Self-adjusted elastic action and its CBM pool-forming effect of the high rank coal reservoir

ARTICLESChinese Science Bulletin 2005 Vol 50 Supp 99-103Before being broken under stress, coal is a kind of elas-Self-adjusted elastic actiontic body with a rather compressible or swellable capacityTaking as a kind of the organic reservoir mainly composeand its CBM pool-formingof organic matters, the response of the coal reservoir tostress is more sensitive than some routine ones such aseffect of the high rank coalsandstone and carbonate rock reservoirs. So the directdynamic factors determining the opening or closing extentreservoirof the natural fissures in the coal reservoir include twoaspects. The first is the effective stress, which is expressedIN Yong FU Xuehai, WU Caifang FU Guoyouas the pressure difference of the overall stress, being perBU Yingyingpendicular to the fissure strike, to the fluid pressure inpores and fissures within the coal reservoir, and with inInstitute of Mineral Resources and Geoscience, China University ofcreasing the effective stress the fissures are compressedMining Technology, Xuzhou 221008, Chinaas to become narrow or even close. The second is theCorrespondence should be addressed to Qin Yong(email: yongqinumtedu.cnswelling-shrinking effect of the coal matrix blockrounded by natural fissures. The fissures becomes narrowAbstract The coal reservoir is a kind of geologic body due to the swelling effect derived from the absorbingwith considerable elasticity, and its response to stress is more while the desorption of gas makes the coal matrixsensitive than the routine ones. In the pool-forming process shrunk so that the fissures become wide. However, theof the coalbed methane, (CBM) the action of many externalgeodynamic factors in the coal reservoir is represented by the clanges of effective stress as well as the adsorp-ability of CBM's diffusion and migration in the coal reservoir. tion/desorption process are influenced by some dynamicTherefore the occurrence of the natural fissures in the ceconditions such as the geological structure, groundwaterreservoir becomes a key that determines whether the CBM and geothermal field. These conditions are put on the coalpool can be formed or not. Based on the principle, the au- reservoir itself, making the coal matrix block elasticallythors have designed a comprehensive method of physical deformedmodeling to study the elasticity of the coal matrix block withThe difference of the lithologic character and chemicaldifferent coal ranks in the light of the solid to fluid coupling, structure of coal leads to a variety of the swelling-shrink-have established the model of the volume deformation(self-adjusted elastic effect) of the coal reservoir under the condiing property under stress. Some previous authors studiedtion of the effective stress-adsorption/desorption, and have the relationship of the effective stress and the permeabilityobtained a correct understanding of the law of the elastic of the coal reservoir or the adsorbing-desorbing effect andthe strain of the coal matrix block. For example, Some-self-closing elastic effect for forming the CBM pool in the ton found that the permeability decreased with increashigh rank reservoir, is put forward, providing an initial point ing the effective stress in an exponential form, and Yee etand a scientifie basis for a further investigation into itsal(1993) once used a method similar to the Langmuir'snamic factors, possible mechanism and role in the CBM en- Equation to describe the adsorption strain of the coal ma-gy-balancing syKeywords: coalbed methane, reservoir, elasticity, self-adjusted action, sive action between the effective stress and adsorbingo-forming effect.desorbing effect and the elastic deformation of coal hasDOI:10.136098zk0017never been referred to up to now due to the testing difficulty. Obviously, the opening-closing extent of the fissuresAlthough the CBM pool-forming process is controlled in coal, which is controlled by the effective stress and ad-by many geodynamic factors, its key depends on the rela- sorption swelling, may be different even under the sametively closed geological environment derived from a fastratum condition. Only the comprehensive effect of bothvorable match among these factors, in which the tectonic the factors can just determine the relative opening orstress field, hydrodynamic field of groundwater, geother- closing characteristics of fissuremal field and strata contribute to the formation of such Aiming at the problems mentioned above, the authors,environment However. these factors first act on the coal in the light of the solid-fluid coupling, have designedreservoir itself, which is represented by the diffusing and comprehensive method of physical modeling to investi-migrating ability of CBM within the coal reservoir. Coal gate the elasticity of the high-rank coal matrix blocks soelf is quite unfavorable for CBM diffuestablish an elastic volume deformation model of themmm凵中国煤化工etesures during the geological history, becomes a key factor law ofCN Provides a foundationthat determines whether the CBm pool can be formed or for studying turther the etfect ot the elastic energy to thenotCBM pool-forming processChinese Science Bulletin Vol 50 Supp. December 2005ARTICLES1 Physical modelingmeasured point for 12 h1. 1 Samples and metho1.2 Modeled resultsThe comprehensive experiment of the physical modelThe detailed data of the modeling can be found in theing for the elastic deformation of the coal matrix block Ph D dissertation by Fu"and Chen"as followswas performed in the Central Fracturing Laboratory,The previous researches show that the adsorption strainLangfang Branch, China Petroleum Exploration and De- of methane can be accurately fitted into the Langmuir'svelopment Institute. The testing device was the rock me- isothermal adsorption model 4. And based on the model,chanics test system made by Terra Tek Company, USA. the Langmuir's adsorption swelling equation can be obThe system, supported by a whole set of the sample-pre- tained through calculating the theoretical maximum strainparing as well as analysis devices, was used to model the emax and pressure P5o that means the pressure when themechanical behavior of rocks under the conditions of tem- strain of the coal matrix block reaches half of its maxiperature, pressure and ground stress of oil pool, with the mum. The volume compressive coefficient and volumeanges of the parameters as follows: the vertical stress modulus under a higher surrounding pressure can be de-from0 to 800 MPa, horizontal stress from 0 to 140 MPa, duced based on the extrapolation from the measured volfluid pressure in pores form 0 to 100 MPa and strata tem- ume compressive coefficients. Therefore, the relationshipperature form 0 to 200C. The data were automatically of the elastic deformation of coal samples respectively tocollected by a computer, and all sensors were demarcated the surrounding pressure, effective stress and coal rank isand corrected by the National Standard Bureau of China acquired(Figs. 1-3)each year.It can be seen from Fig. 1 to Fig. 3 that, the volumAll the coal samples were collected from the coal compressive coefficients of either the natural coal samplesseams in the Lower Permian Shanxi formation at the fresh or water-saturated ones become smaller with increasingworking face of the producing mines in the Qinshui basin, the surrounding pressure(Fig. 1). If the surrounding pres-Shanxi, China. The maximum vitrinite reflectance(Ro.masure is constant, the volume strain increases in the Langranged from 1.89%to 2.87%, which covered the meagre muir curve form as the fluid pressure increases, till it tendscoal, lean coal and anthracite ( Table 1)to become stable(Fig. 2). The maximum strain decreaseThe whole testing flow as well as the sample prepara- with increasing the coal rank(Fig. 3). The results abovetion and processing precision followed strictly the stan- mentioned is identical to the those obtained by Thomasdard recommended by International Society for Rock and Damberger b, Reucroft and Patel "and Levin 8Mechanics in 19814. The samples were immersed into aThe results abovementioned also show that if the for5% KCl solution, then were vacuumized to eliminate the mation pressure keeps constant, the elastic deformation ofgaseous medium and equilibrated to the saturated water coal is strengthened as the fluid pressure increases or thefor 24 to 48 h. During the experiment process, the meth- effective stress decreases. And if the pressure and stressane gas with a purity of 99.99% was filled into the satu- keep the same, the elastic deformation of coal lowersrated water in the case keeping the effective stress con- the rigidity of coal enhances as the coal rank increasesstant. Then the longitudinal, radial swelling and volumeswelling amounts after adsorbing the methane for each 2 Self-adjusted elastic effect model of coal reservoirsample were measured respectively under the methaneDuring the CBM pool-forming process, the coal matrixpressures of 0.5, 1.0, 2.0, 3.0, 4.0 and 4.5 MPa and the block itself will swell due to adsorbing methane, whichcorresponding surrounding pressures of 1.5, 2.0, 3.0, 4.0, compresses the natural fissures in the coal reservoir so that5.0, and 5.5 MPa. The condition was stabilized at each the fissures are relatively closed. The CBM adsorption isTable 1 Geological setting and basic properties of coal samplessetting Wuyang, Luan Changchun, Luan Wangyun, Gaoping Baiseng, Shouyang Shigang, Zuoquang Chengzhuang, Jinchengmple No3Coal seam2.17H中国煤化工1)Fu Xuehai, Physical Experiment of Multiphase Medium Coal Block andCNMHGrmeability, PhD Thesis: ChinaUniversity of Mining and Technology, 2001, 21-462)Chen Jingang, Control and Its Mechanism of Structure and Gas Recovery to Permeability of High Rank Coal Reservoirs, PhD Thesis: ChinaUniversity of Mining and Technology, 2002, 75-81Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLES181235P /MPaP /MPaFig. 1. Plots of volume compressive coefficient(Cve)to surrounding pressure(Ps) for modeled coal samples. a)Raw coal, No 1-2: b)water-saturated coal, No 1-4; c)raw coal, No 2-1; d) water-saturated coal, No 2-2crease under the same tectonic stress, leading to the relative opening of fissures. The two effects are derived from100the responses of the elastic energy in the coal reservoir tovarious geological inducements, and are embodied intothe self-adjusted elastic effect of the coal reservoir. In thispaper, the former is called as the negative effect of self-adjusted elasticity while the latter is named as the positiveeffect of self-adjusted elasticity. Under the formation condition, the variation of the opening-closing extent of thenatural fissures in the coal reservoir is just resulted diP/MParectly in the comprehensive action of the two effects, i.ethe so-called self-adjusted comprehensive effect of thelg. 2. Plots of volume strain(e)to fluid pressure(PL) for modeled coal reservoir.coal sample(No. 18)The model of the self-adjusted effect of the coalvoir is established through probing into the mutual relations between both positive and negative effects ofself-adjusted elasticity and the regularity of the compre-hensive effect. It has the obvious scientific significancefor studying innovatively the CBM pool-forming dynamics to investigate the relation of the self-adjusted effect tosome factors such as the coal reservoir pressure and coal02002302602903.20model ") of fissure for reference. theRom(%)elf-adjustedeffect of the coal reservoir. which isrepresented by the permeability variation resulting from3. Plots of maximum volume strain(emax)to maximum vitri the change of fluid pressure, can be calculated as followctance(Roma) for modeled coal samples.directly resulted in the increase of fluid pressure, whichmakes the effective stress acting on the coal reservoir de中国煤化工-1×0%CNMHG1)FU Xuehai, Physical Experiment of Multiphase Medium Coal Block and Mathematical Simulation for Stress-Permeability, PhD Thesis: ChinaUniversity of Mining and Technology, 2001, 86-89Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLESwhere AK is the change of the permeability induced by the ervoir. Furthermore, the higher theself-adjusted elastic effect, %; K; and K; are respectively weaker the elastic deformation of thereservoir Isthe coal reservoir permeability before and after the fluid during the process of the CBM adsorption or desorptionpressure is altered, x10 um; Ae and Aeki are respectively In particular, the higher the coal rank is, the more diffi-the compressively deformed and shrunk amounts of the cultly the natural fissure in the coal reservoir is pulledcoal reservoir caused by the effective stress when the fluid apart when the CBm desorption takes place, and the muchpressure reduces from p: to Pi; Pi is the pore-fissure ratio possibly the condition for preserving CBM tends to beof coal reservoir under the state of picome betteruperimposing both self-adjusted positive and negativeiii)The variation of the self-adjusted comprehensiveelastic effects based on the physical modeling abovemen- effect with the coal rank is strictly controlled by the evo-tioned, the self-adjusted comprehensive elastic effect and lution of the coal structure. When the fluid pressure in theits model could be gained(Figs. 4 and 5), and some basic experiment is about 5 MPa, the comprehensive effect willunderstandings are suggested as follows.change soon from a positive value to a negative valuethe mavitrinite is over 2.1%(Fig. 5). TheRm=287%▲Rm、=2.17%xR。ms=1.89%°Rm=230%authors in this paper call the point of the coal rank corre-sponding to the conversion as the"balancing point "of the2.24%self-adjusted comprehensive elastic effect of the coal reservoir. The balancing point is not only the line dividingthe meso-rank coal with the high-rank coal, but also thejump point of the third coalification ". Near the balancingpoint, the physical and chemical structures of coal havebefallen a stepped change. For example, the maximum1.01.5202.53.03.5404.55.05.5reflectance of cutinite exceeds that of vitrinite. and thePL/MPachange of the moisture content with carbon or oxygencontent in coal reduces to a minimum . and the diameter ofFig. 4. Relation and model of self-adjusted comprehensive elasticeffect (Ak) to fluid pressure(PL)he basic structural unit of the macromolecule in coal becomes abruptly long, which identifies that the coalifica-tion evolves from the meso-rank period into the high-rank59-4.7MPa▲4.7-3.7MPaFurthermore, the maximum vitrinite reflectance2.7~1.7MPa·3.7-2.7MPaat the balancing point moves towards the higher coal ranka17-0.7MPwith reducing the fluid pressure. However, no matter whatthe move will be. the maximum vitrinite reflectance at thebalancing point will never be over 2.6%.3 Implication to studying CBM pool-forming effectof high rank coalThe geological data of the CBM in region shows that,lo matter what difference in the geological structure,Fig. 5. Relation and model of self-adjusted comprehensive elastic hydrogeology and sedimentation will be, the CBm contenteffect(AK) to coal rank(Romay)is rather high and the CBM weathering zone is respec-() The comprehensive effect is gradually weakened China"4 such as the southeastern Shanxi,northwestemith increasing the fluid pressure in pore and fissure if the Henan, central Hunan and northwestem Guizhou, Thecoal rank keeps unchanged(Fig. 4). When the fluid pres- phenomena implies that the geological conditions favorbelow about 3 MPa, the comprehensive effect de- able to the preservation of CBM occur possibly in the coals-reases quickly with increasing the fluid pressure. How- reservoir itself that has evolved to the higher rank stageer, it decreases slowly when the fluid pressure is over 3 through the favorable matching among various dynamicMPa. In other words, the elastic deformation amount of conditionsthe coal reservoir is relatively high when the fluid pressure It is considered in the traditional theory of coalificationis low, and vice versathat why the high-rank coal reservoirs of China in CBM(ii) The self-adjusted comprehensive elastic effect is conternegatively correlated with the coal rank if the fluid pres- high中国煤化工 micropores and theure keeps unchanged(Fig. 5), which indicates that the normaC Wever, the preservationcoal rank controlled by both tectonic and geothermal dy- of CBM requests primarily enough coal reservoir pressurenamics is a key to the elastic deformation of the coal res- If the pressure lowers remarkably due to the tectonicChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESmovement or groundwater washing, no matter how hiReferencesthe adsorption potential is and how well the micropore is 1. Somerton, w. H, Effect of stress on permeability of coal, Int Jbecause of the desorption and dissipation. Constantly asRock. Ech. min.Sci,1975(12):129-145the four above-mentioned areas with high rank coals ar2. Levine, J.R, Model study of the influence of matrix shrinkageCBM content concern, there exists a great difference inabsolute permeability of coalbed reservoirs, Geological Societythe geological condition such as structure, hydrogeolog.Publication,1996,199:197-212.and sedimentation, either the shallowly buried coal reser- 3. George, J.D.St, Barakat, M A The change in effective stress as-voirs(such as in the southeastern Shanxi), or the very desociated with shrinkageveloped opening faults and strongly active groundwaterJournal of Coal Geology, 2001, 45(2): 105-113(such as in the northwestern Henan)or many independent 4. ISRM, Suggest methods for determining water content, porosity,small synclines formed by violent structural destructiondensity, absorption and related properties and swelling slake dura-(such as in the central Hunan and in the northwestembility index in rock characterization, testinGuizhou). All these show that the external geological con-Revision), in Commission on Standardization of Laboratory anddition is not the only control of the CBM enrichmentField Tests, Oxford: Pergamon Press, 1981, 1-16The coal seam is a geological body with a relatively 5. Ahrpalani, S, Chen,, G, Gas slippage and matrix shrinkage effectsgreat elastoplasticity. The self-adjusted effect model of thecoal reservoir reveals that the mechanical property(espe-n coalbed permeability, in Procedings of the 1993 Intemationalcially the self-adjusted elastic effect) of coal is strictlyCoalbed Methane Symposium(ed. Hood, A. D ) Berminhamcontrolled by the coal rank under the precondition that theCaolbed Methane Association of Alabama, 1993, 257-261coal lithology and the pressure of the CBM containing 6. Thomas, J, Damberger, HH, Intermal surface area, moisture con-system are the same. The self-adjusted result of the elastent and porosity of lllinois coals with rank, Illinois State Geologyticity displays a comprehensively positive effect for theSurvey Circular, 1976, 493meso-rank coal reservoir but a comprehensively negative 7. Reucroft, P J, Patel, H, Gas-induced swelling in coal, Fuel, 1986,effect for the high-rank one, which leads to a depressionof the diversion capacity of the fissures in the high rank 8. Levine, J.R., Coalification: The evolution of coal as source rockcoal reservoir. The much greatly the reservoir pressureand reservoir rock for oil and gas, in Hydrocarbons from Coal (edsdecreases, the more significant the self-adjusted negativeLaw, B. E, Rice, D. D), Menasha: American Association of Petro-elastic effect of the high rank coal reservoir will beleum Geologist, 1993. 39Therefore, the authors in this paper have put forward a 9. Stach, E. Mackowsky, M. Th, Teichmuller, M. et al, Stach'snew academic viewpoint named as the self-closing elasticeffect for forming the CBM pool in the high rank reservoir,Textbook of Coal Petrology, 3rd ed, Berlin: Gebruder Borntraeger,and deem that the effect is an important geological factor1982,38-86by which the CBM is widely enriched in the high rank10. Qin, Y. Micropetrology and Structural Evolution of High Rankcoal reservoirs in China. The self-closing elastic effectCoal in China, Xuzhou: China University of Mining and Technol-counteracts to a large extent the destructive effect of theogy Press,1994,55-76other unfavorable geological conditions on the CBM poI1. Qin, Y, Jiang, B, Coalification jumps, stages and mechanism offorming so that the CBM could be effectively preservedhigh-rank coals in China, in Proc 30th Int Geol Congr, 18B (edunder some unfavorable conditions such as the structureYang, Q), the Netherland: Int. Sci. Perss, 1997, 99-122hydrogeology and sedimentation. The authors also hold 12. Qin, Y, Jiang, B, Jumpy evolution of EPR responese of Chinesethat the abnormal paleo geothermal field with higher tem-high-rank coals and its implication to geochemistry, Scienceperature gradient is the main dynamic factor why theChina,Ser.D.1997,27(6):499-502.self-closing elastic effect can be produced, and its special 13. Qin, Y Fu, X.H. Ye, P. et al Geological controls and theirmechanical property of the coal controlled by the chemical structure is the internal reason. The understandingmechanisms of coal reservoir petrography and physics of coalbedmethane occurrence in China, Journal of China University of Min-might provide an initial point and scientific basis for afurther investigation into the dynamic factors, possibleing Technology, 1999, 28(1): 14-19mechanism and role in the cbm energy-balancing system14. Ye, J. P, Qin, Y,, Lin, D. Y, Coalbed Methane Resources of China,Xuzhou: China University of Mining and Technology Press, 1999Acknowledgements This work was supported by the China National973"Project( Grant No. 2002CB211704)and Key Program of NationalNatural Science Foundation of China( Grant No. 50134040)中国煤化工:005; accepted July 18, 2005)CNMHGChinese Science Bulletin Vol 50 Supp. December 2005103