International Journal of Coal Geology 88 (2011) 75–82

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International Journal of Coal Geology

j ourna l homepage: www.e lsev ie r.com/ locate / i j coa lgeo

Inducing fractures and increasing cleat apertures in a bituminous coal under isotropicstress via application of microwave energy

Hemant Kumar a, Edward Lester b, Sam Kingman b, Richard Bourne b, Claudio Avila b, Aled Jones b,John Robinson b, Phillip M. Halleck a, Jonathan P. Mathews a,⁎a John and Willie Leone Family Department of Energy and Mineral Engineering & The EMS Energy Institute, The Pennsylvania State University, University Park, PA 16802, USAb National Centre for Industrial Microwave Processing, Department of Chemical and Environmental Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK

⁎ Corresponding author at: 126, Hosler Building, DepaEngineering, The Pennsylvania State University, University P863 6213; fax: +1 814 865 3248.

E-mail address: jmathews@psu.edu (J.P. Mathews).

0166-5162/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.coal.2011.07.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 March 2011Received in revised form 27 July 2011Accepted 27 July 2011Available online 4 August 2011

Keywords:PermeabilityEnhanced coalbed methaneCO2 sequestrationInjectivity

For the degassing of coal seams, either prior to mining or in un-minable seams to obtain coalbed methane, it isthe combination of cleat frequency, aperture, connectivity, stress, and mineral occlusions that controlpermeability. Unfortunately, many potential coalbeds have limited permeability and are thus marginal foreconomic methane extraction. Enhanced coalbed methane production, with concurrent CO2 sequestration isalso challenging due to limited CO2 injectivity. Microwave energy can, in the absence of confining stress,induce fractures in coal. Here, creation of new fractures and increasing existing cleat apertures via short burst,high-energy microwave energy was evaluated for an isotropically stressed and an unstressed bituminous coalcore. A microwave-transparent argon gas pressurized (1000 psi) polycarbonate vessel was constructed toapply isotropic stress simulating ~1800 foot depth. Cleat frequency and distribution was determined for thetwo cores via micro-focused X-ray computed tomography. Evaluation occurred before and after microwaveexposure with and without the application of isotropic stress during exposure. Optical microscopy wasperformed for tomography cleat aperture calibration and also to examine lithotypes influences on fracture:initiation, propagation, frequency, and orientation. It was confirmed that new fractures are induced via high-energy microwave exposure in an unconfined bituminous core and that the aperture increased in existingcleats. Cleat/fracture volume, following microwave exposure increased from 1.8% to 16.1% of the unconfinedcore volume. For the first time, similar observations of fracture generation and aperture enhancement in coalwere also determined for microwave exposure under isotropic stress conditions. An existing cleat aperture,determined from calibrated X-ray computed tomography increased from 0.17 mm to 0.32 mm. Thecleat/fracture volume increased from 0.5% to 5.5%. Optical microscopy indicated that fracture initiated likelyoccurred in at least some cases at fusainmicrolithotypes. Presumably this was due to the open pore volumesand potential for bulk water presence or steam pressure buildup in these locations. For the major inducedfractures, they were mostly horizontal (parallel to the bedding plane) and often contained within lithotypebands. Thus it appears likely that microwaves have the potential to enhance the communication betweenhorizontal wellbore and existing cleat network, in coal seams at depth, for improved gas recovery or CO2

injection.

rtment of Energy and Mineralark, PA 16802, USA. Tel.:+1 814

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Coalbed methane is both detrimental and potentially beneficial. Itspresence in mines (coal mine methane) constitutes a danger of rockoutbursts (Wold et al., 2008), explosion, and increases mineventilation energy requirements (Karacan, 2007a). Methane is also amore effective greenhouse gas than carbon dioxide (Forster et al.,2007). Yet, coalbed methane is also a highly valuable fuel thatcontributed 9% (2009) (EIA, 2009) of the U.S. methane production. In

certain mining locations, it is now common practice to drain methanefrom gassy coal seams in advance of mining operations and also toextract valuable coalbed methane (Sloss, 2005). However, the releaseof methane from coal is often a slow process. In canister tests, utilizingcoal cores inches in diameter, it can take months for the gas to bereleased with a significant portion being retained in the coal (Boddenand Ehrlich, 1998). Residual gas has been reported as high as 32% forhigh-volatile bituminous coals (Eddy et al., 1982). Extraction ofcoalbed methane depends on several parameters: the extent ofthermal maturity, gas content, maceral and mineral composition,lithography, coal thickness, fracture density, in situ stress, permeabil-ity, burial history, and hydrologic settings among others (Ayers, 2002;Bodden and Ehrlich, 1998; Bustin and Clarkson, 1998; Durucan et al.,1992; Faiz et al., 1992; Rogers, 1994; Scott, 2002). Coalbeds are

76 H. Kumar et al. / International Journal of Coal Geology 88 (2011) 75–82

commonly self-sourcing and low permeability (often micro Darcyrange) gas reservoirs (Rogers, 1994). Thus, it is often necessary tostimulate the reservoir for economic extraction (Rogers, 1994).“Increasing field permeability in coal seams requires new ideas andgreater research effort” IEA Coal Research (Creedy et al., 1997).Alternative stimulation approaches are desirable for increasing thegas release rate from coal seams. Inducing fractures in coal withmicrowave energy along in-seam boreholes may have the potential toincrease the extraction rate and thus the economics and production ofcoalbed methane.

Typically more than 98% of the methane gas is located in themicropores of the coal (Gray, 1987a, b). It is the methane concentrationgradient in the coal that is responsible for its flow into the cleat systemvia diffusion, after water removal and depressurization (Nandi andWalker, 1970; Rogers, 1994). Coal cleats, are systematic orthogonalfracture systems commonly perpendicular to bedding plane, and areresponsible for coals permeability (Close, 1993; Laubach et al., 1998).Endogenic stresses in coal due to thermal maturation, along withtectonic stresses are likely responsible for the observed cleat orienta-tions (Close, 1993; Laubach and Tremain, 1991; Laubach et al., 1998;Pashin, 1998; Su et al., 2001). The cleats and thus permeability of coals isinfluenced by the stress regime. Direct observation of cleat structurewith fluorescent resin showed preferential loss of the smaller aperturecleats, under regionally representative coalbed methane stress condi-tions (1000 psi), and reduced connectivity between the coal and majorcleats (Soeder, 1991). Cleats undergo aperture reduction and closureunder high hydrostatic pressure (Gash et al., 1993). This has beenobserved using an X-ray computed tomography (CT) approach (Saiteset al., 2006). The abundance and orientation of macroscopic fracturingas well as type, size, shape, density, orientation, distribution, and cleatconnectivity are lithotype dependant (Clarkson and Bustin, 1997). Theduller lithotypes are less prone to fracturing during handling andpreparation (Clarkson and Bustin, 1997) and have lower grindability(Hower, 1998). A higher coalbed gas production rate is expected forcoals withmore cleats (lower cleat spacing) at the same conditions (Cuiand Bustin, 2005).

Microwave energy has been evaluated for improved coal grind-ability, rapid coal coking, and enhanced coal cleaning (Lester et al.,2005, 2006;Uslu andAtalay, 2004).Microwaves heat volumetrically incontrast to conventional heating which heats through conduction(from the outside inwards). The microwave energy will selectivelyheat those components that are more receptive (or ‘lossy’). Theselective heating of a component, within a larger matrix of compo-nents, depends upon its dielectric permittivity and loss factor. Coal isrelatively transparent to microwaves (Lester et al., 2005) thus thepenetration depth is likely to be on a scale ofmeters. However, coal is acomplex matrix of materials which includes some receptive compo-nents that can result in rapid heating (Bogdal and Prociak, 2007). Polarmolecules, such as water, are highly affected by microwaves(Meredith, 1997). Pyrite is also highly receptive to microwave energyand will heat readily, causing differential stresses within the coalmatrix (Meredith, 1997). In a similar manner moisture impartsdifferential stress as it forms steam (Viswanathan, 1990). Thesecomponents may act as “hot-spots” for fracture initiation because oflocal thermal/mechanical stresses (Rowson, 1990). Microwave in-duced differential heating in a heterogeneous ore, will generate newfractures and induces changes at the grain level by increasing the porevolume (Chanaa et al., 1994; Kingman et al., 1999). Within coal, wateris almost certainly super-heated if the microwave power density ishigh enough (Kingman and Rowson, 1998).

Improved coal grindability was thought to occur due to physicalchanges (porosity/fractures) in the coalmatrix (Lester et al., 2005). Nosignificant moisture loss was found after rapid bursts of microwaveenergy so any grindability enhancements were not a result ofmoisture removal. However, to our knowledge no work has examinedmicrowave applications to coal under isotropic stress conditions to

improve permeability through cleat aperture enhancements and theinduction of fractures in situ. It is desirable to observe the existingfracture system pre- and post-microwave exposure.

2. Objectives and approach

The objective was to determine the pre- and post-microwavefracture network under both ambient conditions and simulatedoverburden stress. To accomplish this, a non-destructive approachwas necessary that could be performed in an environmental chamber.X-ray computed tomographyutilizes thedifference inX-ray attenuation(density and atomic number influenced) to differentiate in 3D thestructure of objects. For example, in early coal research Xe gas wasflowed into a lump of coal under 700 Torr gas pressure and imagesbefore and after exposure compared (Mayolotte et al., 1981). TheXegas,with a high atomic number, attenuates the X-rays allowing it to act as atracer for gas flow. The gas preferentially enter the coal in the regions ofhigher electrondensity (Mayolotte et al., 1981), presumably themineralfilled more highly fractured region with preferential flow being parallelwith the bedding plane for work performed on an Illinois coal at highergas pressure (Mayolotte et al., 1983, 1984). Image processing showedthe preferential location of the Xe gas in the fine cracks (cleat system).Other studies have alsoused this approach (Ertekin et al., 1991;Karacan,2007b; Karacan and Okandan, 2000, 2001). Alternatively, but alsodestructively, contrast enhancement has been achieved with Wood'smetal (MontemagnoandPyrak-Nolte, 1999; Pyrak-Nolte et al., 1997), orby observing epigenetic minerals (Kumar et al., 1998). More recently,with advances in resolution, the cleat structure had been directlydetermined (Bossie-codreanu et al., 2004;Mazumder et al., 2006; Saiteset al., 2005, 2006; Tsotsis et al., 2004; Van Geet and Swennen, 2001;Wolf et al., 2008). High resolution X-ray tomography has also been usedto observe cleats and fracture propagation under various stages ofthermal drying of coal (Mathews et al., 2011).

Here microwave exposure is evaluated using both high-resolutionX-ray CT and optical microscopy to identify induced fractures and inanalyzing any cleat apertures enhancement in the absence andpresence of isotropic stress during microwave exposure. The role oflithotypeswas evaluatedwith optical microscopy at locations selectedfrom X-ray CT observations.

3. Experimental

3.1. Coal sample

A bituminous coal core of 50 mm diameter from the Pittsburghseam was obtained. Moisture content was determined as per theASTM standard D3302 (ASTM, 2010) from sub-samples. The coal corewas divided in to two cores of approximately equal length (54 mm)and kept under argon in foil multi-laminated bags to limit anymoisture loss (moisture by mass was 2%) or coal “weathering” (Glicket al., 2005). One sample was used for microwave exposure in theabsence of simulated isotropic stress (core A); the other was exposedto isotropic stress within an X-ray and microwave transparent coreholder (core B) during microwave exposure.

3.2. X-ray computed tomography

An industrial X-ray CT (Universal HD-600, OMNI-X) scanner wasused to obtain high-resolution coal volumetric reconstructions at 160 kVexcitation and 120 or 150 mA current. Temperature wasmaintained viaa specialized air handling system at 76 °F±1 °F. Volumetric scansconsisted of multiple 0.06-mm thick cross-sectional stacked slices.Images were stored as a three dimensional matrix of voxels. Each voxelrepresents a volumetric element of 0.056×0.056×0.06 mmand consistsof a computed tomography (CT) number (anX-ray attenuation number)that is related to the material density and atomic number. The linear

77H. Kumar et al. / International Journal of Coal Geology 88 (2011) 75–82

attenuation coefficient of X-ray is usually reported as CT number(Mazumder et al., 2006; Yao et al., 2009).

CTnumber = Kμ−μ ref

μ ref

!

CTnumber CT numberμ attenuation coefficient for measured sampleμref attenuation coefficient for reference sample, e.g. waterK proportionality coefficient

3.3. X-ray and microwave transparent core holder

An X-ray and microwave-transparent core holder was constructedto enable the sample to be exposed to microwave energy whilst underisotropic stress (core B). A high-strength glass-filled (20% silicateglass) of grade V polycarbonate microwave-transparent vessel wasfabricated to contain the core B (Fig. 1). The core holder consists oftwo parts, a long-threaded portion which contains a one-waypressure relief valve (1000 psi) and a gas charging port. The square-cut stub-acme thread was half the length of the holder to ensureadequate axial strength. The core was well wrapped in Teflon tape to

Fig. 1. Schematic and photograph of microwave- and X-ray-transparent polycarbonatevessel for microwave exposure of core while under gas pressure.

retard/prevent gas penetration into the core. The core rested on thebottomon theholderwith argongas pressurizing the surrounding spaceat 1000 psi. This pressure applied is equivalent to the approximateoverburden stress for 1875 foot depth. Determination of the dielectricproperties of the empty vessel confirmed its microwave transparency.

3.4. High-energy microwave bursts

Coal cores were exposed to short-bursts of microwave energy for atotal of 3 s at 15 kW, at a frequency of 2.45 GHz. Treatment was carriedout using a Sairem GMP15 kW microwave generator, connected to anautomatic E–h tuner for impedance matching purposes. The same wasexposed within a TE01n single-mode cavity, in which the superpositionof the forward and reflected waves creates a single well-defined mode.X-ray CT volumetric image reconstructions were obtained for thesample prior to andafter themicrowave exposure. In the caseof the coreB, itwas containedwithin themicrowave-transparent vessel at 1000 psiargon gas pressure during exposure. Following exposure the vessel wasallowed to cool prior to the slow release of the argon gas.

3.5. Optical microscopy

Following microwave exposure and CT analysis the cores wereimpregnated with an epoxy resin using a room temperature vacuumoven. The resin, low viscosity and negligible contraction on curing,wasallowed to set overnight. In the case of the core B resin impregnationwas performedwithin the vessel to avoidhandlingdamage. Surfaces ofinterest were selected from the X-ray CT volumetric reconstructionand the cores were cut and polished. Surface mosaics were obtainedusing optical microscopy under oil to aid inmaceral contrast (Lester etal., 2002). A Zeiss microscope S100 (Carl Zeiss, Jena, Germany)equipped with Zeiss Axiocam HRC color camera (1300×1030 pixelsper frame, Carl Zeiss, Jena, Germany)was used to acquire the images ofcoal surfaces. Oil immersion objective lens of 32× and eyepiece of 10×were used for magnification. The stage of the microscope wasprogrammed for movement in horizontal plane with automatedcontrol. Cleat aperture, lengths, and orientation were obtained fromdigital image analysis.

3.6. Image processing and image analysis

Image analysis was performed on the polished coal surfaces and onthe volumetric reconstructions of the cores from X-ray CT data pre-microwave and post-microwave exposure. Cleat aperture, lengths,and orientation were obtained from digital image analysis of thesurface using Adobe Photoshop with the ForeaPro4 plug-in. It wasused in image processing of the raw X-ray CT slice data. The X-ray CTdata consists of multiple slices (approximately 600 slices) of X-rayattenuation numbers. As the interest was in the volumetric data, thenon-coal exterior data fromthe slicewas removedusing imagemasking.Slice rotations were also performed to better match pre-and post-exposure orientations. AVIZO software was used for 3-D visualizationand quantification of cleat/fracture volumes and aperture sizes from CTdata.

The fracture volume is dependent on the appropriate X-ray CTthreshold selection (Fig. 2). There is a significant increase in fracturevolume following microwave exposure.

The relative fracture volumes were determined using:

FV %ð Þi =Fracture volumeiTotal core volumei

× 100

FV(%) Fracture volume percentagei 1 (pre exposure sample volume)or 2(post exposure sample

volume)

Fig. 2. Variation of fracture volume percentage of the coal core pre- and post-microwave bursts for core A (left hand side) core B with X-ray CT threshold number.

78 H. Kumar et al. / International Journal of Coal Geology 88 (2011) 75–82

X-ray CT and optical microscopy were used to determine thethreshold selection for fracture volume determination. Optical micros-copy had a much higher resolution (about 1 μm with the optics used)than the X-ray CT resolution of b56 μm. A single large fracture wasselected, and its aperture measured at multiple locations within thevirtual X-ray CT slice and optically. The optical determination wasperformed on a hand polished sample. Bothmicroscopy and CT showedanobvious fracture spanning theupper region (not shown). The averageaperture for this single fracture was 0.41±0.01 mm by opticalmicroscopy. Duplicate values were obtained with a CT thresholdnumber of 1750. The cleat and fracture apertures in the CT work weremeasured perpendicular to the plane of the cleat.

4. Results and discussions

Fig. 3 shows a virtual volumetric reconstruction for the core A pre-and post-exposure. Initially the core had limited fracture volume, withan obvious cleat spanning the core. Following microwave exposurethe fracture volume had increased significantly with multiple newfractures. The fracture volume increased from 1.8% to 16.1%. The coreexpanded as a result of the microwave bursts; hence the fracturevolume determination is relative to the new volume. From this andother volumetric reconstruction views, the induced fractures tendedto be contained mostly in the horizontal (bedding) plane similar toearlier observations (Kingman et al., 2007).

Fig. 4 shows volumetric reconstructions of core B pre- and postexposure. Core B also initially had limited fracture volume in the cleatsystem. Following microwave exposure new fractures were generated.Fracture volume increased from 0.5% to 5.5%. Thus, rapid microwaveexposure was shown to induce fractures in a stressed coal. The coal hadalso expanded. If additional coal surrounded core B, new volumewouldnot be created but rather large aperture fractures are generated at theexpense of reduced/lost cleat aperture and any coal compression

Fig. 3. Fracture map of the unconfined coal core befo

similar to hydrofracing. Here the pressure vessel and the application ofstress likely limited the extent of radial expansion in the core B. Forboth cores the radial expansion was greater at the upper region of thevertically orientated cores.

Natural fractures (cleats) are commonly orientated perpendicularto the bedding plane; however, the new fractures generated here bymicrowave exposure were more frequently parallel to the beddingplane. Induced fracture orientation will be influenced be leastprinciple stress, as in the case of hydrofracing where vertical orhorizontal fractures may occur and is also probably influenced byseam thickness (Colmenares and Zoback, 2007).

Apertures of the cleats, before and after exposure, and inducedfracture were measured utilizing AVIZO software. The apertureincrease 62% for the major cleat present while an increase of 106%was observed for the horizontal fracture (determined from theaverage apertures across 7–10 sections within the field of view,measured perpendicular to the fracture plane) as shown in Fig. 5(a).Thus, existing cleats/fractures were enhanced bymicrowave exposureand new fractures were also created. A normalized fracture lengthwas used for the comparison between pre- and post-measurement toaccount for the core expansion. Normalized length may be expressedas:

Normalized length locationi =Slice number of volumetric reconstructioni

Total number of slice in volumetric reconstructioni

i = 1 pre exposureð Þor 2 post exposureð Þ

The normalized length assumed proportional expansion rather thanthe spatially-specific expansion observed. The aperture enhancementswere variable. The expansion ranged from approximately 400% to 100%.At the core edges, lost material is responsible for the larger gainobserved in Fig. 5(a). The fracture–aperture gains are less extensive incore B, where the gain was 250% for the single existing cleat (Fig. 5(b)).

re (left) and after (right) microwave exposure.

Fig. 4. Fracture map of the confined coal core before (left) and after (right) microwave exposure.

79H. Kumar et al. / International Journal of Coal Geology 88 (2011) 75–82

Following microwave exposure, the cleat aperture was alsovariable (generally decreasing its aperture along the fracture) whilepre-exposure individual cleats had remarkably uniform apertures(optical microscopy observations). Individual natural cleats tend tohave very uniform apertures extending beyond the length scalesexplored here. In the field, the presence of methane reduces thestrength of coal (Aziz and Ming-Li, 1999) and thus the fracturing maybe enhanced beyond these observations. There are also fracturesbelow the resolution of the CT (optical microscopy observations) thatwill contribute to additional enhancements.

The fracture aperture distributions of two nascent fracturescreated by microwave exposure in core B are shown in Fig. 6(a).The fracture had been occluded by the particles/fines generatedduring fracturing; occasionally the fracture apertures may be reduceddue to coal slippages. The core had expanded in the horizontal andvertical directions after the microwave exposure. However, it isunlikely that the occlusion occurs throughout the fracture (may be X-ray CT slice specific) and, indeed, may aid in preventing fractureclosure/healing. Alternatively, excessive fine generation is undesir-able due to fracture plugging.

Fracture aperture distribution for three newly generated fracturesin core B is shown in the Fig. 6(b). Apparently, fractures are likely to beinitiated from the inner coal matrix and propagate outward. Thisbehavior supports the generation of high pore pressure in the innervolumes. However, fracture 3 in Fig. 6(b) has larger values ofapertures at the corners which could be the effect of weak plane inthat location. Fracture apertures show a range of values that could bethe affect by fracture initiation/termination location, lithotypes, cleatpropensity (length of fracture), moisture content, mineral influences,or some combination thereof. The shape of the fracture, any displacedmineral grains or coal particles may aid in retaining open porositywhile under confining stresses.

Different lithotypes have different methane capacities and influencecoals heterogeneous behavior (Bustin, 1997; Clarkson and Bustin, 1996;

Fig. 5. Cleat apertures for three fractures before and after microwave exposure without (left)and post are filled legends. Symbols indicate individual cleats. (a) For the two lower cleats thecore edges during microwave exposure hence observations are restricted to 0.2–0.8 norma

Crosdale et al., 1998; Gamson et al., 1993; Karacan and Mitchell, 2003;Pone et al., 2010; Saites et al., 2006; Ting, 1987). The heterogeneousnature of the induced fractures of the coal is likely dependent on thepresence of multiple lithotypes in each coal sample. Lithotypes areexpected to have different fracturing strength, fracturing tendency, typeand direction of failure, moisture content, porosity, andmineral content.Hence lithotypes are expected to play a role in fracturing induced bymicrowave exposure.

Optical microscopy was utilized in evaluating the role of themicrolithotype during the microwave-induced fracturing process. Amosaic image of 5 assembled micrographs, capturing a portion of thesurface with high-resolution 600 pixels/mm (11,346×5848 pixelsregion), is shown in Fig. 7. The blue is false-coloring to aid inidentification of the resin fill. This area was selected, from volumetricreconstructions, as the likely initiation location for some majorinduced fractures. The fracture area was 24% of the region of view(Fig. 7). New fractures were likely initiated at highly reflectingmacerals at this location i.e. high reflectance inertinite. These fracturesappear to have propagated through vitrinite bands and merged intothe existing cleat system. The fractures in the top right hand regionappear to work along and across the fusinite or semifusinite bandings.Inherently, these are commonly are friable, and can shatter throughthermal shock (Stach et al., 1975). In Fig. 7, the widest fracturesappear between inertinite domains and are the likely initiation points.Microscopy shows the microwave-induced fractures are mode Iopening (Laubach et al., 1998).

It is postulated that the water present in the lithotype rapidlyconverts into super-heated steam during microwaving at high powerdensities. The super-heated steam develops differential pressure insidethe pore network of the coal generate potential sites for fracturegeneration. These fractures propagate in the weak lithotype bandinguntil they intersect thenatural fracture system,where they terminate. Interms of microlithotype composition, the thicker vitrinite bands readilyfracture along and across the bands. It is unclear if it is the bulk water in

and with (right) confining stress during exposure. Pre-exposures are shown in unfilledlength continued in another plane andwas not analyzed. (b)Material was lost form thelized length. Measurements are within ±0.056 mm (X-ray CT resolution).

Fig. 6. Fracture apertures of newly generated fractures viamicrowave exposure under isotropic stressed (left) andunder no stress (right) conditions. Values are accuratewithin±0.056 mm.

80 H. Kumar et al. / International Journal of Coal Geology 88 (2011) 75–82

macropores or the more widely dispersed bound water in microporesthat is responsible for the majority of the fracturing. Elevated porepressure is one of the proposedmechanisms for opening-mode fracturecreation (in a compressive stress state) when hydrocarbon gasgeneration is sufficient (Rogers, 1994).

5. Potential applications of microwave energy for ECBM

Microwave induced fracturingmay have applicability in horizontalcoal drilling where themicrowave burst fractures would allow greatercommunication for methane degasification or to enhance CO2

injectability. In Australia, for gassy seams, extensive borehole networks

Fig. 7. Mosaic of micrographs obtained from a bituminous coal surface following microwavcoloring by their aperture (lower).

may be employed in advance of mining (Black et al., 2010). Theseboreholes are potential access sites for the microwave bursts. Amicrowave antenna could be situated in these boreholes and beremoved while “pinging” bursts of microwave energy. High-energymicrowave exposure to the coal would create higher connectivity andhence a reduced “skin factor”. A low skin factor would allow higherproductivity and reduced pressure drop in the wellbore region in low-permeability coals. However, more work is required to decide theoptimum moisture content of the coal seam for fracture creation withmicrowave bursts. Should it be done as soon as the seam is exposed or atsome later stages of dewatering? An alternative approach is to use areduced diameter drill (lower cost and more rapid drilling) following

e exposure. Cleats and fractures are false-colored in blue (upper micrograph) and false

81H. Kumar et al. / International Journal of Coal Geology 88 (2011) 75–82

with microwave-induced fracturing. Induced fractures may also aid ininjectivity enhancements along horizontal wells prior to CO2 injectionfor enhanced coalbedmethane extraction. It is unclear if fine generationwill be an issue. Along with the other issues raised, economic viability,and safety issues in the employment of microwave-induced fracturingtechnique will also need to be addressed once the extent of impact canbe determined.

6. Conclusions

The potential use of short-bursts of microwaves for developing newfractures and enhancement of cleat apertures in bituminous coal(exposed with and without application of isotropic stress) wasevaluated. It was confirmed that the exposure of microwaves to a coalcore generated new fractures and increase the existing cleat aperturesfor an un-stressed core. Similar observations were found in the coal coreunder isotropic stress during microwave exposure, indicating there ispotential to use microwave exposure for improved connectivitybetween horizontal wellbore and coal seam. Cleat/fracture volume,determined frommicro-focusedX-ray computed tomography, followingmicrowave exposure increased from 1.8% to 16.1% of the unconfinedcore volume. The cleat/fracture volume increased from 0.5% to 5.5% forthe core exposed while under isotropic stress. Induced fractures wereoften horizontal and terminated at the existing cleat system. Cleataperture enhancements were also noted. Fracture initiation occurredselectively in some microlithotypes (fusain). Majority of the fracturespropagated throughweaker lithotypes andwere often containedwithinthose lithotype bands. It is apparent that the high-energy microwavebursts could be used for enhancing the communication betweenwellbore and existing fracture network in unmineable coal seams.These enhancements, in horizontal wells, may be used for the enhancedextraction of coalbedmethane or increased CO2 injectivity in the case ofCO2-sequestration.

Acknowledgements

Funding for this project is provided through the “Ultra-Deepwaterand Unconventional Natural Gas and Other Petroleum ResourcesResearch and Development Program” authorized by the Energy PolicyAct of 2005. This program – funded from lease bonuses and royaltiespaid by industry to produce oil and gas on federal lands – is specificallydesigned to increase supply and reduce costs to consumers whileenhancing the global leadership position of the United States in energytechnology through the development of domestic intellectual capital.RPSEA is under contract with the U.S. Department of Energy's NationalEnergy Technology Laboratory to administer several elements of theprogram. RPSEA is a501(c) (3) nonprofit consortium with more than150members, including24of thenation's premier researchuniversities,five national laboratories, other major research institutions, large andsmall energy producers and energy consumers. The mission of RPSEA,headquartered in Sugar Land, Texas, is to provide a stewardship role inensuring the focused research, development and deployment of safeand environmentally responsible technology that can effectively deliverhydrocarbons from domestic resources to the citizens of the UnitedStates. Additional information can be found at www.rpsea.org.

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