identified the key challengesand critical success factors that needed to be met to successfully execute the proposed extended reach wells. The well had tomaintain sufficient mud weight to keep the wellbore stable, and control stability in several major coal seams. Eight significant

coal seams with thicknesses ranging from 3-8 meters had to be drilled, along with minor coal seams less than 2 meters thick.The team dealt with these demands with changes to the well trajectory, and adapting operational practices to the task. In the

original campaign, the 1997 Moonfish drilling campaign consisted of a traditional extended reach s-turn wellbore profile(Figure 5).

This design basis complicated the drilling problems as it did not separate the wellbore stability and lost circulation

challenges into different hole sections. Toward the end of the 12¼-in. intermediate section, the mud weight could not be

Figure 5.— Comparison of the early-campaign trajectorywith 50-degree inclinations through the coal, and therevised 2008 profile limiting inclination through the coal to30 degrees or less. Note the additional casing stringbelow the “Lakes Entrance Shale.”

Figure 6. –Measurement of cleat width under atmosphericpressure.

maintained high enough to keep the wellbore stable without inducing fracture propagation in the low integrity sands and coals.As a result, a lower-than-ideal mud weight was employed and additional problems caused by the borehole instability wereadded to the challenges. In order to maximize the execution chance of success of the 2008 wells, a tailored wellbore profilewas developed separating the key challenges into two hole sections. The intermediate hole section was designed to address themud weight-dependent wellbore stability issue, and the production hole section was designed to address the coal stability

problems along with the lower formation fracture gradients.The strategy for fluid design was not to compromise on mud weight required for borehole stability. A quantitative risk

analysis determined the optimum mud weight for borehole stability to be 12.0 ppg. Another part of the strategy was toformulate the drilling fluid to prevent coal instability caused by pore pressure penetration. A low fluid-loss non-aqueous fluidwas engineered that would block and significantly slow filtrate from entering the cleats thus preventing near-wellbore pressurization. The most effective way to block the cleat entrance was to utilize a low fluid-loss fluid, combined with anoptimized distribution of engineered particles in the non aqueous fluid (NAF). It may seem to some that the proposedformulation is similar to an engineered particle NAF, such as they are employed by various operators to build integrity while

drilling. However, these applications build integrity by widening the fracture to compress the adjacent rock, and then sealingit at this increased width and closing stress. Fluid entry into the fracture is prevented by having solids in the mud that simplydo not fit within the fracture when it is expanded to the desired width. The desired width depends on the increase in integrityrequired. The wider the width the greater the compression of the adjacent rock and this increased closing stress elevates the pressure required to open the borehole (e.g., integrity). The engineered particle system designed for the coals did not increasethe integrity. This was not necessary as there was adequate stress to prevent the cleat system from opening further, so the

required process is better described as simple plugging of a fixed gap width. The particles only need to block the entrance ofthe pre-existing fractures, creating a superficial bridge at the entrance.

Coal cleat width was estimated using a large piece of coal recovered from a previous well – the estimated fracture widths

were ~10-350 μm, no confining pressure. A microphotograph of cleats is shown in Figure 6.

However, considering overburden at depth, 350 μm can be considered the upper limit in width; actual width could beassumed to be much narrower. It was decided to minimize the spurt loss and filtration loss of the drilling fluid on a slotted

disk, 200 μm wide inside a typical modified high-temperature, high-pressure fluid loss cell. The pictures of the apparatus and

disk are shown in Figure 7. The test conditions mimicked the actual overbalance and temperature of the well to be drilled,

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2000 psi differential pressure and 250° F, respectively. The particle types and distributions selected for testing were a

combination of modeling particle bridging of a 200 μm gap and experience. The results of the fluid loss for several blends of blocking material in the NAF are shown in Figure 8. Blend number six shows the lowest spurt- and filtration loss. Theoptimum particle sizes are a wide distribution of calcium carbonates, graphite and fibers (Figure 9).

Additional testing on aloxite disks (5 micron, 90 micron, and 190 micron) yielded low fluid loss across all permeabilities:5.9 ml, 4.8 ml and < 2 ml (all 30 minute test duration), respectively. The fluid thus optimized was ready for application.

Additional consideration was given to solids control. As is clear from the particle size distribution of the target blend of

the particles, shale shaker screen sizes typically used for NAF (API 140- 200; passing maximum sizes around 69-116.5 μm)cannot be employed without significant loss of blocking material. A model was run to estimate loss of particles as a function

of screen size, and the optimum screen size of API 70 (which, under ideal conditions excludes all particles larger than 196-231

μm) was chosen.

Figure 7. – Set-up for optimizing spurt loss and filtration loss through a 200 micron slotted disk.

Figure 8. – Slot test results using various blends of particletype and particle size distribution.

Figure 9. – Particle size distribution of blend with lowest spurtloss and filtrate.

ResultsThe drilling fluid program was executed without surprises. The specially-formulated drilling fluid was displaced into the

hole at the beginning of the production interval. As drilling continued, rheology and particle size distribution were closelymonitored and remained stable throughout the hole section. The limited solids control allowed the low gravity solids toincrease over 14vol%, but flow properties and gel strengths were not adversely affected. The production hole remained verystable with only small amounts of coal instability. It appears that both mud weight selection and blocking particles to limit

fluid invasion into the cleats were successful in maintaining borehole stability.

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In a drilling operation a single optimized parameter does not make drilling success. The team performed a rigorousoptimization of ECD management and drilling practices to ensure drilling success while drilling coals (Kilroy and Dupriest,2009). Equivalent circulating density was managed and pressure spikes from pump start-up were managed through gradualramping of the flow rate using a controller.

The team achieved an increase of drilling performance in comparison to the earlier 1997 drilling campaign by significantly

reducing the trouble time experienced as a result of the focus placed on wellbore quality. Application of the performancemanagement process addressed the borehole behaviors which had lowered the results of previous wells while simultaneously

achieving record setting performance. The process ensured limiters (such as borehole instability) were addressed with equalrigor, technology, and workflow.

ConclusionsDrilling through coal seams can lead to excessive trouble time caused by coal instability. An important characteristic of coal behavior while drilling is the presence of cleats, a network of fractures that allow fluid filtrate to invade. Such invasion maylead to a pressurization of the near wellbore cleat system, an increase in stress, and instability. Consideration has to be givento designing a drilling fluid that achieves both sealing of the cleat system and mechanical borehole stability (i.e., sufficientmud weight for the level of sealing and far field leakoff that exist).

The mechanisms governing borehole stability in sandstones and shales are equally applicable to coal. A critical mudweight sufficient to avoid excessive stress concentration above the shear failure of the rock has to be maintained.

A notional filter cake design criteria has been established. The leakage rate of fluid into the cleats must be less than the

leakage rate to the far field or backpressure will build that allows the stress to rise at the borehole. This notional criterionis useful in that it allows the designer to understand observed variations in field results.

Filtrate control can be achieved with blocking particles and a low fluid loss drilling fluid. Cleat characterization and particle plugging tests are needed to optimize the fluid before applying it in the field.

If instability is observed, the fluid loss should be reduced further, and/or the mud weight increased.

A recent drilling campaign offshore Australia utilized a drilling fluid that was engineered to the above criteria. The wells

were successfully drilled with little trouble time and achieved record-setting drilling performance.

AcknowledgementsThe authors express their appreciation to ExxonMobil Development Company, and Halliburton Management for allowing

the publication of this paper. Additionally, they express gratitude to the following people for their input and assistance: Jürgen

Schamp, Tommy Graham, Australia Drill Team, execution team on Rig 175 and Don Whitfill, Halliburton. The authorsacknowledge and thank the Gippsland Basin Joint Venture (Esso Australia Resources Pty Ltd, 50% and Operator, and BHPBilliton Petroleum (Bass Strait) Pty Ltd, 50%) for their permission to present this paper.

ReferencesBaltoiu, L.V., Warren, B.K., Natras, T.A.: “State of the Art in Coalbed Methane Drilling Fluids,” IADC/SPE 101,231, presented at the

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Mountain Petroleum Conference Denver, Colorado, April 14-15, 2009.Carmichael, R.S.: “Handbook of Physical Properties of Rocks,” Boca Raton, Fla.; CRC Press, 1982Gentzis, T., Deisman, N., Chalatunyk, R.J.: “Effect of Drilling Fluid on Coal Permeability: Impact on Horizontal Wellbore Stability,” Int.

Journal of Coal Geology, Vol. 78, May 2009, p. 177-191.Kilroy, D.D., Dupriest, F. E.: “Practices to Achieve Record Performance in Narrow-Margin Drilling in Bass Straight Extended Reach,”

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Last, N., Plumb, R., Harkness, R., Charlez, P., Alsen, J., McLean, M.: “An Integrated Approach to Evaluating and Managing WellboreInstability in the Cusiana Field, Colombia, South America,” SPE 30,464 presented at the 1995 SPE Annual Technical Conference and

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Geology, Vol 35, Iss. 1-4, Feb 1998, p. 175-207.Palmer I., Moschividis Z., Cameron J.: “Coal Failure and Consequences for Coalbed Methane Wells,” SPE 96872 presented at the 2005

SPE Annual Technical Conference and Exhibition, Dallas, TX, Oct. 9-12.Santarelli, F.J., Dahen, D., Baroud, H., Sliman, K.B.: ”Mechanisms of Borehole Instability in Heavily Fractured Rock Media,” Int. Journal

of Rock Mech. Min. Sci. & Geomech. Abstr, Vol 29, No 5, 1992, p. 457-467.

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