Study on process and model of CBM dissipating

ARTICLES4. Zhang, J, Wang, H, Zhao, Q., Coalbed Methane Geology of ChinaChinese Science Bulletin 2005 Vol. 50 Supp. 134- -139(in Chinese), Beiig: Geological Press, 20000 9-71.5. Zhao, Q.. Li, w., Sun, F, Distribution and accumulation of coalbedStudy on process and model ofmethane in China (in Chinese), Acta Petrolei Sinica, 1997, 18: 1-CBM dissipating6. Qian, K., Zhao, Q, Wang, Z., The Theory and Experimental Tech-nique of Coalbed Methane (in Chinese), Bejing: Petroleum Indus-HONG Feng, SONG Yan, CHEN Zhenhong,try Press, 1996, 170- 182.ZHAO Mengjun, LIU Shaobo, QIN Shengfei. Fu, X., Qin, Y, Wang, W. et al, Hydrogeological contolled prop-& FU Guoyouerties of coalbed gas on central-southerm Qinshui basin (in Chinese),Research Instiute of Petroleum Exploration and Development, Petro-Coal Geology of China, 2001, 13:31- 34.China, Beijing 100083. Su, X, Zhang, L.. Abnormal high-pressure regime of coalbedCorrespondence should be addressed to Hong Feng (email:methane reservoir (in Chinese), Natural Gas Industry, 2002, 22(4): hfe@petrochina .com.cn)15-18.9. Yu, s., The dynamic valuation and discussion of coalbed methaneAbstract Coal Bed Methane(CBM) occurs in coal seams(in Chinesc), Well Testing and Production Technology, 1995, 16:in the states of adsorption gas, free gas and water-dissolved14-20. .gas. Its dissipating starts with desorption, and then it dissi-10. Su.X. Lin, x, Song, Y.e al. The casfcato and model of pates outwards in the states of free 8S and water dssedgas. The dissipating approach is classified to three patterns:coal-bed methane reservoirs, Acta Geologica Sinica, 2004, 78(3):Free gas in pores dissipates through the cover rocks; hydro-201 -205.11. Qin, Y, Adcances and reviews on research of coalbed gas geologycarbon molecules in the cap-rocks and reservoir diffuse be-in China (in Chinese), Geological Joumal of China Universities,cause of concentration gradient; gas dissolving in water isdirectly taken away by water. According to the controlling2003, 9(3);: 339- -358.factors of CBM conservation and considering the cover rocks,2. Anna, L. O., Groundwater flow associated with coalbed gas pro-soleplate, hydrological region identification and dissipatingduction, Feron Sandstone, eatcentral Utah, Intemational Jounal theory, three geological models of CBM dissipating are built:of Coal Geology, 2001, 56: 69-95.closed system model, lateral hydrological closed model and3. Su, X., Chen, J, Sun, J, Geology, Exploration and Development ofopen system model.Collbed Methane (in Chinese), Beijing: Science Press, 2001, 38-Keywords: CBM, dsspate, cap-rocks, hydrogeology, conservation,40,62-63.model.14. Cas Geological Staff Room in Jiaozuo Institute of Mining Tech-DOI: 10.1360/98zk0021nology, Introduction to Gas Geology (in Chinese), Beijing: CoalIndustry Press, 1990, 82 -95.15. Yielding, G, Freeman, B.. Needham, D. T, Quantitaive fault sealCBM is an unconventional natural gas, and mainly oC-prediction: AAPG Bulletin, 1997, 81: 897-917.curs in coal seams in a state of adsorption gas which is16. Lu, B., Ding, W., Advance of rescarch on fault scaling (in Chinsse),adsorbed on the interface of coal cracks. So CBM conser-Geological Science and Technology Information, 1998, 17: 75-vation is often ignored, even in US, the most succesfulcountry in CBM exploitation. In fact, despite its presence17. Bai, x, Luo, Q, Research of fault seal property (in Chinese), in a state of adsorption gas, CBM still depends on an ad-Journal of Daqing Petroleum Iste, 1998, 22: 89- 92.vantageous condition of conservation. Zhang Jianbo',18. Wang, L, Xu, H, Advance of research on fault sealing (in Chinese, Qian kail!), Qin yong", Zhan Xinming3]) et al. seriouslyJoural of Xinjiang Petroleum Institute, 2003, 15:11- 15.studied Qinshui Basin and put importance on CBM con-9. Zhang, J.. Zhang, L, Yang, H. et al, The scaling mechanism forservation influenced by the upper strata. CBM dissipatingfault and its recognition methods (in Chinese), Henan Petroleum,modes and mechanism are discussed in this paper. Ac-2003, 17:7-9.cording to the difference of CBM condition of conserva-20. Wang, X., Fu, C, Shi, J, et al, Examples of abroad coalbed meth.tion, CBM dissipating geological models are built, andane exploration and development (in Chinese), Beijing; Petroleumthen the significant effct of CBM condition of conserva-Industry Press, 1998, 346- 356.tion is discussed.21. Su, X., Lin, x., Zhao, M, Song, Y., Geology of coalbed methanereservoirs in the Southeast Qinshui Basin of China, Intemaional1 Process of CBM dissipatingJournal of Coal Geology 2005, 62(4): 197- -210.CBM occurs in coal seams in states of adsorption gas,2. Su, X., Zhang, L, Zhang, R., The abnormal pressure regime of thefree gas and dissolved gas. Generated gas in coal seamsPennsylvanian No. 8 coalbed methane reservoir in Linlin-Wupufirstly is adsorbed till the interface of coal cracks is flldDistrict, Eastem Ordos Basin, China, International Journal of Coalperati中国煤化工ges, the desorption ofGeology, 2003, 53: 227 -239.CBM_BM dissipating begins(Rcceived March 15, 2005; accepted June 27, 2005)withHC N M H Gsipates outwards in the1) Qin Yong, Study of formation condition in the mid south of Qinshui Basin, China University of Mining, 1999.134Chinese Science Blletin Vol. 50 Supp. December 2005ARTICLESstates of free gas and dissolved gas. The dissipating ap-floor lithology of No. 8 coal is mud and the gas content isproach is classified to three patterns: Free gas in pores10.43 m'/ts, then northward the Han test I and II roofdissipates through the cover rocks; hydrocarbon molecules lithology becomes fine sandstone and the gas content isin the cap-rocks and reservoir diffuse because of concen-less than 6 m'/t, and even in the Ji test II the roof lithologytration gradient; dissolved-gas is directly taken away bybecomes mid-grain sandstone and the gas content reduceswater(Fig. 1).to0- 1.9 m/r. In the Fushun coal mine, Liaoning Prov-ince the roof and floor are both mud and shale, so theCap-rocksgenerated gas amounts to 9- 21 m/t. However, in JingxiExpansibility forcecoal mine it is only 1-3 m'/t because of the sandstoneroof2.DesopionAdsorption gas-To the conventional oil and gas pool, there is the dis-Concentrationgradientplacement pressure gradient in the cap-rocks and reservoir,L Dissolved gas么Awhich is mainly formed owing to capillary interstice in therocks, and buoyancy force exists in the oil or gas. WhenTaking away by waterthe buoyancy force is over the capillary force, oil and gasdissipate across the cap-rocks. However, to CBM reservoirFig. 1. Three ptterms of CBM dssipating.formation, the gas is mainly adsorption-gas. When free1.1 Free gas dissipating across cap-rocksgas occurs, there are expansibility force and buoyancyforce, and the former is dominant. If the expansibilityIt is an indirect pattern that free gas in pores dissipatesforce of free gas is over the displacement pressure ofthrough the cover rocks. Free gas dissipating often causescap-rocks, the gas will dissipate; otherwise the gas will befluid pressure in pores to decrease, and sequentially theconserved. So gas dissipating across cap-rocks needs todesorption of CBM takes place to resupply for the poresconquer the capillary force. The difference between thetill a new dynamic balance is achieved. Different from theexpansibility force and the buoyancy force is that the for-conventional gas pool, both the source rocks and the res-mer affects not only the upper cap-rocks but also the lowerervoir of CBM are the coal seams. Besides the cover rocks,interlayer under coal seams. This is the distinct differencethe lower interlayer is also very important to close CBM.between CBM and the conventional gas on conservationThe reason is that theoretically enough large CBM gasaffected by cap-rocks. Here, a new conception “advan-content requires coal seams to be obturated in a closedtage-broken”is defined. It means that in a closed system,system. If the cover rocks are closed and the lower inter-if there are different breaking pressures between thlayer is permeable, the generated gas in coal seams willcap-rocks and the lower interlayer, with the increase ofdiffuse across the lower permeable strata and CBM dissi-CBM desorption amount the pore free- gas generates ex-pating will take place, which will affect the gas adsorptionpansibility force that simultaneously acts on the upperamount, and vice versa (Fig. 2). For example, in the southstrata and the lower interlayer, and when the expansibilityof Qinshui Basin, because of the different roof and floorforce is over the displacement pressure of the cap-rocks,lithology, the gas content varied distinctly. With the statis-the free gas will break the cap-rocks and dissipate, mainlytic and analysis of gas content of No.3 coal in Qinshuiinto the cap-rocks with lower breaking pressure, till theBasin where the burial depth is similar but the roof andsystem achieves a dynamic balance. The least displace-floor lithology is different, it is observed that gas contentment pressure between the cap-rocks and the lower inter-is over 18 m'/t when the roof and floor lithology is mud;layer is the critical value.however, it is only 2- 8 m'/t in the sandstone roof andPexpasibily = PoQfm [Vg,(1)floor lithology. So the difference is distinct'4. In the Da-ning-Jixian area of Ordos Basin, the Han test I roof and where Po is standard atmosphere, free is free gas amount,C ap rocksCap rockssPemieable rocksHOCHCoal bedCH4CH Coal bedInterlayerPerinicable rocks中国煤化工uuindHCNMH GFig. 2. Concentration of CBM affected by different cap-rocks. (a) Perfect roof unu rloor - une Imost avantageous counservation condition;(b) osmotic lower interlayer- disadvantagcous conservation condition; (c) osmotic upper stratum- disadvantageous conservation condi-tion.Chinese Science Bulltin Vol. 50 Supp. December 2005135ARTICLES .and Vg is pores volume in unit coal matrix.v_K.?P(3)μ' 8ZL'.2 Dissipating by molecules diffusionMolecules diffusion occurs all the time, which consistswhere V is fluid flow speed (m/s), K is coal permeabilitywith the conventional gas dissipation theory. Because of(m2), and u is fluid viscosity (Pa:s).dissolved-gas occurring in CBM, hydrocarbon concentra-tion gradient is formed, and then molecules diffusion takes2 Geological models of CBM dissipatingplace. Hydrocarbon dissolved in water diffuses across theIn spite of the cap-rocks or the hydrodynamic condition,cap-rocks from the high concentration gradient to the low,so long as the best adsorption amount or gas content ofwhich accords with Fick's Diffusion Low.coal seams can be achieved, it will be available to thedQ.=D(2)CBM conservation. Based on the understanding of CBMlt dx’dissipating mechanism, dissipating models in differentwhere dQ/dt is dissipating speed of natural gas (m'/s), D isgeological settings are discussed. The models are built todissipating coefficient, dcldx is concentration gradient ofevaluate the CBM formation and conservation condition,natural gas (m'/m' .m).quantificationally or half-quantificationally. Because themechanism study on CBM formation and conservation1.3 Dissolved-gas taken away directly by watercondition is not enough, the built models of CBM dissi-This pattern mainly takes place when there is outsidepating may have some limitation and need to be developedwater permeation, which can be the adjacent rock waterconstantly in the later practice and research. The basicpermeation or the atmosphere water afflux along coalpoint to build the models focuses on the main factors ofseams, and the fluid flows in coal seams. According toCBM conservation condition. Macroscopically, the upperQin Shengfei's studyo.7l, the hydrological condition obvi-strata and the lower interlayer of CBM reservoir formationously affects CBM, which is commendably proved by theare the first considered factors. If there were no upperisotope composition of CBM. Compared with conven-strata and lower interlayer with closing ability, the closingtional gas pool, the isotope of CBM generated from theability of CBM reservoir formation could not be studied.coal source rocks with the same maturity is obviouslyOn the other hand, hydro-geological condition affectslighter. The main reason is that water is a polar solvent,evidently CBM dissipating. Due to different tectonic set-and the polarity of "3CH4 is over the "'CH4's, so watertings, there are different hydrological region identifica-dissolves "'CH4 more easily and takes it away, and conse-tions to coal seams. Because the underground water isquently adsorption gas concentrates more "CH2. Inactive and its salinity is low, the supplying water zone andother words, it is another approach that dissolved-gas isthe strong runoff condition form a comparatively closedtaken away directly by water. Dissipating speed is associ-system. So CBM dissipating becomes easier there. In theated with gas solubility in water and fluid flow speed.slight runoff area and retention-water condition, the un-Many examples prove that compared with the strong run-derground water is not active and its salinity is high, sooff condition the retention-water condition is more favor-there is advantage for CBM conservation in the closedable to CBM conservation. For example, in Kaiping syn-system.cline, the underground water supplies from the northwestAccording to the Dissipating Theory and contollingflank and drains to the southeast flank. The reten-factors of CBM conservation and based on the formertion-water condition forms in the syncline axis near thestudies on CBM conservation condition8), three geo-northwest flank, and the gas content is 8.76- -9.63 m'/t.logical models on CBM dissipating are built: closed sys-However, in the southeast flank, the runoff is compara-tem model, lateral hydrological closed model and opentively strong, and CBM gas content decreases obviously tosystem model.4.38- 6.57 m/t. Similarly, in Daning Jixian area of Ordos2.1 Closed system model of CBM dissipatingBasin, south of Qinshui Basin, there is also high gas .con-This model is mainly applied in the syncline axis. Intent in the retention-water condition and slight runoffl.l).Because the capacity, figure and curvature of coalthis area, the hydrological region identification belongs tocracks are quite complex, this status should be dealt withthe retention-water condition, the fluid is not connectedparallel flow permeation in hydrodynamic action. Free gaswith outside environment, and there are high formationdissipating accords with Darcy Low and fluid flow speedfluid salinity (over 3000 mg/L in Qinshui Basin) and lowis in direct proportion to the pressure gradient.metha中国煤化工TYHCNMHG1) See footnote 1) on page 134.2) Liu Honglin et al., Enrichment and accumulation of coalbed methane, Collected papers on basic studies on formation mechanism of coalbed gasreservoir and economic exploitation, 2004.136Chinese Science Bulltin Vol. 50 Supp. December 2005ARTICLES ,Comparatively the closed geological condition isThe mathematical equation relates to hydrocarbonmostly considered in this model. In the retention-waterconcentration gradient which is equal to the methanecondition, CBM is not taken away by fluid and CBM dis-solubility in water and is associated with the temperature,sipates mainly across the upper cap-rocks and the lowerpressure and salinity. To regress the mathematical equa-interlayer. So the upper cap-rocks and the lower inter- tion, the solubility in water of Chinese natural gases oflayerare considered as a key factor and control CBMconservation condition. Therefore, if the cap-rocks and thedifferent geneses under different temperature, pressure andlower interlayer are well closed, the accumulation andsalinity conditions can be used, which is calculated byconcentration of CBM are often promising, and the en-Hao Shishenglo:richment occurs in the axis, e.g. Qinshui Basin and theS(T, M, P)= -3.1670x10-0r2.M+1.1997x10-T.M+ 1.0635x10-l07?.M- 9.7764x10-P.M+2.9745x10-I0T.P.M+1.6230syncline axis in Xishan coalfield".x10 4r.M 2.7879x1027- 2.0587x10 sp2+1.7323x10~P+9.5233x10*TP+1.1937,(5)where S is gas solubility in water(m'/m'), T is tempera-Retention-water arcature(C), P is pressurex10MPa), and M is salinity ofHigh fluid salinityformation fluid(mg/L).Low methane solubilityFree gas dissipating: According to the dynamic balancetheory of free gas dissipating, the least displacementpressure between the upper cap-rocks and the lower inter-layer is supposed to be P. If Paxasilty>Po, free gas willCBM reservoirdissipate across the upper cap-rocks; if Pexpasbiliy =Pd, freegas will be conserved, and then the reservoir pressureshould beP=oyotatP+Fp/ryosatio.Porfre /Vg. (6)Fig. 3. Sketch map of geological closed system model of CBM dissi-The adsorption amount is associated with temperaturepating.and pressure, and increases with pressure increasing.V= VP/(P1+P),(7)Basal theory: CBM is mainly adsorbed on the interfacewhere V is adsorption volume, P is pressure, Pi is Lang-of coal cracks. In appropriate temperature and pressuremuir pressure (the adsorption volume is 50% of the largestcondition, the adsorption amount achieves a maximum.in this pressure), VI is Langmuir volume (it shows theBecause the later temperature and pressure conditionlargest adsorption capacity and it is independent of tem-changes, the originally adsorbed gas partly starts desorp-perature and pressure, controlled by coal composition andtion and occurs in the coal crack in the states of free gasconfiguration).and water-dissolved gas. For the fluid is in retention-waterThus it can be seen that both free gas dissipating andcondition, it does not altermate with the outside water.concentration diffusing are associated with the quality andWith CBM adsorption amount increasing, methane solu-character of cap-rocks. The larger displacement pressurebility reaches the maximum. Owing to the hydrocarbonavails to attain the higher CBM gas content, bccause theconcentration gradient between the reservoir and cap-larger displacement pressure restrains desorption as gasrocks, gas constantly dissipates across the upper cap-rocksexpansibility force increases with desorption volume. Soor the lower interlayer, and gas dissolving in water is thethe larger adsorption amount is conserved.main dissipating approach. However, free gas dissipatesThe“volume”" is introduced here. If the desorptionmainly by way of conquering the capillary force.volume is over diffusing volume, gas saturation will beConcentration diffusing: It is the concentration gradientover 100%. Else the saturation will decrease. The diffus-in the same medium or different medium that inducesing volume is controlled by quality and character ofmolecules diffusing. Supposing the cap-rocks are aquifercap-rocks and the thickness.and gas dissipates by way of dissolving in water', wGeological signification of the model includes: It canhavebe used to estimate CBM enrichment. With numericalQ=2-1 r D(z). G(z)-C.(3)..A.o(z).dz， (4)analysis using the geological model, if the cap-rocks do2-q .Lnot close well, the diffusing amount will be large. So it .where Q is natural gas dissipating volume(m), D is dif-does not avail to CBM enrichment. On the other hand, itfusing coefficient, C is gas concentration (m'/m*), A is may中国煤化. Thold of the effectiveacreage of gas pool(m), L is thickness of cap-rocks(m), 2|closedYH_nfirm the quality andis depth(m) at t, and z is depth(m) at t2.characlC N M H Ghess which satisfy the1) Sce footnote 2) on page 136.Chinese Science Blletin Vol. 50 Supp. December 2005137ARTICLESrequest of the least gas content of CBM formation.pansibility force = hydrostatic pressure + capillary force2.2 Lateral bydrological closed model of CBM dissipat-(the gas buoyancy force could be ignored for the quite lowpermeability of coal seams).ingPoQ/Vg= Pd+PwghThis model is mainly applied in the syncline flank or(there is a water-sealing critical value of free gas)，(8)the monocline with coal outcrop. In this area, hydrologicalregion identification belongs to the slight runoff area orwhere Q is free gas volume (m), Po is normal atmosphere,the retention-water condition, the fluid is partially con-Vg is hole volume (m), Ps is capillary force, and h is wa-nected with the outside environment, and there are mid-ter column height (m).dle-high formation fluid salinity (1000- 3000 mg/L inIn the desorption, if the desorption volume is over theQinshui Basin) and low methane solubility in water (Fig.dissipating amount, the CBM saturation will be over100%, or the CBM bound will extend. Else the saturationCoal seams are a half-open system and the flank onwill decrease, or the CBM bound will reduce.Geological signification of the model: Firstly it can beboundary such as fault limits the bedding fluid flow. Forused to estimate CBM enrichment degree. On the otherthe conservation condition, gas dissipating across thehand, it may be used to confirm the depth threshold ofcap-rocks and the lower interlayer should be consideredCBM, which is mainly deduced by the gas- water interfaceand the closed cap-rocks are very important, and the updipof the CBM reservoir formation and the updip aquiferoussealing of water is also a key factor. In this area, becausecoal seams. If the CBM enrichment degree is high, theof the lateral sealing of water and the downdip fluid re-bound will extend and the depth threshold of CBM willtaining，CBM can be enriched and formed, e.g. inFanzhuang area of Qinshui Basin, in 300- 1500 m burialbecome lower. However, if the dissipating amount is toodepth of No.3 coal seam, there is a high saturation ad-large, the gas-water interface will reduce towards theCBM reservoir formation and the depth threshold of CBMsorption zone and CBM formation is enriched.Basal theory: On one hand, the desorption gas dissi-will become deeper.pates across the cap-rocks and the lower interlayer. On the2.3 Open system model of CBM dissipatingother hand, for the coal outcrop is in the supplying-waterThis model is mainly applied in the margin of synclinezone and belongs the coal-weathering zone, the filter-and monocline, where there are coal outcrops or whiching-water seals the downdip gas and conduces to the dy-belongs to the coal weathering zone. In this area, hydro-namic balance. With the desorption volume increasing,logical region identification belongs to the supplying wa-free gas increases, and the dynamic balance is broken orter zone or strong runoff condition, the fluid is active, andthe superfluous free gas breaks the interface of dynamicthere are low formation fluid salinity (less than over 3000balance and diffuses. So the CBM bound extends or gasmg/L in Qinshui Basin) and low methane solubility indissipates across coal seams. As a result, it is consideredwater (Fig. 5).as a part of the total dissipating volume.In this model, coal seams are in the open system andDissipating across coal seams: According to the dy-are connected with the outside fluid with the strong runoff,namic balance theory, there is gas buoyancy force + ex-and the conservation is highly associated with the hySlight runof area or retentionwater conditionHigh fluid salinityWaler supplying arcaLow methane solubility”①+②.业③+④中国煤化工MHCNMHGFig. 4. Sketch map of geological lateral hydrological closed system model of CBM dsspating.①Hydrostatic force;②cpilary force;③gasbuoyancy force;④gas expansibility force.138Chinese Science Bulletin Vol. 50 Supp. December 2005ARTICLESWater supplying arca or strong runoff areaWalerLow fluid salinitypermeationHigh methanc soubility↓↓7:个Drpihing? water业Coal bedFig. 5. Sketch map of geological open system model of CBM dssipating.gdro- geological condition. In this area, for the disadvantahydrological closed model is controlled by the roof andgeous CBM conservation condition, it does not avail tofloor and the updip sealing of water, and CBM enrichmentthe CBM formation, e.g. in Fanzhuang area of Qinshuiand formation need the good obturating condition andBasin, it belongs to the coal weathering zone in upper 300sealing of water. To open system model, the large dissi-m burial depth and the fluid is active. So it induces thepating amount does not avail to high gas content forma~large gas dissipating volume and low gas content.tion, and such an area is not the promising target.Basal theory: For the coal outcrops and the low burial(3) CBM conservation condition relates to many factors,depth, CBM formation is affected distinctly by under-so the analysis of CBM dissipating mechanism should beground water. Influenced by total head gradient, CBMmore comprehensive.transports from the high total head area to the low andAcknowledgements This paper was supported by the National“973”dissipates. Due to the active fluid and the low salinity, theBasal Study of Chinese CBM Formation Mechanism and Economicaldesorption gas is momentarily taken away.Development (Grant No. 2002B2110).To this model, if the diffusing volume is less than theReferencesdesorption volume, gas saturation will be high. Else the1. Zhang Jianbo, Wang Hongyan, Zhao Qingbo, CBM Geology insaturation will decrease. 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The dissipating of adsorption gas is indirect,tion and fractionation mechanism of stable carbon isotope of coaland takes place in the dynamic balance condition of thebed methane, Science in China, Ser. D, 2006 (in press).8. Li Mingcheng, Migration of oil and natural gas, Beijing: Petroleumthree gas states.Industry Press, 198(2) In the three built geological models of CBM diss-9中国煤化工-al, Formation Condition of_cience Press, 2000.pating, the closed system model is the most advantageous10._ler of Natural Gas Solubil-one to CBM conservation. The coal seams with the ap-tyYHC N M H G Geologial Sigicatioe,npropriate roof and floor, lying in the syncline axis, avail toActa Petrolei Sinica, 1993, 14(2): 18- 20.CBM enrichment. The conservation condition of lateral(Received March 15, 2005; aceped July 25, 2005)Chinese Science Bulletin Vol. 50 Supp. December 2005139