Coal type, microstructure and gas flow behaviour of Bowen Basin coals

Coal type, microstructure and gas flow behaviour of BowenBasin coals by P.D. Gamson and B.B. Beamish CoalseamGas

JamesCook Unirrersit

ABSTRACT Methaneproduction from coal sermsrather than from porous sandstonereser,rcirsis now recognisedas a rm.luable and recowrableenergysourcein Australia. The Permo-Triassic BowenBasin,Queensland,possesses well defined coal seams,wbich containmajor methaneresources.Howerrer,commercialg:rsproduction to date has been ha-urpered by the low permeabilitiesof coal seams. This study exa-minesthe relationshipsbetween coal tyte, microstructure and gas flow behaviour. ScanningElectron Microscope examinationof bright and dull coal" shows tbat there existsa hierarchyof micron-sized fractures (microfractures)and cavities(microcavities)betweenthe microporesand the cleat network which vzlry in width from 0.05 Fm to 20 pm. The microfracturesare generally associatedwith the bright coals and the microcavities are associated with tbe dull coal lalcrs. Tbe size,continuityand connectivity of the microstructuressuggesttbat they play a significantcontributionto orerall (micro)permeability, and are likely to play a major role in the flow of metbanethroughcoal at a leral betweendiJfusionat tbe micropore leveland laminar flow at the cleatlewl.

behayiourand coals which hare a slow sorption behaviour. It is believedthat the continuingfuture commercial return of the methaneresourcewill dependupon a fundamentalunderstanding of the phpical structuresin coals and their influenceon the strorageand releaseof methane.

INIRODUCIION

Natural gas (methane)production from coal seamsratber than from porous sandstone reser"oirs is now recognisedas a valuable and recorerableenergysourcein the U.S.A. (Duel and Kim, 1975) and Australia (Betl, 1987),A recentdetailederaluation(Johnson and White, 1988)of tbe natural gasresource in the coal me:Buresof tbe nortbern part of tbe Permo-TriassicBowen Basin, Australia, bas indicated a methane gas-in-placeres)uroe of 136 trillion cubic feet (TCF). In comparisonfigg6alia'g conventionalgas resen€safc estimatedat about 25 TcF (DPIE, 1988)and tbe largestU.S.A. coalbedmethane reservein the PiceanceBasin at 84 TCF (Kuuslcraaand Brandenburg, 1989). AlTo determine the influence microstructures thougbAustralia'scoal sean reservoirscont"in \€ry large resourcesof metbane,combar,eon tbe flow of gas tbrough the coal mamercial gas production to date has been trix, sorption experiments were carried out hanpered by the low permeabiliriesof coal on small solid blocks of coal, using a new gravimeuic technique. This new technique seams. enables the simultaneousdeterminationof Methaneprimarily residesin the coal as an adsorption/desorptionisotherms and methadsorbedlalrr on the internal coal surfaceor ane diJhrsivities of tbe coal. The resultsdemas ftee gasin largepores and fractures(Curl, onstrate that there exists a clear distinction 198). As noted by Harpalani and Schraufbetweenthe diffusivityof dull and bright coal nagel(1990a)a " Knowledgeof conventional typesin responseto the coal microstructure. gasresenoir modelling is of little ralue in the The gassorption data suggeststhat both dull case of coalbedmethanereservoirs,due to and brigbt coalscan be dividedinto two cate- the uniquemechanism gas of storagein coalgories:coalswhichharc a rapid sorption bedsand the unusualflow behaviourof gasin coals". The transport of metbane through

I9-2I Novewmber Townsville 1992

CoahedMethaneSynposium

44

PD. GamsonandB.B. Beamish

coal is usually modelled as two phases. Firstly, tbe adsorbed metbane must di.ffirse througb the micropores of the coal matrix until it reachesa natural fracture(cleat),and secondly ulethane flows through the cleat network to the well bore in responseto a pressuregradient (Darcy flow), (Harpalani and Schraufrragel, 1990a).Methaneflow from tie coal is dependentupon the effectiw permeability of the coal. According to mathematical and computer models of methane drairage, Oe spacingof the macrofractures (cleats)play tbe primary role in tbe transport of metbane through coal (Harpalani and Schrauftagel"l990a,b;Harpalani and Zhao, l99l). Howe'rer,studiesof coalsusingScanningElectronMicroscopy(SEM) hara sbown that a bierarchyof micron-sizedftacturesand cavitiesexisB between the microporesand the cleatslrtem in coalsfrom the BowenBasin (Gamson, 1991; Gamson and Beamish, l99l; Gamsonet al., in press). This is important in terms of modelling gas flow through coals,as it suggeststhat microstructuresmay play l) a significantcontribution to orcrall permeability,and 2) a major role in the flow of methanethrough coal at a le'rcI between diffusion at the micropore lerel and laminar flow at the cleat le'rel. The aim of tbis study is to inrrestigate the relationship between coal tyle, coal microstructure and gas flow behaviourin Bowen Basin coals. This paper detailsthe effect of coal microstructure on the dilhrsivity of methanethroughdull and brigbt coal.

COAL TYPE AI{D IVtrCROSTRUCTURG Introductlon In this paper ScanningElectron Microscopy is used to examine the microstructure in coals. Although there has been little use of tie SEM towards the investigation of coal microstructure,the SEM provides an ideal me0od of examiningcoal structure in relation to coal resenoir properties. In tbis study suitably sized (- lcrrr), oriented.unpolishedspecimenscut from hole seemcores from a current drilling programme, provide the best method of exanining a small specific part of a gil'en sample in three dimensions and at high magnifications. Tbe major ad\anLages of using this techniqueare that it alCoalbed M ethane Syn p o sium

lows observationof: l) coal microstructurein 3-dimensionsand, therefore,provides information on the shape,sizeand cross-sectional area of the microstructuresand tbeir bear_ ings to the face and butt cleat and bedding, 2) &acture density,orientationand continuity, 3) connectivityof cleats",microfractures and cavitieg4) individualbrigbt and dull coal bandsorrerser,eralcentimetres,5) connectivity betweendult and brigbt coal bands,and 6) greatertexturaldetail of the macerals. Coal Mlcrostructures ScanningElecuon Microscope examination of both bright and dull coal types from the Bowen Basin sbowsthat the two coals comprise two principal microstructures:micron_ sized fractures(microfractures)and micron_ sized cavities (microcavities). The micro_ structuresvary in width from 0.01 prr to 20 prn, thus, according to pore classifications, are of a size belongingto either meso- or macropores,and are differentiatedby size, .&ommicropores(< 0.0012p:n). In rerms of coal tlpeg the microfracturesare generally associatedwith the bright coals and tend ro form a continuous structural fabric tbrough the coal laprs. In contrast, tbe dull coals predominantly contain microcavitiesthat are part of the phyteraland matrix porosity. (The term microstructure refers to those structuresnot visiblein hand specimen,and separatesits use from tbe terms macrofracture or cleat that are commonlyused to describe macroscopic fractures seen in hand specimen.The microstructuresobserradarc considered to represent original structures, and are not due to saurplingor stressrelease, since:l) most macro- and microfracturesare infilled with secondarymineralg 2) fractures are rarely obserrcd betweenthe coal and the minerelisationinfilling the pore space,and 3) tbe obserwd microstructureshave not been obsened in all tbe samplesstudied.)

BRIGHICOAL In terms of microstructurebright coal predominantly contains a fracture porosity of microftacturesthat occur betweentbe cleats. Two fracture sizes are recognised in bright coals: the larger macrofractures or cleats, and smaller mic:oftactures between the cleats. Townntille19-21Novemben1992

The deet netmrk Comnonly tbree difrerent tnes of cleat (fac€, butt and a t-hird cleat direction) are presenf-in Bowen Basin coals (Table 1). Cleat in cenrimetre scaled lump semplesis being distinguished here from large scale joints and &actures (e.g. masterjoints) whicb are widely spaced (0.1-10n apafi) in coat ser'ni. Figure 1, illustrates tbe axial (along core) direction and face cleat, butt cleat and a third cleat directions with respect to the size and shape of each of the nicrostuctures. Botb the face and butt cleats conmonly occur iu ortbogonal sets oriented to the bedding (Figures la, e-0. The face cleat is the more conspicuous fracture seen in hand specimen and penetrates the coal at rigbt anglesto the bedding (Figures la, e). 1s comparisonthe butt cleat is lesspervasive and is confined to areas between tbe face cleaa (Figure 1a). The face and butt cleats are not continuous throug! tie coal seam, but insteadtend to penetrateonly tbe brigbt coal bandC rather tban tbe dull coal bands. Thc preferential restriction of cleats to the brigbt baods can be seen clearly in thin section (Figurc le) and in side view (Figure lf). Cbaracrcristically,face and butt cleatsare of a planar shape,u/hich l/ariesin vertical eEent or height (ranging from 1-5mm) and width (rangng from 0.01-2mm) and are usually spaced 05-rmm apart. Commonly a third curviplanar cleat direction is present whicb cuB acrossboth tbe face and butt cleat directions (Figurc la). Mlcrofrectures Firc dificrent tlpes of microftacture are recogpiscd h bctween tbc cleats of the brigbt coals:Vertical microcleats,Horizontal m icrocleat$ Bloclry fractures, Conchoidal &actures and Striac. Figure lb ilustrates the size and shapeof eac,hof tbe microfractures with respect to tbe axial and cleat directions. The rariatiou in microftacture present in coals is summaris€d in Table I according to their orerall size, spacing, orienLation, ftequency and their associationwitb coal type. Vertical microcleats parallel the face and butt clcat direction and are commonly 5-20 Fm wide, 5&500 pm long and are spaced3& 100 pm apa$ (Figures lg, 2a-b). Anaiogous to cleats,microcleatsare associatedonly with

Townnillc19-21N ovnwnben 1992

the brigbt coal bands as they terminate ar boundarieswith the dull coal bands (Figure 1e). Horizontal microcleats occur betweeu tbe vertical microcleatsand parallel tbe bedding (Figures1g,2c-d).Thesemicrofractures are commonly 0.5-2 pm wide, 5G300 pm long, spacedat regular intermls pf 5-10 um and tend to be continuousin between the wrtical microcleats. Commonly,the smootb,homogeneous, angular blocks of bright coal exhibit an irregular fracture pattern termed blocky fractures whichgirasthe bright bandsa blockyappearance (Figures 2e-0. Blocky fractures are usually 1-15 pm wide, 5G200 pm long and spacedless tban 100trm apart. In addition, the angular blocks of brigbt coal commonly exhibita conchoidalfracturepattern (Figures 2g-h)showingno regularrelationshipto bedding or the cleat direction or regularity in spacing. At maximum magnilication of 80,00Oxbrigbt coalsexhibit a striated fracture pattern tbat is made up of a numberof closelypackedparallel laminationsor sbeet-likelayers (Figures X-jl. These structures termed striae are on awrage 0.1 pn wide, 10-100pm long, and are denselypacked, with on a\€rage a spacing of 0.1-0.3pn. It would appear ftom their small size, continuity and their constant spacingthat striae representplanes of weakness which appear to correspondlayers of secondarytbickening obsenad in cell walls and basiclapr-plane structuresrecognisedin vitrinite. Commonly the microfractures are infilled with minerals (Figures2b, d, f, h, j).

DT'LL COAL In terns of microstructuredull coal predominantlycontainsa pbyteraland matrixporosity of microcavities. Mlcrocsvitl€s and phyteral porosity Phyteral porosity represents the 'roid space associatedwitb the original plant ftagments. Generally microstructures associated with these organic components are abundant in tbe dull coal bands and form microcavities. as opposedto micro&actures. Figure lc, illustratestbe size and shapeof thesecavities witb respectto tbe cleat directions. A common componentof the pbyteral s8uctures CoalbedM ethaneSymposium

4

P.D. Gamsonand B.B. Beamish

&r's nrrmetougShgglJike structures arranged in a series of stacked layers parallel to bedding (Figues lC h). ThesestructuresreDresent remnantsof wood fibres. In cross_iection the sheEtsare t)rpicaUy24 pn thick, smoottr and apparantly homogeneous(Figures3a-b). The sbeetsare separatedbyloug, cylinderJike microcavities (called ciU tomen), that are commonly 24 pm high and 1G30pm wide (Figures lc, h).

COAL TTPE AND GAS trLOW BEHAVIOUR Introduction In Austratiatbere has been considerablere_ searchrelated to methanedrainagefrom coal seans to allow safe mining (Hargraves,l9g2; famberry and Depers, 1991),but relatively little on the stand-aloneproduction of tbe gas,particularlyin the Bowen Basin.An early study by lfargraves (Lg6Z) showed that rari_ ation in tbe gas emissionexistsbetweenwrious coal typeswitbin one seamat any one location. In addition, Lana and Mitchell (1981) showed using surface and porosiry data of coalsftom tbe Gemini sea-, Lei"nardt colliery, Bowen Basin, that tbe brigbt coals,wbicb hare a greater capacity to store gas, have a lower perureability than the dull coals.

Although the original cavitieswere rectangular in cross-sectionand form sievestructures (Figures3a-b),mray of the structures,hower€r, har,€ siuce been broken and compressed,in order to accommodatechangesin stress. This bas resulted in .anious morphological structures, na:nely needle (tug_ mented cell walts resulting in network of pointed, needle-shapedsplinters;Figures 3cd), compresscd (cell walls tbat haraebeen crushed togettreratnost to lines of compresTbe detailedexaminationof coal microstrucsion, but haw are not broken;Figuresie_f), ture usingSEM presentedhere and in otber bogen (ceU cals that haw been brokeu and pusbed into one another; Figures 3g-h), bostudies(Gamson and Bearnisb,1991;Gangen-compressed(bogen structure tbat has son et al in press) showsthat a hierarchyof micron-sizedfractures and cavities exists rn been compressed;Figures 3i-) ana higblydull and brigbt coals at scalesberweentbe compressed. Com'nonly tlese structuresare microporesand the cleat system. The size, filled wit! minerals (Figures3b, d, f, h, j). continuityand connectivityof the microstrucMlcrocavldes end maHx poroslty turessuggests that theyplay a significantcon_ tribution (micro)permeability,and to orarall Matrix poro sity reptesentscavitiesassociated are likely play to a major role in the flow of with pore spirc€scparatingparticles. Matrix porosity occurs in tbree forms: 1) in between methanethrough coal at a lerrclbetweendifa coarse (1G50 pm) granular matrix of mac_ fusion at the micropore leral and laminar flow at the cleat lerrcl. eral fragneats (Figures 7g-b), such as vitrinire and fusinirc, 2) within a chaoricmass Mlcrostructure and gas flow behavlour of minutc (1-5 pm), angularto roundedpartiTo understandthe relationsbipbetweencoal cleg collectiwly lnown as micrinite, anA 3) t1pe,micostructure and gas flow bebaviour, as microcavitics in between clay particles (Figure ft). In rcrms of coal types,the ctays sorptione4eriments were carried out on selected samplestbat were previouslyexamined in the briglt coals occur predominantly as in tbe SEM for their microstructure. Using a frsglurg iDfitl. In contrast, the clays in dull new gravimeEic technique de,rclopedat the coals occtlr in tbe form of either clay bandMenseg 8s fne particles disseminated CGRI (seeBeamishand ODonnell tbis rotume), sorption eryeriments were carried out through the coal as cavity infill, and/or clay particles interbcdded betweeu maceral fragon small (1g), solid blocls of coal rather rban ments. The cavitiesvfiich occur betweentbe crushedsamples,using a microbalance. This techniquehas been dewloped to test small gtaf qarlcles and granular aggregates\ary from 0.1-2pm wide and up to 20 p.urtong. samplesand allowsa closerunderstandinsof the influencecoal microstructurebas on-tl" diffusivityof tbe coal, which controlstbe gas flow rate througb tbe coal matrir This new approachto gas sorptionstudiesby using sma[ solid sanples of coa], contrasts Coah edM etltate Syn p osium

Townsyille19-21N ovemben 1992

with previous studies (e.g. Patcbing and Michail, 1986') u/hich bave measured coal sorption using a bulk sample (8G150g)of a tnocn crushedsize ftaction (< 250 pm)' and are perfurmed to obt4in faster sorption equilibrium. The new tecbniquefollows on ftom initial tests carried out at the CGRI (Beamish et 4l, 1991)to determine the effect solid and crushed samples has on gas sorption' Tbe initrrl sorption tests whicb were performed on tbe two size fractions (solid and crushed), sbowed that the crushed size frac' don has & more rapid methane uptake and reaches equilibrium more quickly than their solid counte{patts. Althougb this indicates tiat the higber uptake of methane observed in the crusbedcoal is more representatiw of tbe coals'maximum gas storagecapacity'the rapid uptqke of methane shown by tbe cru.nea iize fraction, tells us little about the rime it takes for netbane to flow through a solid coal i.c. the coals difftrsivity' In contrast, thc sarne initial tests showed tbat the rate of methane uptake for the solid coals raried, and tlat there existsa clear difference in tbe sorption behaviour between $e dull and brigbt solid coals. Using theseinitial re' sults, ii was evident that the use of smalt solid sanples in sorption measurements'provides a method of understandingthe lvay gas flows tbrougb a solid coal. Tbe individuat pieces of dull and bright cod were desorbedfrom l.lMPa to atmospberic' and tbc alnouat of gas released measured witb timc. To undersuntl the effects of microstructulc on sorption, parallel samples were analSed in the SEM. Samplesdecdon In lotal thirty coal samples were used to study sorption behaviour and coal microsEucnre. Of thoseunples sixare reported oD herc (Table 2) as representing tpical sorption behaviours of dull and brigbt coal 16s srmflcs consist of tlree setsof dull ancl brigbt coalsthat were selectedftom the saine seamand tbe samedepth. To reduce the influence of the effect that sa:nplesize has on difr.rsivity,all the sampleswere cut into similarly sized cubic blocks.

er, 1992 T ownsvilk19'21Novevtmb

Sorptlon behaviour of dull and bright coal The desorption rate of the six coal samples are shown in Figure 4, wbich plots the fraction of gas desorbedversusthe squareroot of time. The desorption data for the six sa:nples (Figure 4) indicates that the rate at c&ich metbanedesorbsftom a solid coal ra'ries between coal samplesand more impor' trntly betweenthe dull and brigbt coals. The results show that both the dull and brigbt coals show two distinct fields of behaviour: fast sorption and slow sorption. The faster desorbingdull coals are characterisedby a ong stage sorption process' that is characterisedby a rapid releaseof the total gas( lB; Figure 4). In contrast,the slowerdesorbing dull coals (sa:nple 28 and 38) are characterisedby a two stagesorption process:a fust stageduring which tbere is a rapid releaseof gac followed by a second stagewbere sorp' tion is much slower (Figure 4). Sinilarly tbe bright coalsare characterisedby two tlpes of sorption behaviour. The faster desorbing by a bright coal (sa:nple1A) is cbaracterised durstage frst process: a two stagesorption gas, folof release rapid is a ing whicb there is sorption where stage lowed by a second the contrast, In (Figure 4). mucb slower much slower desorbingbright coalsare cbaracterisedby a oue stagesorption process' The difrerencesin tie sorption rate sbown between the sa:nples can also be seen in terms of tbe time it takes for tbe dull and brigbt coals to desorb 63Voof' tleir total gas Tsorp ltJorp; Table 2). The table sbowstbat minutes 5 of low a ftom coals, between nries to as mucb as 1309 minutes (- 22 bours). Typically Tsorp for the faster dull coals rrar' ied aom 5-10minuteswbereasTsorp for the slowestdesorbingdull coal, was 189'210min' utes (Table 2). By comparison,Tsorp for the hster brigbt coalswul in tbe order of 15&160 minutec and 30GBm minutes for the slow' est desorbingbright coal. The differencesin sorption bebavioursbown by tbe two coal classescan best be erylained in terms of the macropors and micropore componentsof tbe coal. Tbe simplestform of comparisonof desorptiondata is to apply a spbericalunipore model. Of tbe six coals boieraer,only three of the coals,lB-duU' 2A and 3A-bright fit tbe unipore spberical model. To iplain tbis, the releaseof metb' sium Coahed M ethaneSYmPo

4 ane in suc.hcoel$ may be considereda one stagesorption proc€ss. In the dull coal, 18, only macropore sorption occurs due to ttre domination of macropores. In contrast,the one stagesorption processin the brigbt coals, 2-A.and 3A, frt a unipore model due to the dominatiooof micropore diffrrsion. The three other coal$ lA-bright, 28 and 38du[ hovc'rer, do not fit a unipore spherical model due !o a distinct curvature in the desorption currc (Figure 4). To explain tbis phenomenon,tbe sorption behaviour is divided into a microspbere (micropore) component v/bicb iE surrounded by a macrospherc (macroporc) component (Ruckenstein et aL,l97l'), Tbe releaseof methanein tbese coab may be considered a two stage sorption procesq where sorption in the macroporesis much faster than in the micropores so that equilibrium is essentially achiewd in tbe macropores before any appreciable releasc by thc micropores is obser.red. Consequently a first stage is obsened duriog which only macroporesorption occurs, foloc/ed by a much slower second stageduring which macropore sorption is at equilibrfu:n, 8nd only micropore sorption occurs.

MICROSTRUCII]RE AI\D TEE EF'FECT ON SORSTIONBEHAVIOUR Tbe difrereuces in gas sorption bebaviour and the effect of a macropore and micropore component shown between the dull and brigbt coal samplesdescribed above,can bc eplained in tcrms of microstructure and extent of mineralisation. Mlcrmtmcture of the brtght coals In rcrms of sorption the bright coals release their gas 8t different rates. The faster brigbt coal lA bngnt, which has a two stagesortion behaviour, is characterisedby having a well defined cleat grid netvDrk that comprisesa dominant frcc cleat (Figure 5a), a lessregular butt ctreatand a tbird curviplanar cleat, tbat cuts a6osn the face and butt cleat. All three cleats arc infilled with clay which occurs as individual plates (Figure 5c) and booklets and are packed moderately tightly (Figure 5b). Betcrcen the cleatsare a series of misofractues cfrich consist of closely spacedErtical microcleats (Figure 5d), hori-

CoalbedM ethutcSymposium

P.D"Gamsonand B.B. Beanish zontal microcleats,blocky fractures,concboidal fracturesand striae (Figure 5e). Tpically all the roicroftacturesare open and unmineralissd. The macroporosityand permeability associatedwith: 1) cavitiesin between the clay particles iafilling the cleats, 2) continuous horizontal microcleatg, vertical microcleats and blocky fractures between the cleatq and 3) the presenceof unmineralised conchoidal ftactures and striae, indicates there is probablysome degreeof macropore space and tberefore permeability, whicb explains the moderatelyrapid releaseof gasof this sa:nplewhen testedin the microbalance, These faster bright coals contain a well defined cleat glid network, where cleats are moderatelytightly infilled with clay and connecting pore space exists between tbe clay particles iafilling the cleag. In addition, the coals contains open, unmineralised blocky ftactureq microcleats,concboidal ftactures and striae. The rapid sorption behaviour characterised by thesebright coalsappearsto be related to gas sorption throughcoalswith a higberproportion of macropores. In contrast, tle slower sorbing 2A and 3Abright coals,c/hich show a one stagesorption process, concain 'rery different microstructures. 2A-brigbt is dominated by closely spaced(0.5-1.5mmapart) face cleats(Figure 6a). The face cleats are 30-50pm wide, extend througb the core and are slightly curved (Figure 6a). Tbe cleatsare infilled witb large (3-5 Fm wide), niurow (1-3 ptrr higb) tightly packed,plates of calcite 'ffbich elcend across and at riglt anglesto the cleat (Figures6b-d) which are separatedby tbin (0.01-0.05pm high) &actures. Perpendicular to the face cleatsare discootinuousbutt cleatswhich are 1G20pn wide, and generallyeseDd between the face cleats. The butt cleats,hower€r, are not mineralised. In between the face and butt cleat$ there is a densenetwork of narrow (1-20 pm wide), closelyspaced(1G200 pm apar$ bloclcyfractures, tbat are perpendicular the face cleat and parallel tbe butt cleat (Figures6e-0. The blocky fracturesare generally open and unmineralised. Tbe blocky ftacturesare also joined by conchoidal fractures and striae. As well as a dominant fracture porosity, 2A brigbt also contqins;l) a minsl (17o)phyteralporosityassociated with cell structurc in tbe vitrinite, and 2) a number of circular (1S200 pm in diame-

1992 Townrville 19-21November,

ter) Fc-Mg-rich pods (Figures 6a, e'D which haw a matrix porosity of minute (0.05-0.5pm wide) subangular grains which are porous and afford 8 higb permeability. Altbougb the face cleats ate tightly infiiled witb calcite, iodicating low porosity' tbe open and unmineralised butt cleats, blocky ftactureg cihicb sepalate the cleat infill ftom the brigbt coal, conchoidal ftactures and striae, indicatetbat 2A ssstrins a macroporeporosity. The connectivity of the butt cleats and bloclry ftactures betweenthe face cleas indicates 2A probably has a high permeability' urticb crplrins the rapid releaseof gasshown by 2A whea tested in the microbalance. 3A brigbt is characterisedby a dense network of closely spaced (1'2 mm apart) face and butt cleats arranged perpendicular to each otber. The face cleatsare narrow (5-10 p.mwide), planar (Figure 7a) and are discontinuous tbroug! tbe core width. In comparison,tlc butt cleats are e)dremelynarrow (1'3 pn wide), and are shorter in length. The facc cleae are tigbtly infilled with sstau (12rnm wide) platelets of illitic clay (Figure ?b), wbereastbe butt cleatsare tigbtly infilled rvitb mtnutt (0.14.5 pn wide) angular,illite gains (Figr:rc 7c). Microfractures are also a coInmoucomponent of 3A bright. The most conspicuousare '/ertical microcleatsthat are 0.t1.5 pm wide, are spaced 1CS500 ptut apart and haw a planar appearance(Figure 7d). Likc the cleats, the vertical microcleats are tigbtly infilled (Figure 7e) with illitic clay tbat comprisesplatelets with an irregular outline (Figurc 2h). In between the wrtical microcleats arc a series of smaller fractures: conc,boidalftactures and striae. The concboid^l fractures are 0.05-1pm wide and are tigbtly infilled with illitic clay (Figure 2b). Striar, tbe smallest microftactures seeu in brigbt coat, are couunon in 3A-brigbt, and are typicaUy0.05-0.1Fm wide, 1-20pm long and are regularly spaced 0.1 pm apart (Fig' ure 2j). Tvo striac direction were uoticed: 1) parrllel o the face and butt cleats,2) at rigbt anglesto tbc tbe face and butt cleats. Both sets of striac sbow signs of infilling (Figure 2j) with minute grains of illitic clay. The lack of macropore spaceresulting from tbe tigbtness of cleat and microcleat infill' and tbe prescucr of mineralisation in tbe conchoidal fractures and striae, erylains the e:cremely

T ownntllc19'21Novnnnber, 1992

slow releaseof gas shownby 3A brigbt wben testedin jlg 6lsrelnlqnsg. The sl,owsorption behaviour sbown by tbese bright coals follow a more linear rate, which appearsto be related to gas sorPtionthrougb coals with few mactoporesand a higber proportion of micropores. This is probaplydue to the presenceof more tightly infilled cleats antl microftactures, and/or absence of mi' softactures. Mlcrosbrrcture of the dull coals Variation in sorption bebaviour shownby the dull coals can also be related to the microstructue of tbe coals. Tbe faster dull coal' lB-dull, is dominatedby bands of wood fibres (Figures3q, 5f'j), that are interspersed with tbin bandsQS100 pm wide) of discontinuousbright coai (Figure 5i), fragmentsof cell wall (Figure 5i) and cavities associated with micrinite. Tbe wood fibres are predominantly open and unmineralised (Figures 5fh). Althougb many of the fibres are broken and compressed(Figure 5h)' the cavitiesas' sociatedwitb tbem are wide and connecting' This is illustratedin figures 5i-j, whicb showa bigb number of compressedwood fibres be' tween tbin bright coal bands and ftagments of cell wall. Tbe phyteral porosity associ' ated witb wood fibres offer a high macropore porosity vhich would appear ftom tbeir continuity and connectivityto offer a higb degree of permeability and tberefore offer excellent conduits for gas flow. The bigh percentage of open macroporesin lB'dull explains tbe rapid release of gas sbown by this sample when testedin tbe microbalance. Altlougb 2B-dull also has a fast sorption be' baviour (Figure 4), the sorption process is cbaracterisedby two steps. The microstruc' tures of 2B-dull is dominated by layers of wood fibres oriented parallel to the bedding' wbich are broken and baw been pushed into one another (Figures6h-j) resulting in a typi' cal bogdn-sructute. The cavitiesin between the wood fibres are generally0.5-10pm wide' 0.5-5 Fm high, and are unmineralised. The wood fibres howsver, are not compressed. The large cavitiesassociatedwith the bogen' structure in 2B-dull, offer a higb macropore porosity. Howercr, becausethe wood fibres are broken and haw been pusbed into one anotbet, the continuity of tbe cavities haw been reduced,resulting in lower permeability

Coalbed Methane SYm Po sium

50 tban unbrolcencompressedwood fibres. Neverthelessin 28 it is probable tbat the pores are connectcd whic.h e4lains the rapid releascof gas shorn by 2Bdull when teited in the microbaf ice. In comparison,tbe slower desorbingdull coal (3B-dull), comprises a \€ry different micro_ structural asscmblage. The coal comprisesa seriesof thin (5G200 tm), alternating Iaprs of dull (60% of tle assemblage)and homogeneousbrigbt bands. Unlike tB and 2B_dult, the dull Iayers in 3Bdull are composed of two porosity tpes: phyteral and matrix The phyteral porosity b dsminxrcd by Iayers of woody fibres that are either broken and pushedinto one another,resultingin bogenstructures (Figures Zi-j), or are compressed, resulting in compressed structures (Figures 3t, 7D. Apart from a minor proportion of open unfilled bogen-structures (Figure Zj), typically the bogen- and comptessed-struc_ tures in 3B-dull are tightly infilled with sma[ (0.5-l pm wide) angular grains of illite clay (Figures 7f, i), resulting in a low porosity and low permeability. Tbe mauix porosity in 3Bdull is associatedwitb cavities occurring betc,Eeneitber: l) snall (l-5 pn wide and 1-15 pm long), angular maceral fragmeuts (Figures 7g'h), 2) cby particles infilling pore space between maceral fragmens (Figures 7g-h), or 3) cavities in between angular-subrounded clay particles associatedwi& clay bands. The low percentage of macropore space resulting from: l) compressionof the wood fibres, 2) breqr.rgg of the wood fibres, 3) infilling of cavity space between ttre compressedand broken cood fibres, and 4) bigh degee of mineratisafigninfilling the phyteral and matix porosity, infers tbat permeability in 3A dull ir rery low, and explains the extremely slorr rclesso of gas shownby 3Bdull c/hen testcd in thc microbalance.

P.D.Gamsonand B.B.Beanish both macroporous flow and microporous flow.

GAS FLOW TEROUGE COALS IN TIIE BO\ryEI{BASIN A combinationof ScanningElectron Microscopy and sorption testing using a microbal_ ancehas showutbat the tine taken for methane to trawl tbrougb the coal matrix varies according to the microstructure at the small scaleand is reflected by signilicantmriations in Tsorp. hevlous modelsof gas flowthrougfi coal Previousmodelsare basedon the assumDtion that metbaneprimarily resides in the coal as a &ee gas in Iarge pores and ftactures,or as an adsorbed lalcr on the internal coal surfaces(Curt, 1978;Gray, lgg7). Gas in coals is thougbt to occur mainly in the adsorbed state,as a monomolecularlayer 4 Angstroms thick (Wynan, 1984),on the pore surfaces. The adsorbedtapr accountsfot 9\9g/o of togal methane (Barker-Read, l9g4; Gray, 1987) witb the remaining small a.mountof gar,2-t07o(Barker-Read,1984;Gray, 19g7), in the gaseousstate, within the open pore spaces(e.g.macropores,fractures).

Presentmodels of methane flow throusb a coal sea:n,indicate tbat the adsorbednietnane after desorption into tbe gaseousphase must difr:se througb the pore structureof the coal matrix until it reachesa cleat (King, 1985;King and Ertekin, 1989;Harpatani and Schraufragel,1990a,b), followed by ftee flow through the cleats to a well in responseto a pressuregradient (Figure 8). The di.ffusionis usually modelled using Fick's Law and tbe free flow is modelled using Darcy,s Law. Darcy's Law describes the flow of water The rapid sorption behaviour characterised througbporous media that coutainsporesof uniform cross-sectionand of uniform packby lB and 2Bdull coalsappearsro be related ing, so tbat pore size,shape,distributionand to gas sorption tlrough coals with a higber tbe connectivityof pores are grouped toproportion of open, unmineralised macrogether under the one puameter, permeabilpores, and probably relates to macroporous ity. Frou Darcy'sLaw the generalbehaviour flow. In contrast, the slow sorption behavof metbaneflow through macroscopicstruciour characteriscd by 3B-dull follows a two tures in coal such as the natural cleat netstep proccss, vftich appears to be related to work has been modelled, since tbe cleatsare gas sorption tlrough coals with few macro_ regarded as having a uniform pore geomery pores and e higher proportion of micropores (size, shape and spacing) that is repredue to minerelisalion, aud probably relaies to sentatil€ of coal as a whole. Moreover, he model assumesthat the madx blocks. as de-

C.o albedM ethanc Syn posium

Townfl)ille 19-21Novemben 1992

fined by thc cleats comprise nicropores of thc sanc di-meter. According to this model of gasflow througb coals (Figure 8), gasmigration is go',eraed by two main factors. First, tht disr^nce methane has to difftrse is dependentupon tbe face and butt cleat spacing that delineates the size of the matrix blocks in the coal. Second, tle Amount of gas flowing througb the cleat is dependent upon the width, length, continuity and permeabilityof the cleats. Althougb this model of methane transport througb coal may well apply to predominantly brigbt coal seans where tbe cleatsare open and gnrninglafi5gd,the nature of gas flow through coals in the Bowen Basin, as indicat€d by studies of sorption behaviour and SEM analpis, is perhaps more complex than this. Gas fiow through coals ln the BorrenBasin-a newmodd Scanning electron nicroscopy has showu, tbat at a scale between the micropores and cleat systeo, Bocrcn Basin coals contain a rangc of nicroftactures in the brigbt coals, and a range of microcavitiesin the dull coal of nrious pore sbapesand sizes which vary according to the degree of minerelisalisr. Conscquently,it is likely that methane flow tbroug.h thc matrix will take place not througb pores of similar geometry as previously suggested,but rather through a complicated network of interconnected microftactures and microcavitieg of rarying size, sbape and cross-section,and often witb ming{s filting pore space. According !o models of methane flow (Figure 8), thc flow of methane from the coal is dependcntupon the effectire permeability of the coal, Le. the cleat networt. Assuming diftrsion of methane begins and presumably finishes at the micropore level, as previous models of gas flow suggest,(where micropores 420 Angstroms in dia:neter are joined by minutc passages5-8 Angstromsin width, tlat allow only tbe diffrrsioo of gas molecules) then metlane flow from the micropore s)6t€m!o tbe cleatsmust rely upon the effectiwnes of tbe microstructure s)6tem in coal to transport methane. The rarious microfracturesand microcavities, and thc different sorption behavioursexhibTownsvilh 19-21N ovevwnb et 1992

ited by the dull and bright coals,suggeststhat tbere may be four ratber than two steps in.CIlrrcdin the flow of methane through coal s€rmsin the Bowen Basin, at a ler/elbetween diffirsioo at the micropore lerrcl and laminar flow at tbe macroftacture or cleat le'rel. A four tier model is proposedthat incorporates methane flow in boti dull and bright coals (Figure 9): Step 1. Diffrrsionftom and througb tbe microporesto midofractures in the brigbt coal and microcavitiesin the dull coal. Step 2. Diffirsion and flow of merhane through mic:oftactures and microcavities partly blocked by diageneticminerals suchas clay, quartr; pyite or carbonates. Consequently, the no\Ement of methane at this lewl is likely to inrolw a dual mechanismof diffrsion and flow dependingon the size and connectivityof the pore spaceremainingin the infilling. Step 3. Flow through open, unmineralised micofractures in the brigbt coal and microcavities in the dull coal. Because the microfractures and cavities are relatively large (0.05-20 pm wide) transport of merhane would primarily involrrcDarcy's la.minarflow. Step 4. Gas no\€ment througb cleats and joints to the well base.Open cleatsare generally large and we expectmost mo\€ment wiil be Ianinar flow. Howercr, in tle Bowen Basin cleau are generallyinfilled by clay mineralC atrd mo-stlythis infill forms a tight seal. Cousequently,gas mo\€ment will be eitber completelyblocked or be by diffusion. This new model is proposed tb accounrfor the transport of methane in both brigbt and dull coals. The etrra steps sbow that microstructures are likely to play an important rate-li?nitingstep at scalesbefore gas transport at tbe cleat or ftacture le'rcI. Mlcrostructure and gas flow The effectircness of methane flow tbrougb the microstructures (as opposed to the cleats), would be ultimately influenced by sercral microscopicconsiderations,whicb includc tbe shape and size of microstructures, microstructure disuibution (density, orientatioo and continuity), connectivity of tbe micf,osttuctures and the cleat s)6tem, the amouDt of ftacture infiling with secondary CoahedM ethaneSynposium

52

P.D. Gamsonand B.B.Beamish

mi.eral$ and clay dispersedtbrougb the orgadc matrir Eaci of tbesemicoscopic conditions will barr a different effect on tbe quantity, rata atd direction of gas flowing tbrough the c,oal Tbus we introduce tbe term micropermeability to refer to the conductivity of &e mir.rss611g1s1eg of the scale 0.05-20pm width.

rosity. Wbere cavitiesare associatedwith uubroken wood fibres, it is commou for the pore spaceto conrinue, borizontaily tbrougb the dull coal bands for se'rcral centimetres, and therefore offer a higb degreeof micopermeability. Wbere wood fibres bare been compressedand/or broken ald pushed togetber, howeraer,the connectivity or micropermeabilityof the coal to gas flow, will be re_ Il *Ts of gas flow tbrougb coats it is likely duced. This is becausethe arrangementof that the qrrrnlify of gas that will flow througb wood fibres tead to be aligned/orerlapparalthe coal q/ill ulrirnrgslybe determinedby the lel to rhe bedding whicb wouid probaUiyim_ shapc and cross+cctional area of the micropose a strong directional permeability whicb structures. Typically, cleats and microftacwould causegas to flow in one direction, that tures arc ptqnrr, urying in width (0.1-2000 parallel is to thc bedding and at rigbt angles height (10pm-3mm) and length (10 pn1n), to the cleat network I! contrast,microfer6co) and tosd to fora a grid network in meability perpendicular to the dominant brigbt coals. In cotrtast, phyteral pores assowood fibre directioo is likely to be extremely ciated with u,ood fibres are cylindiical, rarysmall. Ifowerrr, wherc cavities in dull coals ing in width (lO3O pm), beight (2a pm) and occur between maceral &agments and clays leng$ (10 pm-6co), and tend to occur in of the matrix porosity, tbe random orienta9g9t-1il.r layers that parallel the bedding in tion of these pores is likely to causegas to dull coals. By comparGon, cavities associflow in rarious directions, whicb combined natrix ated witb porosity in dull coals and with tbe tortuosity tbrougb the matrix, will clayc, rcnd !o har,p a less regular shape and prolablyresult in a low micropermeabiiity. Comparcd !o tbc phyteral pores and l*. fracturcs, Estrix cavities are generally Gas flow through the coal seam will be furqnatler (0.05-10 Fm wide) and rary more in ther consEainedby the degree of connectivity shapc &om simplc angular pores in between between the different types of microstrucmaccral fragmeutg to more complex, contures of the individual coal bands. Altbougb torted porcs betweenfibrous clayparticles. the cleatsand microcleatsform a continuous Iu terms of the directiou of gasflow, microp_ structural fabric through the bright coal layermeability will be controlled by tbe densiry, ers, and the pbyteral and matrix porosity is continuousin the dull coal lalers, it is evident oricntation and continuity of thl microsuucftom SEM that fractures rarely connectboth turcs. Charactcristicaly, bright coah are tbe brigbt and dull coal bands. Tbis is imcomposcd of a ftactr:rs porosity, that is portatt as it suggeststhat microperoeability domiaatcd by frce and buit cleats, vertical and tberefore tbe flow of gas througb a coal and horizontal microcleat& concboidal miseamwill changeaccordingto coal t;4re,such croftacurrer and striac. Thc microcleaa that flow will probably bc rrrtical and horicommonly form e densc ortlogooal network zontal within tbc bright coals and borizontal of fractrrrer bct*tcn the cleat$ and offers the in the dull coalsand clay bands. pot€rt-tl.l for htgh le\Dls of connectivity and thereforc micopermeability. Becauseof the ditrerent cJeatand microcleat directions,gas flow is likely o occur in two or more diricMPLICATIONS OF MICROSTRUCTURE ON GAS FLOWMODEI-S tious. Thc clcab and microcleats are also linleef, by s'nrlter conchoidal fractures and Considering tbe rariety of microstructures striae vfiich arc not as continuous as the aud tbeir effect on a coals'-difrrsivity,rhe nacleats and microcleatsbut may provide add! ture of gas flow througb coals in the Bowen tional porc spacc for metnanJ flow &om tbe Basin is probably more conplex than preuucroporesto tbe largcr microcleats. viousnodels of gasflow haw suggested.AlIn connast, dull coals arc dominatedby a difthougb thc idea of gas diffusion through the ferent microstructual distribution, that is matrix blocks and leminar' flow througb tbe gorrrued by either a phyteral or matrix pocleatsnuy well apply to predorninantlybrigbt coal sermswherethc cleatsare open and un-

CoalbedM ethanc Syn po sium

Townsville 19-21Novemben1992

Coaltwc, mioosfiactun and psJlow behaviour mins1trti56{,in the Bowen Basin coals commonly comprise: 1) both bright a:rd dull coal typec 2) dull and brigbt coals which contain a'rariety of rnicrostructures,3) dull coalswith few or n-o clcats, 4) cleats, which to varytng degreeg arc infillcd with clays and carbouat€s. At pres€nt, cuEent models of gas flow do not accountfor thescnriations. In terms of gas flow modelling, such rariations il coal serms have important implications for pressnt models of gas flow tbrougb Presently, models subdivide matrix coal blocb c&ere diffirsion dominstes,ftom that in.olwd s,{tb lqminar floq on thg cleat spacing. Becauscthe effectsof microitructure on dimtsivity are considercdto be real, the presencc of open and continuousmicrostructures in coal suggcstft1t lamiasr flow is likely to begin at trevelssmaller tlan those identified at prcscnt by the spacingof cleats. To accomodatc the effects of mioostructure into models of gas floq it is necessaryto redefine the effecti'c block size inside which di.ffusion @gmin4t€9.

In brigbt sortr, u&erctbereis a hierarcbyof opcn,condnuousand connectingmicrofracturesbctupcn thc clcaLs,the effectiveblock sizcis probablynot thatbeingdefuiedat presentby thc clcat spacing,but somewhere betweentbe cleatsrud tbe microfractures.In contrast,wherecleatsandmicofracturesare infilledwit! minslal$ and tbepore spacefor lamiDsrflow is reduced,tbe effectiveblock sizcmaybc larger tbanpreviouslyassumed, definedby otber andmaybc moreeffecti'aely morc widely spaced,open ftactures(oins) in thc brig[t coals.Ultimateiythis couldreflecf frrcturc sctsbc)Dndthe core. In dull coalshowe!€r,wherethereare usuallyno c.lcatqit is moredifiicult to definetbe effcctir,pblock sizefor gasflow. The microstructual and sorptionresultsindicatethat c/terc dull @alscoatainopen,unmineralised strucnEegand macroporous flow dsminsigg, tbe cttectirt blocl