Thermal simulation of the formation and evolution of coalbed gas

ARTICLESChinese Science Bulletin 2005 Vol 50 Supp 40-44ome researchers have simulated the yields ofThermal simulation of thegases and hydrocarbons, geochemical characteristics ofproducts and kinetics of hydrocarbon generation by meansformation and evolution ofof pyrolysis experiments on total coals. In addition,many simulation experiments of coal macerals have alsocoalbed gasbeen reported and a lot of new findings are obtained5-7Coalbed gases are mainly composed of methane and otherDUAN Yi, WU Baoxiang 2, ZHENG Chaoyang.2hydrocarbons and non-hydrocarbon gases, and the gener-WANG Chuanyuanating process of each gas component and the relationshipsbetween those gases are important for understanding the1. Key Laboratory of Gas Geochemistry, Institute of Geology and Geo- feature of the formation and evolution of coal-derived gas2. Graduate School of Chinese Academy of Sciences, Beijing 100039,So far, studies on these problems are still lacking. Pre-vious pyrolysis simulations of the evolution of coal matuCorrespondence should be addressed to Duan Yi (email: ration usually adopted isothermal heating and only pro-Duany @ns lzb ac cn)longing time in dealing with the influence of heating timeAbstract Thermal simulation experiment of gas generon the evolution of hydrocarbon generation from coalstion from the peat and the coals were performed using the However, the evolution of coal maturation is a dynamichigh temperature and pressure apparatus, at temperature process in geological environment, so it is controlled byranging from 336.8-600'C, a pressure of 50MPa and two temperature and geological time. Thus, this experimentheating rates of 20'C h and 2 C/h, and the evolution and method cannot follow the kinetic process of gas generaformation of coalbed gas components were studied. Results tion from organic matter well. To date, no experimentshow that for the coals, the gaseous products are mainly studies on the evolution of coalbed gas generation fromcomposed of hydrocarbon gases. However, for the peat the peat have been reported. Peat has been considered as thecontent of hydrocarbon gases in gaseous products is lower original matter of coal formation without t undergoingthan that of non-hydrocarbon components. In the gener- deep burial and strong degradation. So peat contains lotsated hydrocarbon gases methane is predominant and heavy of original information important for understanding thevolutionary characteristics of coal-forming organic matated non-hydrocarbon gases, and hydrogen(H) and sul- ter during the cause of coalification. On the basis of thefurated hydrogen(H S)are existent in trace amount. It is above-mentioned facts, the peat and low-maturity coals inalso observed that temperature is the main factor controllingg this study were used in the simulation experiment and thethe evolution of coalbed gas generation. With increasing process and characteristics of gas generation from thesevitrinite reflectance, methane rapidly increases, COz sightly experimental materials were studied. The experimentalincreases, and C2s hydrocarbons first increase and then de- system were performed at the heating rates of 20 and 2Ccrease. The peat and Shanxi formation coal have a higher h to reveal the role of time in the process of gas genera-generative potential of coalbed gases than coals and Taiformation coal, respectively, reflecting the effect of the proption. The potential uses of results obtained in the studyerty of organic matter on the characteristics of coalbed gas coalbed gas components in geological environment arecomponent generation. In this study, it is found that low discussed. This provides a scientific reference for furtherheating rate is favorable for the generation of methane, H2 understanding the genesis and characteristics of coalbedand CO, and the decomposition of C2s hydrocarbons. This gas in geological environmentshows that heating time plays an important controlling rolein the generation and evolution of coalbed gases. The resultsThe results 1 Samples and experimentalbtained from the simulation experiment in the study of In this experiment, two coals and a peat were collectedcoalbed gases in natural system are also discussed.to simulate the formation and evolution of coalbed gasKeywords: coalbed gas, generation and evolution, pyrolysis, peat, components. The coals came from the No. 2 coalbed in theLiyazhuang Coal Mine of the Qinshui Basin(ShanxiDOl:10.1360/98zk0002Formation(Ps), bituminous coal)and No 6 coalbed inTanggongta Coal Mine, Inner Mongolia(Taiyuan Forma-Coalbed gas is a mixture of various kinds of hydrocar- tion(Cat), bituminous coal). The peat was collected frombon and non-hydrocarbon gases generated from the Ruoergai Marsh of Sichuan Province at a depth of 3.0coal-formed organic matter during its geological evolution. 3.2 m, of which the organic carhon content was approxiThermal experiments of organic matter was widely carried中国煤化工 hydrogen, oxygen andout to simulate the process of hydrocarbon generation, nitrogeCNMH were 59.8%,4.8%assess the generative potential of hydrocarbons from dif- 24.20wopwou,vj, and the carbon isotoferent types of organic matter and study the influence of composition was -27. 1%o. Other analytic data for the peatmedia on generation, evolution of hydrocarbons and so and the coals are listed in Table 1Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLESTable 1 Analytic data of content of macerals, H/C(atom)ratio, and vitrinite reflectance( Ro)of the peat and coalsType of kerogenContent of macerals(%)TOC(%)H/C(atom)ratio Ro(InertiniteexiniteTaiyuan Formation( Cyl)coal0.79051Shanxi Formation(P:s)coal272945.00.750.34The simulation experiments were finished in the State drocarbons and H2 having a relative content of 0.5%Key laboratory of Organic Geochemistry of Guangzhou 39.6%,0.4%-14.4% and 0. 1%-2.3%, respectivelyInstitute of Geochemistry, the Chinese Academy of Sences. Samples were first crushed into 120 meshes, and 2.2 Generation and evolution of gas componentsthen were extracted by MAB(methanol: acetone: benThe formation and evolution of coalbed gas compozene=1: 2.5: 2.5)solvent. Kerogen was obtained from the nents can be understood through the simulation experi-extracted samples Pyrolysis experiments were carried out ment. In this simulation experiment, representative Tai-using a pressurized apparatus. With the protection of ar- yuan and Shanxi formations coals of north China weregon gas, kerogen samples of 25-50mg was sealed into 13 collected because coalbed gases in China are mainly produced from these coal seams. In addition, a peat was alsogold tubes respectively placed into the 13 high pressure used in the simulation experiment in order to follow thecells and into one pyrolysis oven. Tubes were held at con-stant 50MPa in pressure and were programmed to rise whole process of the evolution of coalbed gases.Thefrom 200 Cat heating rates of 20 and 2 C/h. The 13 higlresults in Fig. I give the relationship between the matura-pressure cells would be taken out corresponding to 13 tion(R )of the coals and the peat and the amount of thegenerated gases. This is favorable for the study of coalbedtemperature points between 336.8-600C. During the gases in geological environmentprocess of experiments, errors in temperature and pressurewere 1.2%. These results showstandard calibration. At every given temperature, the solid that the amount of the generated methane was controlledreaction residuals were recovered and treated to measure by the evolutionary maturity of coals. Consequently, thevitrinite reflectance using a Leize MVP Ill microscopic accumulation of coalbed methane. In addition, the amountmotometer systemof the methane generated from the peat was higher than2 Result and discussionthat from the coals at the same Ro value. For example, at2.1 Composition of gaseous products600C the amount of the generated methane was 41l mL/gorgC for the peat, far higher than 198mL/g orgC for theThe relative content of gas components generated from Shanxi formation coals and 201mL/g orgC for the Taiyuanthe coals and the peat calculated by data given in Fig. 1shows that the gaseous products from the coals wereformation coal at the heating rate of 2C/h. This showsmainly composed of hydrocarbon gases in which methane that the assessment of coalbed gas resource in natural en-(CHA), C2-Cs hydrocarbons and hydrogen(H))were vironment using the amount of the coalbed methane gasgenerated from coal with a certain maturity became lower3.5%-86.7%,0.4%-25.0% and 0.1%-3.9%, respec- than that of the peat and this result would not reflect thetively. Non-hydrocarbon gases had lower relative contents original amount of gases generated from coal-formingthan hydrocarbons. The percentage of carbon dioxideorganic matter. Under the same Ro value and the different(CO2)ranged mainly from 10. 1%-77.8% apart from the heating rates conditions, there was an obvious differencefact that it is higher at earlier stage of maturation. Hydro- in the amount of methane generated between the coals andgen disulfide(H-S)was present in low concentration, with the peat. Generally, the amount of methane generated ata relative content of 0%-0.4%. However, for the peat, the higher heating rate was higher than that at the lowernon-hydrocarbon components prevailed in gaseous prod- heating rate The difference was between 0 and 120mL/gucts.For example, the relative content of CO2 was as high org中国煤化工 Ug orgC for the coalsas 54.0%-98.8%. HS had a 0.4%-0.8% relative con- ThisCNMHGheating time, the mortent similar to that of the coals, In contrast the relative metlgUiwu wiui ui same coal maturity, recontent of hydrocarbon gases was lower, CH4, Cr-Cs hy- flecting that the heating time of coal is another importantChinese Science Bulletin Vol 50 Supp. December 2005ARTICLEScontrolling factor during the formation of coalbed meth- tive content of heavy hydrocarbon gases in thermal ge-ane gas. Therefore, in geological environment the older netic coalbed gases in natural environment can help uscoal bed should contain more coalbed gases than the understand the evolutionary stage of the C2-Cs hydrocarnewer ones with similar maturity between therbon formation2.2.2 C2-s hydrocarbons. In this experiment, the 2.2.3 Non-hydrocarbons. As Fig. 1 shows, theamount of Cr-Cs hydrocarbon generation from the sam- amounts of CO2 generation from the coals and the peat arevalue was <05mL/g orgC for the coals and <5mL/g orgC increase with the increasing Ro value. ae ary stage andples was extremely low when Ro was less than 1.0%. The already high at the earlier thermal-evolutionle same Rofor the peat(Figs. 1 and 2), and corresponding relative value and the heating rate, the production of CO2 genercontents in gaseous products were all less than 3%(Fig. 3). ated from the peat is much higher than that from the coalsThen, the amount of C2-Cs hydrocarbon generation in- (with a difference of >250 mL/g org C generally)and thiscreased with the increasing Ro value. It reached the maxi- value is higher from the Taiyuan Formation coal thanmum of about 83 mdg orgC for the coals and about 11- from the Shanxi formation coal. This is probably related17 mL/g orgC for the peat corresponding to the Ro value with the difference in maturation among the studied samof 2.5%. However, the amount of the generated Cz-Cs ples Peat is considered as the original coal-forming matterhydrocarbon decreased as Ro value was more than 2.5%. It and preserves a large amount of oxygenous compoundswas not more than 0.38 mLg orgC corresponding todue to the fact that it was heated slightly under naturalRo value of 4.5%o. Among three samples, the differencesproduction of C2-Cs hydrocarbons were remarkable. At the composition during the earlier experimental stage formedsame Ro value, the amount of C2-Cs hydrocarbon genera- a large amount of CO2 and the percentage of CO2 gener-tion was the highest for the peat, middle for the Shanzated can reach 60% of total CO2 amount in this study. Orformation coal and the lowest for the Taiyuan formation the other hand. the Ro value of the Taiyuan formation coalcoal. The difference between the highest and the lowest is lower than that of the Shanxi formation coal. so thevalue was 7-8 times in production(Figs. I and 2). These Taiyuan formation coal generated more CO2. Therefore,results showed that more organic matter have been pre- the concentration of CO2 in thermal genetic cogasserved in the peat than the coals. Therefore, the amount of in natural environment is mainly controlled by the evoluCr-Cs hydrocarbon generation might be underestimated tionary stage of coal maturation. If the coalbed gases werethrough the experimental simulation on coals. In addition, formed during the earlier stages of coalification and wereit was observed that the geochemical properties of coalspreserved well without degradation, the concentration ofcould influence the amount of Cz-Cs hydrocarbon genera- CO2 should be considerably high in coalbed gases. In adtion. For example, Shanxi formation coal with higher exdition, the contents of cO obtained from thenite component than Taiyuan formation coal (Table D) higher-temperature experimental stage in this study aregenerated more C2-Cs hydrocarbon, although Taiyuanstill higher than those in coalbed gases generated duringformation coal has a higher Ro value. Under the same Ro the high maturity of coals in natural system. For example,value and different heating rates conditions the evolutionary trends of the production of Cz-Cs hydrocarbonsthe content of Coz in coal beds with Ro value of aboutwere not obvious before its peak generation. However, the 3.6% in South Qinshui Basin, China is from 0.03%to0.17%. These contents of CO2 are obviously lower than2C/ is lower than that at the heating rate of 20 C/ after that of 13%0-58% obtained at the Ro value of 3.7-4. 1%in this study. This difference is probably because COits peak generation, showing that the longer the heating from the coal beds in geological environment dissolvedtion. This reflects that the heating time plays a certain into water and expelled out of coal beds or COz was partlcontrolling role in decomposing Cr-C, hydrocarbons. In depleted due to the formation of methane by its reactionthis study, the relative content of C2-Cs hydrocarbonswith hydrogen in natural systems. Fig. I also shows thaterated from the coals and the peat ranged from 0the production of CO2 is affected by the heating rate. For a2. 5%(Fig. 3). The relative contents of C2-C, hydrocarbons given sample, the amount of CO2 generated at the 20 C/hgenerated at the different evolutionary stages of the sam- heating rate is lower than that at the 2 C/h heating rateples are obviously different. Under this closed-system This is similar with case of methane, reflecting that thecondition, the high amounts of C2-C, hydrocarbon genera- longer heating time, the more COz is generated. Thereforetion appear within the Ro value range of 2%-3%, conse- the中国煤化工 nt factor controlling thequently implying that there is a certain quantity of C2-Cs amoCNMHhydrocarbon in thermal genesis coalbed gases. This differsonents in thermal gefrom the viewpoint that C2-C, hydrocarbon is absent in netic coalbed gases. Especially, H2S is regarded as abiogenetic coalbed gases. These data show that the rela- harmful gas in coalbed gases. In this study, the amounts ofChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESYield (mL/g org C)Yield (mL/2004501001500.50.5(d)2.51001503.54555Fig. 1. Evolution of the yield of gaseous products generated from the peat and coals with vitrinite reflectance (a) Peat 20C/;(b) peat, 2C/h; (c)Shanxi formation coal, 20C/h; (d)Shanxi formation coal, 2C/h;(e)Taiyuan formati中国煤化工coa,2℃hH2 and H2S generated from the coals and the peat are very amorCN Gh heating rate is lowersmall(Fig. I). The amounts of H2 generated roughly holds than that at the 2 Ch heating rate. These characteristics arean increasing trend with the increasing maturation, and the similar to those of methane and CO2 For H2S, the fluctuantChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESYield(mL/g org C)variation of its production is present with the increasing Rovalue, but the amplitude of its vibration is very smallexperiment perder closed system condition at high temperature and pres:2sure as well as different heating rates using the peat andcoal samples reproduced features of the formation andevolution of thermal genetic coalbed gases. Results showthe existence of the remarkable differences in compositionand production of thermal genetic coalbed gases at different maturity stages. It also is found that temperature,heating time and geochemical property of the studiedsamples are the major factors that control the characterisc2.5tics of the formation of coalbed gases. These provide thescientific basis for studies of genesis and exploration ofcoalbed gases in natural systemAcknowledgements This work was supported by the 973of China( Grant No. 2002CB211701), and the Impreau of Intermational Co-operation, the Chinese(GJHZO5)anin Resource and Environment Field, the Chinese Academy of SciencesGrant No. KZCX3-Sw-128)Referene1. Orem, W.H., Neazil,S G, Lertch, H E, Experimental early stage4.55coalification of a peat and a peatified wood sample from Indonesia,Fig. 2. 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P et al., Pyrolysis simulation ofthree kinds of colas: Part(I)the composition and evolution ofgaseous products (in Chinese)Coal Conversion, 2000, 23(4):7. Rohrback, B. G. Peters, K. E. et al., Geochemistry of artificiallyheated humic and sapropelic sediments-IL: oil and gas generationA.A. PG Bull,1984,8:961-9708. Smith. J. w, Pallasser, R J. et aL., Microbial origin of Australiancoalbed methane, A.A. PG Bull, 1996. 80: 891-89729. Clayton, J. L, Geochemistry of coalbed gasA review International Journal of Coal Gcology, 1998, 35: 159-17310. Dai, J. X, Qi, H. F, Song, Y. et al., Component, type of carbonisotopic of Chinese coalbed gas and their genesis and significandScience in China (in Chinese), Series B, 1986, 12: 1317-13261. Liu J. Z, Tang Y C,Anof predicting the production ofmethane using the kinetic method for hydrocarbon generation froFig 3. Evolution of the relative content of heavy hydrocarbons(C2s) 12中国煤化工.4x11:1897-19ctance.1,Peat,20℃al, Comparison of the characCNMHGfrom/h; 2, peat, 2C/h; 3, Shanxi formation coal, 20C/h; 4, Shanxi formationpes of coal (incoal, 2C/h: 5, Taiyuan formation coal, 20C/h; 6, Taiyuan formation coal,Chinese), Geology and Geochemistry, 2003, 30(3): 92-96Received March 5, 2005; accepted July 1, 2005)Chinese Science Bulletin Vol 50 Supp. December 2005