International Journal of Coal Geology 87 (2011) 87–96

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International Journal of Coal Geology

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Determination of in-situ stress direction from cleat orientation mapping for coal bedmethane exploration in south-eastern part of Jharia coalfield, India

Suman Paul, Rima Chatterjee ⁎Department of Applied Geophysics, Indian School of Mines, Dhanbad, India

⁎ Corresponding author. Tel.: +91 326 2296658; fax:E-mail addresses: sumanpaul.jugeo@gmail.com (S. P

(R. Chatterjee).

0166-5162/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.coal.2011.05.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 February 2011Received in revised form 10 May 2011Accepted 10 May 2011Available online 15 May 2011

Keywords:Cleat volumeCleat orientationCoal seam porosityCoal seam permeabilityStressJharia coalfield

Accurate prediction of in-situ stress directions plays a key role in any Coal Bed Methane (CBM) explorationand exploitation project in order to estimate the production potential of the CBM reservoirs. Permeability isone of the most important factors for determination of CBM productivity. The coal seams in Jharia coalfieldgenerally show low permeability in the range of 0.5 md to 3 md. To estimate the in-situ stress direction in thestudy area, an attempt has been made to undertake the cleat orientation mapping of four regional coal seamsof two underground coal mines located at south-eastern part of Jharia coalfield, India. Cleat orientationmapping is critical to determine the maximum principal compressive horizontal stress (SH) direction for CBMexploration and exploitation, which in turn controls the direction of maximum gas or water flow though coalbeds. From the field study it is found that the average face and butt cleat azimuths are towards N15°W andN75°E respectively. Average permeability of the four above-mentioned major coal seams has been calculatedfromwell logs of nine CBMwells distributing over an area of 7.5 km2, adjacent to the underground mines. Thecleat orientations are congruous with the regional lineament pattern and fits well with the averagepermeability contour map of the study area to infer the orientation of in-situ maximum horizontal stress.Goodness of fit for the exponential regressions between vertical stress and permeability for individual coalseams varies between 0.6 and 0.84. The cleat orientation is further validated from the previous fractureanalysis using FMI well log in Parbatpur area located southern part of the Jharia coalfield. The major coalseams under the study area exhibit directional permeability, with the maximum permeability, orientedparallel to the direction of face cleat orientation.

+91 326 2296563.aul), rima_c_99@yahoo.com

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Exploration for CBM in India has evolved over the past decades to apoint where the focus is very largely on the targeting of regions ofrelatively higher permeability. Coal acts as both source and reservoirrock for natural gas and it has been commercially exploited in thedevelopment of CBM projects. Any CBM project would be economicallyviable only if gas content and permeability of the reservoir meet certaincriteria,which include 150 scf/ton of gas content and 1 mdpermeability(Mandal et al., 2004). Cleat orientationmapping is critical to determinethemaximumprincipal compressive horizontal stress (SH) direction forCBM exploration and exploitation, which in turn controls the directionof maximum gas or water flow though coal beds (Wang et al., 2009;Wold and Jeffrey, 1999). Cleats are naturally occurring fractures in coalwhich are formed mainly due to shrinkage during the coalificationprocess (Laubach et al., 1998; McCulloch et al., 1974, McCulloch et al.,1976 and Prasad, 2009).

According to the previous researchers, cleats can be developed indifferent periods of the coal formation of various mechanisms, likedehydration or desiccation or devolatilization processes. Cleatformation also depends on coal rank, coal thickness, maceral content,coal lithotype, coal environment, thermal contraction, tectonics, andgeological structure (Frodsham and Gayer, 1999; Jeremic, 1985;Laubach et al., 1998; Paul, 2003 and Ward, 1984). Sometimes miningactivity may result some local changes in cleat orientation in theimmediate vicinity of the mines. A minimum permeability is requiredto successfully interconnect the natural micro-fractures and cleats tocreate migration pathways for gas before conducting hydraulicfracturing process. The mechanisms for the gas flow in the coalinvolves: a) desorption of gas from the coal surface inside the micro-pores; b) diffusion of the gas through the micro-pores, governed byFick's law; and c) Darcy flow through the cleat system and of thefractures to the wellbore (Gray, 1987 and Meng et al., 1996).

For CBM production the main flow is determined by the cleatattributes and the orientation pattern (Close and Mavor, 1991). Fig. 1is illustrating a typical cleated coal sample from the Jharia coalfield,India. From this figure it is clear that the face and butt cleats areoriented almost perpendicular to each other and the spacing betweenthe cleats and length of the cleats are not equal everywhere. The coal

Fig. 1. Depicting the general cleat pattern observed in Jharia coalfield, India. Face, buttcleats and their orientations are indicated in the figure.

88 S. Paul, R. Chatterjee / International Journal of Coal Geology 87 (2011) 87–96

samples observed at different places of the study area reveals that thecleat spacing and the length of the cleats vary from one place toanother. The regular reticular cleat pattern as observed in Fig. 1 is thecharacteristic of the cleat geometry in Jharia coalfield. The tectonicstress field controls the cleat geometric patterns. The control of thetectonic stresses on the cleat formation is based on the intrinsic tensileforces and fluid pressure in coal beds. The formation of regularreticular cleat geometries under the in-situ stresses (horizontalmaximum principal compressive stressNhorizontal minimum princi-pal compressive stress) had been discussed by Su et al., 2001. The facecleat extends along the SH direction and the butt cleat along thedirection of horizontal minimum compressive principal stress (Sh), so

Fig. 2. Illustrating the distribution of nine CBM wells and two underground mines in Jharia c

the regular reticular cleat pattern is formed in Jharia coalfield. Theextensional stress regime existing in Jharia coalfield (Ghosh andMukhopadhyay, 1985) is favorable to develop fractures and cleats inthe coal seam. These cleated networks in a coal matrix create channelsfor fluid flow through a CBM reservoir.

In the Jharia coalfield opening mode fractures propagate in a planeperpendicular to the least compressive stress when the pore fluidpressure exceeds Sh. The cleated coals can have varying permeabilityalong its length (Philip et al., 2005). Coal bed permeability through thecleated network is sensitive to fracture aperture as well as to thefracture length distribution. Permeability of coal increases with cleatdensity and cleat aperture. High cleat density in coal seams isfavorable for higher fluid flow in CBM reservoirs (Ali et al., 2008;Dabbous et al., 1974 and Lingard et al., 1984).

The optimal orientation of cleat or openingmode fractures is alongthe maximum principal horizontal stress and influenced by regionalcompressive stress direction (Frodsham and Gayer, 1999; Paul, 2003;Ryan, 2003 and Simon, 2000). Though the examples of opening modefractures in sandstones (N3 km depth) from west Texas showedmisalignment of SH and open fractures (Laubach et al., 2004). Theeffects of cleat orientation and confining pressure on coal rockproperties have been studied in many coal basins worldwide; like LaPlata mine in San Jun Basin of NewMexico (Gash et al., 1992), Mine inKushiro Coalfield in Japan (Li et al., 2004), mines at Zonguldak Basinlocated on the Black Sea coast of Turkey (Karacan and Okandan,2000), RECOPOL project in the Upper Silesian Coal Basin, Poland (Wolfet al., 2008), mines in the Pittsburgh coal bed Pennsylvania (Diamondet al., 1975), coalmines in southwestern Indiana (Acosta et al., 2007),as well as many coal basins including Appalachian, San Juan, PowderRiver and BlackWarrior Basins in United States (Ayers, 2002; Flores etal., 2008; Pashin, 1998, and Pitman et al., 2003). Well monitoring andporosimetry tests from the BlackWarrior and San Juan Basins indicatethat permeability is commonly higher along the face cleat than alongthe butt cleat (Koenig, 1989 and Pyrak-Nolte et al., 1993). Therefore,reservoir drainage may be facilitated by spacing wells more closelyalong the butt cleat than along the face cleat. Recently, the authorsMeng, Zhang and Wang had indicated the effect of stress onpermeability in the Southern Qinshui basin, China (Meng et al.,2009 and Meng et al., 2011). They demonstrated the relationship

oalfield. Inset shows the simplified geological map of Jharia coalfield (Sengupta, 1980).

Table 1Generalised stratigraphic sequence of Jharia coalfield (after Chandra, 1992).Modified after D. Chandra (1992).

Age Group Formation Litho-type Max. thickness

Recent and sub-recent Weathered Alluvium, sandy soil, clay, gravel etc. 30 m

UnconformityJurassic Deccan trap and other igneous activity (intrusives) Dolerite dykes, mica lamprophyre dyke and sillsUpper Permian D Raniganj Fine grained feldspathic sandstones, shales with coal seam 800 m

AMiddle Permian M Barren Measure Buff coloured sandstone, shales and carboneceous shales 730 m

ULower Permian D Barakar Buff coloured coarse to medium grained feldspathic sandstones,

grits, shales, carboneceous shales and coal seam+1250 m

AUpper Carboniferous Talchir Greenish shale and fine grained sandstones 245 mArchaeans Metamorphics

89S. Paul, R. Chatterjee / International Journal of Coal Geology 87 (2011) 87–96

between permeability of coal reservoir and the in-situ stress in thatCBM reservoir of the Qinshui Basin, China. There were several casestudies in Britain, Canada, Mexico, Australia, China and USA wheremaximum horizontal stress orientation has been correlated with thecoal cleat orientation in coal basins (Bell, 2006; Enever and Clark,1994; Gentzis et al., 2006; Gentzis et al., 2007; Hickman and Davatzes,2010; Meng et al., 2011 and Rippon et al., 2006).

Recent studies of cleat orientation patterns and fracture stylesuggest that new investigations of even these well studied parameterscan yield insight into coal permeability (Laubach et al., 1998). For thispaper original field data collected from two underground mines: WJArea and PB Area of Bharat Coking Coal Limited (BCCL), India has beenused to determine the cleat orientation pattern for south-eastern partof Jharia coalfield. All cleat and joint orientation data collected from

Fig. 3. Rose diagrams showing the directions of fac

four different coal seams of the two underground coal mines of Jhariacoalfield have been used to determine the stress direction in the studyarea.

To assess the potential of any CBM reservoir, many researchersused different well logs like density, gamma ray, resistivity, acousticand image logs for quantitative estimation of permeability andmapped cleat orientation and density (Ali et al., 2008; Bachu andMichael, 2003; Bell, 2006; Bell and Bachu, 2003; Chatterjee and Pal,2010 and Pashin, 1998). In view of above, a fresh attempt is required todevelop a new approach to infer the in-situ stress direction acting atmajor coal seams from the available conventional well log data. Thecontent of the present paper will include (a) the study of cleatorientation for four major coal seams from two underground coalmines located at south-eastern part of Jharia coalfield, (b) estimation of

e cleats for seams (a) K, (b) L, (c) O and (d) P.

Fig. 4. Rose diagrams showing the directions of butt cleats for seams (a) K, (b) L, (c) O and (d) P.

90 S. Paul, R. Chatterjee / International Journal of Coal Geology 87 (2011) 87–96

in-situ stress magnitude from well logs from nearby nine CBM wellsadjacent to those underground mines, (c) evaluation of initial coal bedpermeability for fourmajor coal seams to infer the in-situ SH orientationand (d) correlation of average permeability contour with the cleatorientation plots for fourmajor coal seams. This paperwill also describethe cross checking of the predicted SH orientation from well logs withthe predicted SH from face cleat orientation data.

2. Study area

India has huge coal reserves and a large portion of it is deep seatedwhich is not economically minable at present. This factor providesenough scope for the exploration and development of CBM in Indiancoal basins. The study area is located near the Dhanbad town which is

Fig. 5. Rose diagrams showing regional directions o

known as “Coal Capital of India”. The study area covers the parts ofSingra, Kapuria, Moonidih and Jarma blocks of the Jharia coalfield,located in eastern India (Fig. 2). The Jharia coalfield is situated about260 km northwest of Calcutta (now Kolkata, West Bengal, India). Thecoalfield is roughly sickle shaped, its longer axis running northwest–southeast. The coal basin extends for about 38 km in an east–westdirection and maximum of 18 km in north–south direction, andcovers an area of about 456 km2. The dip of the Formation in general issoutherly (10°). A simplified geological map of Jharia coalfield isshown in Fig. 2 (Sengupta, 1980) and the general stratigraphicsuccession (modified after Chandra, 1992) is given in Table 1.

The formations generally follow the boundary of the coalfield,striking E–W and NW–SE and dipping towards the centre of thecoalfield. The basement metamorphic rocks are overlain by the Talchir

f (a) face cleats, (b) butt cleats and (c) joints.

91S. Paul, R. Chatterjee / International Journal of Coal Geology 87 (2011) 87–96

Formation followed by the Barakar Formation which is the main coal-bearing horizon. The Barren Measures overlain the Barakar formationfollowed by the coal-bearing Raniganj Formation. The coal seams ofpresent study belong to Barakar Formation, which does not show anyevidence of high intensity tectonic deformation except normal gravityfaults of different magnitudes — both minor (throw less than 10 m)andmajor (throw 10 m to N100 m) (Ghosh andMukhopadhyay, 1985and Sengupta, 1980).

The Barakar Formation consists of buff coloured coarse to mediumgrained feldspathic sandstones, grits, carbonaceous shales and coalseams. High density igneous intrusives rocks (Mica Peridotities) arealso found in wells under the study area in the Jharia coalfield. Datapertaining to organic petrology and thermal maturity of Barakar andRaniganj Formations are of considerable importance in determiningthe CBM potential in the Jharia coalfield. The vitrinite macerals aredominant in the shallower coals of Barakar Formation (LowerPermian). The vitrinite content range from 40% to 80%, exinitecontent vary in the range of 1% to 4% and vitrinite reflectance valueranges from 0.83% to 1.69% in Barakar coal seams of Jharia (Hazra etal., 2003). The volatile matter on dry ash free basis indicate that theBarakar coals of Jharia are high volatile ‘A’ bituminous to low volatilebituminous in rank (Rudra and Hajra, 2009). As the depth increases, ingeneral it has been observed that ash content increases withconsequent decrease in vitrinite content and increase in inertinitecontent for the Barakar coals of Jharia coalfield. The range of inertinitecontent for deeper coals in Jharia coalfield is from 35% to 80% andvitrinite content range from 20% to 62% (Hazra et al., 2003).

3. Identification of cleats and analysis of cleat orientation data

For cleat orientation measurement at two underground mines ofJharia coalfield the following methodology are adopted: (1) magni-fying glass has been used for cleat identification, (2) all data arecollected either from roof or from bottom of coal seams as cleatpatterns are most prominent/exposed in those parts, (3) only the coalseams having prominent vitrinite (bright) bands are preferred for thestudy as cleats are easily identifiable in such seams and (4) all data arecollected wherever both face and butt cleats could be identified.

The underground mines are situated in and around Moonidihblock, located at the south-eastern part of the Jharia coalfield. Tilltoday no cleat orientation data or reports are available with us inpublic domain. All cleat and joint orientations have been identifiedand measured in four regional/major coal seams, namely; K, L, O andP, of two working underground mines: WJ Area Mine and PB AreaMine of BCCL (Fig. 2). The working depths of the seams are typicallybetween 400 m and 550 m. The studied seams K, L, O and P aredipping towards S33°W, S31°W, S45°W and S43°W respectively. Forthis study a total 25, 72, 28 and 20 numbers of cleat orientation datafrom different locations have been collected for coal seams K, L, O andP respectively and plotted in rose diagrams for each coal seam. Basedon the observations at different locations, one set of mega cleat trendis found almost NNW–SSE direction in major part of seams. This set isrunning almost parallel to the strike of the bed and face of the coaldevelopment. Out of the collected data, the field data amounting 14

Table 2Orientations of cleats and joints in the studied coal seams for WJ area and PB area undergr

Seam Face cleat direction Butt cleat

Percentage (%) of maximumdirectional data

Orientation Percentagdirectiona

K 56 N25°W 56L 36 N15°W 36O 20 N20°W 20P 17 N15°W 17Combined (K, L, O and P) 41 N15°W 41

(56%), 36 (50%), 20 (71%) and 17 (85%) showing maximum face cleatorientation varying between N15°W and N25°W (Fig. 3) andmaximum butt cleat orientation varying between N65°E and N75°E(Fig. 4).

Spacing of face cleat is observed at an interval of 0.2 m to 0.3 m. Toget the composite face and butt cleat azimuth, all face cleat and buttcleat data of seams K, L, O and P are plotted on two different rosediagrams and it is observed that the resultant face cleat and butt cleatorientations are aligned towards N15°W and N75°E respectively(Fig. 5a and b). All joint orientation directions are also plotted on aseparate rose diagram and it has been found that the dominant jointorientation direction is towards NNW–SSE followed by a sub-dominant orientation towards ENE–WSW (Fig. 5c). Summarisedcleat and joint orientation data for individual seams and compositeof all four seams are given in Table 2. The resultant face cleatorientation from the four major coal seams provides the in-situ SHorientation (N15°W) in this part of the study area.

The lineament study based on satellite imagery in Jharia coalfieldand the directional analysis of lineaments in Moonidih area hadindicated the most dominant trend towards NNW–SSE and secondprominent trend towards ENE–WSW (Tiwari and Rai, 1996; Verma etal., 1989). The previous authors (Ali et al., 2008) had also observedthat the face cleat orientation from FMI log in Parbatpur area locatedat the southern part of the Jharia coalfield (Fig. 2) is directed towardsNW–SE, varying between N30°W and N60°W. The present studyindicates the rotation of 10° to 30° of face cleat orientation from theParbatpur area to the Moonidih area in the Jharia coalfield. The localrotation of face cleat orientation as well as in-situ stress (SH)orientation are also observed in other coal basins of the world(Bachu and Michael, 2003; Bell, 2006; Bell and Bachu, 2003; Laubachet al., 1998; Montgomery, 1999; Pashin, 1998; Wolf et al., 2008 andZhang et al., 2000).

4. Estimation and analysis of in-situ stress magnitudes

Coal seams including the major seams K, L. O and P from bottomtop of nine CBMwells, i.e., K1, K4, K8, K10, K12, S5, S8, M1 andM3, areidentifiedwith the help of available density, resistivity and gamma raylogs. The density logs for two CBM wells (K1 and K4) displayed inFig. 6 indicate the major coal seams of Jharia coalfield. For propernomenclatures of the identified coal seams and for coal seamcorrelation, geophysical logs of all nine CBM wells have been usedin combination with litho-logs of the same wells.

The coal seamcorrelation for nineCBMwells hasbeengiven in Table3.The vertical section of major coal seams for section K8 to S8 is plotted inFig. 7. Reducedfloor of themajor seamsareplottedwith faulting.Wehavealso extrapolated seams without log data. The thickness of seams variesbetween0.85 mand3.24 min seamK, 1.55 mand3.5 m in seamL, 1.05 mand 5.6 m in seam O and 0.85 m and 9.28 m in seam P respectively.

Well logs of nine CBM wells have been considered to determinethe magnitudes of in-situ stresses of same four major coal seamscovering an area of about 7.5 km2 area of Singra, Kapuria andMoonidih blocks located at west and southern parts of theWJ area andPB area mines. No borehole well log data are available at the eastern

ound mines, Jharia.

direction Joint direction

e (%) of maximuml data

Orientation Percentage (%) of maximumdirectional data

Orientation

N65°E 60 N25°WN75°E 42 N15°WN70°E 25 N20°WN75°E 22 N15°WN75°E 43 N15°W

92 S. Paul, R. Chatterjee / International Journal of Coal Geology 87 (2011) 87–96

side of these mines. Estimation of in-situ stress for coal-bearing stratahas been applied widely in underground coal mines as well as CBMexploration in many coal basins of USA, Australia, Canada and China(Bell and Bachu, 2003; Bustin, 1997; Gentzis et al., 2008; Meifeng etal., 2008; Montgomery, 1999 and Zhang et al., 2000).

A database of in-situ stress measurements from underground coalmines exists for several of the world's coalfield for world stress mapproject (Heidbach et al., 2008); though no in-situ stress measurementand the measurement of cleat volume/porosity and permeability hadbeen carried out in this part of the study area. Themagnitude of verticalstress/overburden load (SV) at any depth is produced by the pressureexerted by the rocks above that point. Vertical stresses are calculated byintegrating density log values for nine CBM wells distributed in thestudy area (Fig. 2). The hydrostatic pressure gradient with mud density1.1 g/cm3 is normal in the nine CBM wells drilled in the study area. Tocalculate effective stress, pore pressure has been assumed equal to thehydrostatic pressure at that particular depth. The effective minimumhorizontal stress (Sh) in the tectonically relaxedbasin such as in this partof Jharia coalfield for the four coal seams has been calculated using theequation (Chatterjee and Pal, 2010):

Sh = γ SV−Pð Þ= 1−γð Þ ð1Þ

where P is the pore pressure equivalent to hydrostatic pressure and γis the Poisson's ratio of coal (0.32).

The vertical stresses for these nine CBM wells have beencalculated. The plot of vertical stress against depth for the majoridentified coal seams of two CBM wells (K1 and K4) are displayed inFig. 6. There is a decrease in stress gradient inside the coal seams.Stress gradient changes with the density of rocks. Variations in stressgradients are also observed due the presence of high-density igneousintrusive rocks in all wells. The overall trend of the vertical stressincreases linearly with depth with a slope of about 45°. This pattern issimilar to that in the other coalfields of India and foreign coal basins(Mucho and Mark, 1994; Townend and Zoback, 2000). There is avariation of slope inside the coal seams. It is possible to know thevertical stress magnitude at the roof of the individual coal seams. Thevertical stress magnitude at the major seams roof varies from13.28 MPa to 31.97 MPa from 491.95 m to 1063.90 m in the studyarea. The large variation of stress magnitudes are observed due thehigh-density igneous intrusive as a sill parallel to the coal beds.Table 4 is listing the vertical stress, vertical stress gradient, effectivevertical stress gradient and effective horizontal stress gradient valuesfor roofs of four coal seams in this study area and Table 5 is listing thevertical stress gradient and effective horizontal stress gradient withinthe four coal seams in the study area.

The vertical stress magnitude values show the variations in thevertical stress magnitudes for roof of each seam. Stress magnitude isincreasing towards south in this part of the study area because coalbeds are dipping in this direction. The vertical stress gradients areincreased from ESE to WNW for seam K, L, O and P, while it is less forroof of L seam at the NW corner of the study area (Fig. 2 and Table 4).The effective vertical stress gradient value within seams increasesfrom SE to NW part of the study area. The effective horizontal stressgradient on the roof of coal seams also increases from wells located atSE to the wells located at NW part of the study area (Fig. 2 andTable 4). The effective horizontal stress gradient within coal seamsincreases noticeably from SE to NW for all seams in the study area(Table 5).

Fig. 6. Density logs indicate major coal seams: (a) O and P of well K1, (b) K and L of wellK4 and (c) O and P of well K4. The variation of vertical stress with depth against thesecoal seams are also plotted in a, b and c.

Table 3Coal seam correlation for nine CBM wells in south-eastern part of Jharia coalfield.

Seam K Seam L Seam O Seam P

Well no. Well Collar (m) From (m) To (m) T (m) From (m) To (m) T (m) From (m) To (m) T (m) From (m) To (m) T (m)

K1 182.39 777.10 781.80 4.70 765.40 770.35 4.95K4 190.81 639.83 641.00 1.17 631.46 634.39 2.93 505.00 508.80 3.80 490.05 495.89 5.84K8 181.66 643.22 644.84 1.62 633.72 636.05 2.33 506.95 510.55 3.60 491.95 497.60 5.65K10 191.91 712.50 715.74 3.24 703.15 706.10 2.95 563.77 568.10 4.33 548.44 555.10 6.66K12 171.42 839.55 841.55 2.00 834.10 837.60 3.50 706.94 711.95 5.01 694.72 704.00 9.28S5 163.31 1063.90 1069.50 5.60 1052.70 1057.25 4.55S8 170.29 1102.84 1104.25 1.41 1098.42 1101.77 3.35 960.08 962.70 2.62M1 173.95 882.82 886.97 4.15 731.17 732.32 1.15M3 170.92 961.07 961.92 0.85 951.17 952.72 1.55 804.82 805.87 1.05 680.87 681.72 0.85

Well Collar (m) with reference to Mean Sea level (MSL), T = thickness of coal seam.

Fig. 7. Indicated vertical section across the line connecting wells K8 to S8. The majorcoal seams are plotted only in this section. Mean sea level (MSL) is the reference depthlevel for all five wells along the line K8 to S8 (refer Table 3).

93S. Paul, R. Chatterjee / International Journal of Coal Geology 87 (2011) 87–96

5. Evaluation and analysis of initial coal bed permeability fromwell logs

Shallow resistivity logs of the nine CBMwells show that coal seamsare typically characterized by high electrical resistivities (700 Ω-m to1705 Ω-m), and low density (1.44 g/cm3 to 1.56 g/cm3). It had beenobserved that the resistivity, measured by shallow resistivity loggingtool across the coal seam decreases substantially in the wells filledwith high salinity fluids compared to those, filled with low salinityfluids (Yang et al., 2006). This indicates replacement of pore fluids in

Table 4Vertical stress, vertical stress gradient, effective vertical stress gradient and effective horizontsouth-eastern part of Jharia coalfield.

Seam K Seam L

Well name VS VG EVG EHG VS VG EVG E

K1K4 17.92 27.50 17.67 8.11 17.78 25.14 15.71 7K8 17.27 27.00 16.66 8.22 17.12 27.00 17.50 8K10 18.03 28.30 19.30 8.57 17.86 24.70 18.16 8K12 26.12 26.93 17.14 7.92 26.00 27.41 17.14 8S5S8 30.22 28.57 17.14 8.00 30.16 27.42 18.28 8M1M3 21.03 26.31 15.54 6.50 20.840 25.27 14.00 5

VS = Vertical Stress (MPa), VG = Vertical Stress Gradient (KPa/m), EVG = Effective Vertic

the cleats (invasion zone) by the borehole fluid. The cleat volume/porosity of cleated coal is given by the previous authors (Chatterjeeand Pal, 2010) as,

Cleat volume or Porosity Фð Þ = 100 × 0:65=Resistivityð Þ0:6 ð2Þ

Using a matchstick model of cleating, initial porosity and initialpermeability of coal in the Jharia coalfield area can be expressed as afunction of cleat spacing and aperture (Harpalani and Chen, 1995).

Porosity Φð Þ = 2b= s and Permeability Kð Þ = b3= 12s ð3Þ

PermeabilityðKÞ = Ф3s2 = 96 ð4Þ

where b and s are the cleat aperture and cleat spacing respectively.In the WJ area and PB area of underground mines, the spacing of

mega face cleat has been observed at an interval of 0.2 m to 0.3 m. Itwas not possible to measure aperture at the underground mines.Using the size distribution model provided by Ortega et al., 2006, thecleat aperture is found as 1.8 mm with the fracture intensity of9 fractures/m. The interconnected cleated network will provide thecoal bed permeability for fluid flow in the CBM reservoir for nine CBMwells under the study area. Therefore Eq. (4) has been used forcomputation of permeability from log derived cleat volume/porosityfor four coal seams of nine CBM wells with cleat spacing of 0.2 m asobserved at the underground mines. Permeability varies with thecube of porosity as used in Eq. (4). The total cleat volume has beenconsidered for computation of permeability because, the cleats,observed in the underground mines and in the drilled core samplesof exploratoty boreholes located in the vicinity of the undergroundmines, are mostly clean and devoid of secondary in-fillings. However,cleats and joints, observed in the outcrops/opencast mines located in

al stress gradient data for the roof of fourmajor coal seams for nine CBMwells located at

Seam O Seam P

HG VS VG EVG EHG VS VG EVG EHG

18.79 25.55 15.16 6.57 18.57 25.55 16.42 7.66.33 14.03 28.23 17.77 7.61 14.01 25.88 17.78 8.22.22 13.63 23.93 16.47 8.05 13.28 23.33 16.47 8.23.75 16.78 27.54 14.73 8.84 13.69 27.50 17.50 7.29.53 22.53 28.00 18.51 7.25 22.32 27.22 18.82 7.05

31.97 28.82 18.52 7.41 31.73 25.62 14.82 7.24.91 26.26 26.66 15.82 8.34

20.51 24.49 14.28 6.29 17.17 21.62 10.81 5.05.18 17.80 21.29 11.83 5.57 15.16 23.00 11.03 5.34

al Stress Gradient (KPa/m), EHG = Effective Horizontal Stress Gradient (KPa/m).

Table 5Vertical stress gradient and effective horizontal stress gradient data within. four majorcoal seams for nine CBM wells located at south-eastern part of Jharia coalfield.

Seam K Seam L Seam O Seam P

Wellname

VG(KPa/m)

EHG(KPa/m)

VG(KPa/m)

EHG(KPa/m)

VG(KPa/m)

EHG(KPa/m)

VG(KPa/m)

EHG(KPa/m)

K1 14.458 1.926 12.650 1.643K4 17.924 2.616 15.693 2.64 15.680 2.284 15.280 2.285K8 18.825 3.163 18.720 3.602 15.400 2.352 16.220 2.196K10 17.640 2.990 18.280 2.305 19.369 4.165 15.910 2.030K12 12.470 2.880 16.820 2.64 16.210 2.370 16.320 2.280S5 16.330 2.390 14.620 1.970S8 16.820 2.69 15.530 1.800 12.010 2.240M1 12.120 3.660 12.480 1.680M3 13.110 1.71 11.640 2.560 11.010 2.560 15.910 2.500

VG = Vertical Stress Gradient, EHG = Effective Horizontal Stress Gradient.

94 S. Paul, R. Chatterjee / International Journal of Coal Geology 87 (2011) 87–96

and around the study area, are in few places filled with oxidized debrisand secondary leached materials.

Well logs of nine CBM wells, i.e., K1, K4, K8, K10, K12, S5, S8, M1and M3 have been analysed to determine the coal seam permeabilityof four regional coal seams; K, L, O and P. Permeability and verticalstress magnitude for each seam and combined seams are correlatedand plotted in Fig. 8. Permeability of each coal seams decreasesexponentially with the in-situ vertical stress. Coal bed permeability in

Fig. 8. Permeability versus vertical stress magnitude plots for coal seams

general decreases with depth. The goodness of fit (R2) for best fitexponential curve between permeability and vertical stress variesbetween 0.60 and 0.84. The regression relationship for all seams fromall wells has been computed. The exponential relationship betweenpermeability and vertical stress has a goodness of fit of 0.49 whereasthe best fit polynomial equation of degree two has better goodness offit (0.65) in this part of Jharia coalfield.

Average permeability of major coal seams has been provided inTable 6. The average permeability value varies from 0.7 to 1.6 md, 0.8to 1.7 md, 0.98 to 3 md and 1 to 3.06 md in coal seam K, L, O and Prespectively. The maximum (3.06 md) and minimum (0.7 md) valueof average permeability have been observed in coal seam P at491.95 m in well K8 and in coal seam K at 1102.84 m in well S8respectively. Average permeability data for coal seams K, L, O and Pindicate the anisotropic behavior of coal bed permeability. It isobserved that the average coal seam permeability increases from SE toNW direction in the study area.

The vertical cross section across the well K8 to S8 indicates faultedcoal seams at this part of the study area. Vertical stress magnitudevalue at coal seams' roof is comparatively less at the NW part than thevertical stress magnitude value at the roofs of the same seams at theSE part of the study area. The cleated network along with the fault willprovide the major fluid flow path in the study area (Fig. 7). The seampermeability will be more along face cleat orientation of the majorseams under study. The fault orientation as we observed from thesection (Fig. 7) is towards ENE–WSW which is along butt cleat

(a) K, (b) L, (c) O and (d) P at south-eastern part of Jharia coalfield.

Table 6Average coal seam permeability for nine CBM wells located at south-eastern part ofJharia coalfield.

Wellname

Seam K Seam L Seam O Seam P

Averagepermeability(md)

Averagepermeability(md)

Averagepermeability(md)

Averagepermeability(md)

K1 1.70 1.80K4 1.20 1.30 1.85 2.00K8 1.60 1.70 3.00 3.06K10 1.10 1.20 2.40 2.76K12 0.90 1.00 1.30 1.40S5 0.98 1.00S8 0.70 0.80 1.50M1 1.65 1.85M3 0.80 0.85 1.70 1.91

95S. Paul, R. Chatterjee / International Journal of Coal Geology 87 (2011) 87–96

orientation. Therefore we can say that the maximum fluid flowdirection is from SE to NW at the south-eastern part of Jharia coalfield.

The permeability increases with the increase of effective horizon-tal stress gradient and decreases with the increase of vertical stressmagnitudes in all seams (Tables 4, 5 and 6). It is true that we did nothave all nine CBMwell data for permeability computation and no dataare available in southwest part of the study area. This directionalvariation of coal bed permeability follows the interconnected openingmode fractures in coal beds. Cleat orientations, measured at theunderground mines of WJ area and PB area, indicate two sets ofopening mode fractures (face and butt cleats). The increases ofpermeability of four coal seams, coincide with the face cleatorientations as described earlier.

The data, used in this study, are primarily based on the explorationholes, drilled for the CBM project adjacent to the underground coalmines of WJ area and PB Area. Cleat orientation data are collected forfour major seams K, L, O and P from two underground mines.Information relating to coal seam permeability and in-situ stressmagnitudes has been derived from the well log data for the same coalseams. Integration of the cleat orientation data, stress gradient andaverage permeability values suggests significant correlation andconsistency between the horizontal stress field existing in the coalseam and in the immediate roof strata. In-situ stress magnitudegradient increases slightly with depth but decrease towards south-eastern part of the study area. Since the directional permeability ofcoal is directly related to face cleat orientation and is parallel to the SHorientation, the face cleat orientation for the study area provides theorientation of maximum in-situ horizontal stress direction which isalong SSE to NNW.

6. Summary and conclusions

It is essential to map the cleat patterns of the coal seams acting asCBM reservoirs for precise estimation of the CBM reservoir behaviorand determine potential flow directions. The cleat characterizationadds critical input for optimum well planning. The direction of facecleat and direction of coal seam permeability have been correlated forfour major coal seams in south-eastern part of Jharia coalfield. Coalseam permeability decreases with the vertical stress/sedimentoverburden load. Regression analysis between permeability andvertical stress of the coal seams show second order polynomial, asthe best fit curve with R2=0.65. Goodness of fit for the exponentialregressions between vertical stress and permeability for individualcoal seams varies between 0.60 and 0.84.

The vertical stress gradient and effective horizontal stress gradientincrease from SE to NW direction for all these seams. The lineamentdata over the Jharia coalfield and FMI log data from awell in Parbatpurarea also support the face cleat azimuth in this study area. There is anaverage rotation of face cleat orientation of about 25° from Parbatpur

area to Moonidih area. The face cleat orientation for major coal seamscollected from the underground mines under study area provides theorientation of maximum in-situ horizontal stress direction which isalong SSE to NNW. The coal seam permeability of the study area isshowing the prominent increasing trend along SE–NW with maxi-mum fluid flow direction from SE to NW.

The direction of face cleat oriented towards N15°W indicates themaximum horizontal principal stress direction. The direction ofmaximum coal seam permeability will also indicate the SH orientationin absence of subsurface cleat orientation data. Cleat orientationrepresents the palaeo-stress orientation direction and maximumpermeability direction obtained from the well log data indicate thepresent in-situ SH stress orientation. Therefore we can conclude thatthe stress (SH) orientation has remained practically same along SSE–NNW direction. Clear understanding of cleat systems along with itsrelationship to the near wellbore stresses is a key factor for selectionof appropriate completion method for optimized production fromCBM wells.

Acknowledgements

The authors express their sincere gratitude to Coal India Limited(CIL) and Central Mine Planning and Design Institute Limited(CMPDIL), Ranchi for giving us the financial support for this entirework under its R&D Scheme. Supports extended by Mr. A. K. Singh,CMPDIL, Ranchi; Mr. A. N. Sahay, Director (RD&T), CMPDIL Ranchi;Chief General Manager (Exploration), CMPDIL, Ranchi; Mr. A. Saha,Manager (Geology), CMPDIL, Ranchi and Mr. P. K. Hazra, GeneralManager (Geophysics), CMPDIL, Ranchi are gratefully acknowledged.Authors are thankful to Sri P. K. Pal, Ex-Chief General Manager(Exploration), CMPDI, Ranchi for his kind support in improving themanuscript. Kind help and inputs by the General Manager (P&P),BCCL; the General Manager (WJ Area), BCCL; the General Manager (PBArea), BCCL and Mr. S. K. Thakur, Zonal Manager, MECL areacknowledged, as without such help the work could not be carriedout.

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