The characteristic curve of methane adsorbed on coal and its role in the coalbed methane storage res

ARTICLESChinese Science Bulletin 2005 Vol 50 Supp 86-92namically varying parameters. Combining the coal seamThe characteristic curve ofburied history, geotemperature variation course, coalmetamorphosed process and gas generation process,methane adsorbed on coal and to model the gas storage history of a coal seam at ditterentits role in the coalbed methane sent reservoir characteristic, is a ditticult problem in theCBM gas storage mechanism research. In addition, in thestorage researchCBM resource prediction process, the adsorption data ofcoal at different temperatures are often needed to calculateCUI Yongjun, LI Yuhui, ZHANG Qunthe methane content at different buried depths. When preJIANG Wenpingdicting the gas content in deep coal seams, it becomesmore important to understand the synthetic effect of tem-Xi'an Branch of China Coal Research Institute, Xi'an 710054. Chinaperature and pressure on the adsorption capacityCorrespondence should be addressed to Cui Yongjun(email: yongjun@tom. com)he method usually used to understand the rules of thesynthetic effect of temperature and pressure on the adAbstract Based on the adsorption potential theory, this sorption capacity is to perform the isothermal adsorptionpaper processes and analyzes the isothermal adsorption data test at different temperatures. Because there is no equationat different temperatures of four coal samples with different that can express the relationship of adsorption capacity,ranks. The research results indicate that the adsorption temperature and pressure, usually the effects of tempera-characteristic curve of methane adsorbed on the coal sample ture and pressure on the adsorption capacity are discussedmethane adsorption capacity, temperature and pressure is adsorption capacity in his literature, but the calculationachieved from the characteristic curve. According to thecalculating routine and method presented, the adsorptionmethod reported was still an empirical equation. Based oncapacity at other equilibrium conditions can be calculated the adsorption test results at several temperatures, Zhongfrom the adsorption data of one temperature. The results of Lingwen et al. 2I discovered that the relationship of tem-this paper are a significant contribution to the research on perature and adsorption capacity under different pressuresthe storage mechanism of the coalbed methane.was linear. From this linear relationship, the adsorptionKeywords: coal, coalbed methane, adsorption, characteristic curve, capacity of coal seam at different buried depths was predicted and thus the gas content of deep coal seams wasDOI:10.136098zk0011obtained. Zhao Zhigen et al. also took the isothermaladsorption test results at different temperatures as the ba-1 Introductionsis to establish the expression between the Langmuirvolume(saturated adsorption capacity) and temperature,The coalbed methane( CBM) is stored predominantly then predicted the methane content in deep coal seamsin the form of an adsorbed layer on the intemal surface of During the research on the adsorption capacity in coalthe coal structure rather than as a free gas in the pore with different reservoir conditions, Scott indicated thatspace. Therefore, the research on the adsorption of gas on with the uplift of the strata, both the reservoir pressure andcoal has an important significance to understand the CBm temperature declined. He explained the reasons why CBMstorage mechanisms. In addition, the adsorption data of was unsaturated after the strata uplifting, but the syntheticcoal are also necessary parameters in other analysis such effect of temperature and pressure on the adsorption ca-as the CBM resource prediction and productivity evalua- pacity was only quantitatively discussed. Hu Taosugtion. These reasons have shown the importance and neces- gested that it might adopt the isosteric heat to predict thesity of studying the coal adsorption behavior. The iso- adsorption capacity at different temperatures when hethermal adsorption test is the testing method most com- studied the adsorption behavior of other solid-gas systemsmonly used for these studiesAlthough a better prediction result was achieved, tlThrough the normal isothermal adsorption test, only the prediction method needed the adsorption data of at leastrelationship between the adsorption capacity and pressure two temperaturesat the fixed temperature can be obtained. However, theHow to obtain the mathematical expression of theCBM geologists also want to know the synthetic effect of methane adsorption capacity, the temperature and pressurepressure and temperature on the coal adsorption capacity. based on one isothermal adsorption test result is an urgentIn the evolution process of coal seam, the buried depth of demat中国煤化工 this problem could bethe coal seam has changed continually over time, which resol-nt contribution to themeans that the reservoir temperature and pressure are dy- problCNMHorical course. resource1)Kim, A G, Estimating methane content of bituminous coalbeds from adsorption data, US Bureau of Mine Report of Investigations 8245, 1977Chinese Science Bulletin Vol. 50 Supp. December 2005ARTICLESevaluation, optimization of favorable block and reservoir pling information and coal property results are listed incharacteristic description, etc. The adsorption potential Table 1. An equilibrium moisture content measurement isheory would provide a route to resolve this problem Re- performed before the isothermal adsorption test is carriedsearches have proved that the behavior of methane ad- out on the samples, with the aim to recover the watersorption on coal belongs to the field of solid-gas physical bearing status of coal at reservoir conditions. The condi-adsorption. The interaction force of physical adsorption is tions of the equilibrium moisture content measurement aremainly the dispersive force, while the dispersive force is 30C, 96%-97% humidity with about 5 days for equilib-the only one force unrelated to temperature Therefore, rium. Then, at the temperatures of 20, 30'C, 40'C andthe characteristic curve calculated from the adsorption 50C the isothermal adsorption tests are conducted re-potential theory would help to achieve the mathematical spectively. The adsorption test adopts the static volumeexpression of adsorption capacity, pressure and tempera- method, and the instrument is made by the raven ridgeResources Corporation of USA. The adsorption isotherms2 Samples and testsof 4 samples are shown in Fig. 1. The curves display thatIn this paper, the coal samples are selected from four for each coal sample, at the same temperature, the adsorpdifferent regions, including Huainan gas coal, Hedong tion capacity increases with the pressure rising; while atcoking coal, Lu'an meager coal and Jincheng anthracite. the same pressure, the adsorption capacity decreases withEach coal sample represents a different rank. The sam- the temperature rising. This conforms to the exothermalTable 1 Sampling information and coal property resultCoal sampleSampling siteCoal seamCoal property analysis (%)Reflectance of vitrinite(%)Ad(%)Xinji, Huainan1910.34Coking coalMiaowan Liulin0.644.7220.261.32Meager coalQuzhuang, Tunliu1.06l.1113.897.091620100Pressure/MPaPressure/MPaon 32/ Meager coalAnthracite24306200rV凵中国煤化工CNMHGFig. 1. Isotherms of four coal samples at different temperatures.Chinese Science Bulletin Vol 50 Supp. December 2004ARTICLESprocess feature of physical adsorption. At the same tem- the adsorbed phase density value Pad of methane adoptsperature and pressure conditions, the adsorption capacity 0.375 g/cm, for the detailed discussion on the adsorbedincreases with the coal rank, e.g., anthracite meager coal phase density refer to ref [8]3 Method to obtain the adsorption characteristic Before using eq.(1)to calculate the e under the equi-coking coal gas coarium condition, the saturated vapor pressure parametercuryPo in the equation needs to be determined. If T< Tc, PoThe adsorption potential theory considers that in the Tc, it can only be calculated through the extrapolationadsorption space exits the adsorption potential e whichdecreases with distance from the adsorbent surface. TheGrom the empirical equation because above the criticaltemperature point the gas cannot be liquefied, and theadsorption potential e is defined as the isothermal work saturated vapor pressure does not existdone in transferring one mol of adsorptive molecule gas The method mostly used is the empirical equation sug-om the free gaseous state to a point above the adsorbent gested by Dubinin %n, which is used to calculate thesurface. The predominant interaction force of the physical saturated vapor pressure above the critical temperaturesorption is the dispersive force, while the dispersiveforce is the only one force unrelated to the temperatureFrom this understanding it can be inferred that for thePo=Pcsame adsorption system, the relationship between e andVad(adsorption phase volume)will not change with the where Pc is the gas critical pressure and Tc is the criticaltemperature. That is to say, the curve of &Vad for thetemperaturegiven adsorbent-adsorbate system should be a single curve. During the research on the adsorption characteristiccalled the characteristic curve". Many solid-gas physical Amankwah o) discovered that by using the modi-adsorption researches have proved that this assumption is fied-Dubinin equation the prediction results were better.tenable, e.g, adsorption of benzene and carbon dioxide on The calculation equation of the saturated vapor pressurethe activated carbon. The behavior of the methane adsorp- refers to eq. (4), in which k changes with the adsorptiontion on coal belongs to the physical adsorption, and its systeminteraction force is also mainly the dispersive force. Thus,the characteristic curve of the same coal adsorbing methane should be singlePo=PcTwo parameters are needed to obtain the characteristiccurve of one adsorption system, the adsorption potential eIn addition, some researchers suggest that if the adsorpand the adsorption phase volume Vad. The method to cal- tion temperature is above the critical temperature, itculate the e is that when the gas molecule is adsorbed by should not only predict the Po value, but also adopt thethe solid surface, the e is equivalent to the work needed to fugacity f and fo to replace the pressure P and Po respeccompress the gas from the equilibrium pressure P in the tively. Accounting for the practical situations of the CBMfree gas phase to the saturated vapor pressure at the ad- adsorption tests, it is more convenient to adopt P and Po insorption temperature T, that isthe calculation. This paper adopts the Amankwah,'s modRTfied-Dubinin equation to calculate Po, because it has beendP=[-dP=RTIn(1) found that &vad points of Tunliu meager coal and Qinshianthracite are scattered if Dubinin equation(or Amank-where E is the adsorption potential, vf is the gas volume wah s modified-Dubinin equation, when k=2)is used, andin the free gas phase, P is the equilibrium pressure, Tthe correlative coefficient of e-Vad curve is below 0.99 bythe equilibrium temperature, Po is the saturated vapor non-linear regression for these two coals (Table 2). Thepressure at temperature T and R is the gas constantcorrelative coefficient of the characteristic curve increasesAnother parameter Vad(adsorption phase volume)could with the k value increasing except for Huainan gas coalbe calculated by the following equationTable 2 k value and correlative coefficient of characteristic curveVxI2.00.99750.994809856pad2240×pau中国煤化工0.9903where Vad is the adsorption phase volume under the equi0.98980.9910librium condition, m is the mass of the adsorbed gas, V isCNMHG09900917the gas adsorption capacity under the equilibrium condi2.80.9919tion, and Pad is the adsorbed phase density. In this paper,3.009983Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLESThe result also shows that when k>2.7, the correlative samples are shown in Fig. 2. During the calculation, the kdiscussion is based on Po when k=2.7. Because the reser- each kind of coal at four temperatures almost fall o, orcoefficient of 4 samples is higher than 0.99, so the next value adopts 2.7 and Fig. 2 displays that eVad curvevoir temperature of the coal seam far exceeds the methane single curve. This indicates that the evad curve is irrelecritical temperature (826C), Po term does not have any vant to the temperature. It indicates further that the interphysical significance and is just a parameter to fit the action force between the coal surface and methane moleequation of the characteristic curve. As to the relation be- cule is a dispersive force, and the adsorption process is atween k value and the correlative coefficient, the current physical adsorption. According to the adsorption potentialresearch on the high pressure gas or the supercritical gas theory, when e=0 the space between the adsorption po-adsorption theory cannot give a suitable explanation at tential surface and adsorbent is the maximum adsorptionthis timespace, and it is equal to the saturated adsorption phase4 The characteristic curve of the methane adsorbed volume. according to the relationship of e and vad, whenon coal and its application valuee=0 the saturated adsorption phase volume is at itsBased on the method stated above the characteristic maximum, expressed by the circle in Fig. 2. To convertcurves of the methane adsorbed by four different coal Vad to the methane volume at standard conditions(0C,y=-39532nx)-12279y=-4997n(x}-14.3750.06002004dem3·gVat/cm.gy=-5.887ln(x}-12.71y=-4403n(x)-9.5948Anthracite00.0200406008▲20℃30℃◆40℃Fig. 2. Characteristic curve and expression equation of four coal samplesTable 3 Langmuir volume(Vo) and results of characteristic curveGas coalCoking coalMeager coalAnthraciteVL from Langmuir equation(cm/g)rango:25.47-27.95 range:28.34-29.06 range:40.04-45.27range:5493-5793average: 26.24average: 28.79average: 41.79average: 56.81Saturation capacity from CC(cm/g) 24中国煤化工E=-4.9997Expression equation of CCE=-39532h(V)oHe=-4.4033In(Vad12.279CNMHGCorrelation coefficient of CC(%)0.99740.99800.99100.9917a)CC means characteristic curveChinese Science Bulletin Vol 50 Supp. December 2005ARTICLES101.3 kPa), the saturation adsorption capacity is obtained the constants A and B. Then, Ti is used in eqs. (8)and(9)CTable 3). From Table 3 it can be seen that the saturation to calculate the constants A and B respectively. Next, toadsorption capacity calculated from the adsorption poten- calculate the adsorption capacity corresponding to thetial theory is very close to the results of the average satu- different pressures by using T2 in eq.(), the adsorptionrated adsorption capacity(VL) from the Langmuir equation. isotherm at temperature T2 is obtained. According to thisThe saturation adsorption capacity calculated from the method, only relying on one set of isothermal adsorptionadsorption potential is only one, while the vi obtained data, the adsorption capacity corresponding to any equi-from the Langmuir equation is slightly different with the librium conditions of temperature and pressure can betemperature changingtheoretically calculatedhe aim of drawing the characteristic curve of methaneIn this research, based on the adsorption test data atadsorbed on coal is not just to prove that the adsorptionmperature of 30C, the above method is used to predictbehavior is the physical adsorption or to obtain the satura- the adsorption isotherms at20℃,30℃,40℃cand5o℃tion adsorption capacity. It has very important application (Fig 3). from the figure it can be seen that the predictionvalue in the CBM exploration and development, For ex-curves and measured points are very close, except the preample, based on the adsorption data of a coal sample at a dicted curves: the gas coal sample at 40C and 50C, thecertain temperature, it is possible to predict the adsorptioncapacity at other different temperatures and pressures meager coal sample and anthracite sample at 20'C haveger deviations. If taking the absolute value of the dif-from the characteristic curve. In addition, the mathemati- ference between prediction value and testing value as thecal expression among three parameters, adsorption capacity, pressure and temperature, could be obtained. The route absolute error, and averaging the absolute error for eachto achieve the mathematical expression is described as sample at all pressures and temperatures, the sequence ofaverage absolute errors of 4 coal samples is: coking coalThe characteristic curves of various coal samples in Fig. 0.23 cmlg gas coal 0. 29 cm/g meager coal 0.782 display that the relationship of the adsorption phase cmlg< anthracite 0.94 cm/g. If taking the ratio of thevolume and adsorption potential is logarithmic, and the absolute error to the testing value as the relative error, thecorrelative coefficients are all higher than 0.99(Table 2). sequence is: coking coal 1. 69%o< gas coal 3.08 %< meagerIf the adsorption phase volume and adsorption capacity at coal 3.57%< anthracite 3.87%. These results are relativethe standard conditions are linear relationship, using the to k value. In order to be convenient in using the methoddsorption capacity to replace the adsorption phase vol- stated above, this paper suggests using the same k value;ume, the relation among the adsorption capacity, pressure however, for each sample the k value may not be optimumnd temperature can be expressed asThis can be verified by the correlative coefficient of thecharacteristic curve(see Table 2). For pressure, the errorRTIpa Inv+b(5) has not the obvious feature, for example, the error of gascoal is larger in the range of 3-5 MPa, the errors of otherwhere a and b are the constant terms in the equation. Sub- 3 samples are larger when P<3 MPa or P >8 MPa. Anastitute the calculation eq (4)of Po into eq (5), we obtain lyzing the adsorption capacity error of 4 samples at theInV=AT[2.7lnT-InP-126603]+B(6) same temperature, it shows that the average error at 20Cis maximum, the average absolute error of 4 samples isture Tis fixed, eq (6)could be further simplified empera- 1.06 cmlg and average relative error is 4.34%.Thewhere A' and B' are the constant terms. If theminimum average error is at 30C, and two kinds of errorsInv=A lnP +B(7) are 0.33 cm/g and 1.74% respectively. This is because theequation of the characteristic curve is achieved from thewhere A and B are the constant terms and the relationship adsorption testing data at 30C. These error analyses showthat although the same k value(2.7)is adopted, the error is(8 not large enough to cause the obvious effect when usingB=A7×27nTA'T×12.6603+Bthe predicted adsorption capacity. Moreover, if the larger k(9) value is used, the higher precEqs.(6)-(9)imply that taking the adsorption data at paci中国煤化roal samples are discussone temperature as the basis, it is possible to predict the aboablished and parametersadsorption capacity at another temperature. The calcula- seleCN MHGfor other coal samplestion details are The isothermal adsorption data at tem- More testing results should be analyzed in the further re-perature Ti is known, expression(7)is used to calculate searchChinese Science Bulletin Vol. 50 Supp. December 2005ARTICLESGas coalCoking coal16Pressure/MPaPressure/MParessureMPa20℃ test value■30℃ test value◆40℃ test value·50℃ test value20℃ predicted…………30℃ predicted40℃ predicted50℃ predictedcurveFig 3. Predicted isotherms of different temperatures from the characteristic curve of 30C.Due to the limitation of the equipment test condition, it coal samples with different ranks at 20C, 30C, 40C andcannot conduct the verification on the prediction effects at 50C respectively, and then, based on the adsorption po-even higher temperature and pressure. However, it is tential theory the characteristic curves of methane ad-theoretically possible to predict the adsorbed by each kind of coal were obtained. During theany other temperature and pressure from the adsorption calculation process, the calculation method of Po adoptsdata at a certain temperature if using eqs. (6)-(9). But in the modified-Dubinin equation( value adopts 2.7 in thisthe practical application it is suggested that the tempera- paper), and the adsorption phase density value adoptsture and pressure should be as close to the measured con- 0.375 g/cm. The results display that the characteristicditions as possible. The larger the difference between the curves at different temperatures almost fall on a singlemeasured conditions and prediction conditions is, the curve. This indicates that the characteristic curve of thepoorer the accuracy of the predicted results is. Even so, coal adsorbing methane is unrelated to temperature. Thefor the coal seams without the adsorption test data or relationship of the adsorption phase volume and adsorpwhere the reservoir temperature is different from the test tion potential is logarithmic, and from this relationship thetemperature, the method stated above still can contribute mathematical expression among the adsorption capacity,to the aspects such as researches on the CBM storage his- pressure and temperature is achieved. According to thetorical course, resource prediction, optimization of favor- method of calculation, only relying on the adsorption dataable block and reservoir characteristic description5 ConH中国煤化工 o predict the adsorptionand pressure conditionsCNMHGthe coal seams withoutThe isothermal adsorption tests were conducted on four the adsorption test data or where the reservoir temperatureChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESis different from the test temperature, it could have sig- Chinese Science Bulletin 2005 Vol. 50 Supp. 92-98nificant contribution to the research on the CBM storage The key stage and momenthistorical course, resource prediction, optimization of favorable block, reservoir characteristic description, etc, of coalbwhen combined with the coal seam buried history, geo-gas reserviortemperature variation course, coal metamorphosed process evolution in the qinshuiBasin ChinaAcknowledgements The authors would like to express thanks to thesupport to the research project"Adsorption characteristic and ZHAO Mengjun, SONG Yan, SU Xianbo"gas-storage mechanism of coal seam"(Grant No. 2002CB211703). The LIU Shaobo, QIN Shengfei, HONG Fengauthors would also like to acknowledge the support and assistance from LIN XiaoyingCoal Research Institute(CCRD)1. Research Institute of Petroleum Exploration and Development, Petro-ReferencesChina, Beijing 100083, China;2. Henan Polytechnic University, Jiaozuo 454000, ChinaCorrespondence should be addressed to Zhao Mengjun(email: zmj@L.Bustin,R.M.,ClarksonC.r,geOlogicalcontrolsoncoalbedpetrochina.com.cnmethane reservoir capacity and gas content, International Joumal ofAbstract The evolution of coalbed gas reservoir is charCoal Geology, 1998, 38: 3-26.acterized by coalbed gas geochemistry and gas content On2. Zhong, L. w, Zheng, Y. Z, Yuan, Z.R. et al, The adsorption capa- the basis of burial history and thermal history, the formingprocess of coalbed gas reservoir and the gas accumulativebility of coal under integrated influence of temperature and prehistory in the Qinshui Basin are discussed in this paper. Thesure and predicted of content quantity of coal bed gas, Journal of difference of the thermal history, geochemistry characteristic,China Coal Society(in Chinese), 2002, 27(6): 581-585and gas accumulative history between Yangcheng andHuozhou areas shows that the formation of coalbed gas res-3. Zhao, Z. G, Tang, X. Y, Zhang, G M, Experiment and signifi- ervoir in the Qinshui Basin is controlled by the geologicalcance of isothermal adsorption of methane on coal under higher process in the critical stage and the critical moment. Thetemperature Coal Geology and Exploration (in Chinese), 2001. components and isotopes of coalbed methane are determinedby the stage at which the coal maturation reaches its maxi-94):29-30mum rank. The coalbed methane accumulative history isrelated to the temperature and pressure of the coal burial4. Scott,A.R,Hydrogeologic factors affecting gas content distribu- history, because the coalbed gas is mainly in adsorptive state.tion in coal bed, International Joumal of Coal Geology, 2002, 50: It is stated that the gas content in the coal seam is controlled363-387.by the moment when the coal seam is uplifted to the shallow5. Hu, T, Ma, Z. F, Yao, H. Q, Prediction of adsorption isothermsest posKeywords: burial history, key stage and moment, gas accumulativeith isosteric heat of adsorption, Journal of Nanjing University of process, Qinshui BasinTechnology(in Chinese), 2002, 24(2): 34-38DOI:10.136098zk00196. Zhu, B. Y. Zhao, Z. G, Basis of Interface Chemistry (in Chinese1 IntroductionBeijing: Chemical Engineering Press, 1996, 336-342Duong, D. D, Adsorption Analysis: Equilibria and Kinetics, LonThe geochemistry characteristic of coalbed gas is thekey aspect of coalbed methane geology, and has been endon: Imperial College Press, 1998, 149-17deavored by many researchers for a long time!-.Up to8. Cui, Y. J, Yang, X. L, Method of volume correction fornow,the origin of coalbed gas has been well under-multi-component gas isothermal adsorption, Coal Geology and Ex- stood.9, but few investigations relate to the relationshipploration(in Chinese), 2001, 29(5): 25-27.between the geochemistry characteristics of coalbed gas9. Dubinin, M. M, The potential theory of adsorption of gases anand the formation and evolution of coalbed gas reservoirThe difference between coalbed gas composition andapors for adsorbents with energetically nonuniform surface, methane 8C value is very great. This is caused by coalChemical Review, 1960, 60(2): 235-241maceral, coal rank, gas generating process, burial depth,10. Amankwah, K. A. G. Schwarz, J.A. A modified approach for es. temperature. and pressure,. The coalbed methane 8Ctimating pseudovapour pressure in the application of the Dubinin-中国煤化工 atural gas with similarmatuAstskhove Equation, Carbon, 1995, 33(9): 1313-1319CNMHGmaturity, the isotopicion-diffusion-migration,Received March 9, 2005:June 30, 2005) the generation of secondary biogenetic gas, and the iso-Chinese Science Bulletin Vol 50 Supp, December 2005