Science in china ser d earth sciences 2005 vol 48 No 8 1197-1202Study of simulation experiments on the TSr system and itseffect on the natural gas destructionYUE Changtao, LI Shuyuan, DING Kangle ZHONG NingningState Key Lab of Heavy Oil Processing University of Petroleum, Beijing 102249, ChinaCorrespondenceshouldbeaddressedtoLiShuyuan(email:syli@cup.edu.cn)Received April 16, 2003Abstract The TSR process may be one of the reasons to result in the natural gas destructionin deep carbonate reservoirs. The work on this field has been the subject of much research inrecent years. In this paper, thermal simulation experiments on the reaction of CH4-CaSOa werecarried out using autoclave at high temperature and high pressure. The products were characterized by some advanced analytical methods. the thermodynamics and kinetics were discussedand the reaction mechanism was investigated tentatively. It is found that the reaction can proceed spontaneously to produce H2S, H2O and CaCO3 as the main products at the temperaturerange of 550-700. The activation energy and geological reaction rate calculated from the kinetic model can be compared with those from previous work. The results obtained in this papercan provide important information on the explanation of geochemical depth limit for natural gasand on the exploration of gas reservoirsKeywords: natural gas destruction, TSR, simulation experiments, kineticsDOI:10.1360/03vd0133In recent years, much work on the natural gas de- suggestion that the reactions involved in thermo-struction in carbonate reservoirs has been carried out chemical sulfate reduction (TSR) could result in theat home and abroad. The classification of source rocks, natural gas destruction in sulfate-rich carbonate reser-the estimation of natural gas resource and the explora- voirs. Therefore the investigation of TSR process istion of gas reservoirs can be related to the determina- necessary to determine the geochemical depth limit fortion of the geochemical depth limit for natural gas. On natural gas and to evaluate the hydrocarbons generathe basis of reaction kinetics and lab simulation extion for carbonate rockperiments, the kerogen decomposition proceeds, substantially, to completion at more than 5% vitrinite re-According to the previous work on the natural gasflectancel. However, the natural gas in reservoirs destruction, the accumulation of Hs in deep sour gascould not detected at lower thermal maturity (3.5% reservoirs can be explained by the TSR process2-71vitrinite reflectance). These observations support the The hydrocarbons are branched and n-alkanes, cyclicNomenclature: AHm, standard enthalpies of formation; AGm, standard Gibbs energyAGm, standard reaction Gibbs energy: a, b, c, parameters for heat capacity; E, apparent activatH中國(guó)煤化工CpC N MH Grequency factor,s-lR, gas constant, 8. J. mol".K, N, reaction order; K, reaction rate constant; T, reaction temperature, K; t, reaction time, S; x, reaction conversion;B, constant heating rateCopyright by Science in China Press 20051198Science in China ser d earth sciencesand mono-aromatic species, in the gasoline rangeThe gaseous products were characterized by miSulfate is almost from gypsum or anhydrite.. Meth- crocoulometry and gas chromatography while theane and solid calcium sulfate are the most stable of all solid products were analyzed by FT-IR and XRDpossible reactive compounds. In addition, the pub- methodslished experimental data are focused on the liquid hy-drocarbons. There are few studies that methane is theThe microcoulometry with a model of WK-2B wasused to determine the h,s concentration. The tem-only organic reactant with the solid calcium sulfate toperatures of entrance, exit and vaporization zone werebe investigated. In this paper, thermal simulation experiments on the reaction of CH4-CasO4 were carriedcontrolled at 500, 850 and 60, respectively. Theflow rates of oxygen, nitrogen, and sample gas wereout at high temperature and high pressure. The prod- 40 mL/min, 160 mL/min and 30 mL/min, respectivelyucts were characterized by microcoulometry, gas The gas chromatograph with a model of Hew-chromatography, FT-IR and XRD methods. Based on lett-Packard 6890 was equipped with a TCD detectorthe experimental results, the thermodynamics and kiand five packed columns. The temperature of detectornetics of the CH4-CaSOa reaction were discussed andthe reaction mechanism was investigated tentativelyFor X-ray diffraction(XRD), the solid products1 Experimentalwere mechanically crushed and ground to a size ofII Apparatus<200 meshes. The measurements weyThe experimental apparatus includes the autoclaveing a diffractometer with CuKo radiation. All FT-IRthe gas pipeline and the analytical equipments. The spectra were recorded from 4000 cm"to 400 cmautoclave is a vertical reactor made of a stainless-steEach spectrum gave the average value of 32 scans ustube with a model of FDW-01. The temperatureing a spectral resolution of 4 cmontrolled with the precision of tIC and the concen- 2 Results and discussiontration of methane is 99.9%0 because hs is a corro-sive gas, it undergoes further reactions with the auto-2.1 Analysis of the reaction proclave wall. The sample basket of a quartz glass cylin- The concentrations of gaseous product, hydrogender is placed on the bottom of the reactor to contain sulfide, at the different final temperatures are shown inthe CaSo4 powder to escape the effect of H2S-wall Table 1. The gaseous components from the GC analyreaction. After vacuumization of reaction system, the sis include CH4, C2H, H2, CO2 and CO. Theirmethane was charged into autoclave to a given pres- compositions at different temperatures are listed insureTable 1 as well1.2 Experimental conditions and analytical methodTable 1 The compositions of the gaseous products at different tem-The initial pressure of the methane was 6 MPa andperatures for the reaction of CH4-CaSOamperature/℃Hthe final pressure of the reaction system ranged from5502.4960.8610.61495.9820.0060.04141 4 MPa to 16 MPa. It is hard to detect the reaction5.5781.1100.83692.1650.1550.1558products under the temperature of less than 4006506.3161.2220.89191.1760.3100.2649ThereforeItoclave was first heated to 400 di6707.1371.4700.98289.6300.4440.3375rectly andto550,600,650,670and7008.3931.6851.16987.7590.5120.4822700 using a programmed heating in different timesof 166 h, 120 h, 66 h, 60 h, 50 h, respectively. Duringthe simulation experiments, the weight of solid reaclvmerioatiliv中國(guó)煤化式mmetant and solid product was measured by electron bal- drogenationCNMHGis mainly fromancethe reaction between h s and the autoclave wall or theStudy of TSR system and natural gas destructiondecomposition of methane. CO2 and CO may be theintermediate of the reaction of CH4- CaSO4. It is obvious that with the increasing temperature, the contentsof C2H6, H2, CO2, CO and HS increase but the con- 75tent of CHa decreasesElemental sulfur was not found in the X-ray diffrac≌0m:0/Fig. 1. FT-IR spectra of calcium carbonate and calgcium sulfate are given in Figs. 2 and 3. In Fig. 2, thepeaks at 1418.77 cm, 877.85 cm and 714.85 cm350030002500200015001000500Wavenumbers/crcorrespond to the vibrational band of the carbonic acidFig. 2. FT-IR spectrum of calcium carbonateradical. Especially, the appearance of peak at 1418.77cm can be explained by the formation of carbonate.In Fig. 3, the vibrational band of water in the spectrumgives the peaks of 3547. 32 cm and 1688.96 cmEThe peaks of 1117.70 cm, 463. 55 cm", 669.70 cm",600.87 cm result from the vibrational band of vitriolradicalo. By comparing the peaks in Figs. 1, 2 and 3,it is shown that the peaks in Figs. 2 and 3 can be found導(dǎo)in Fig. 1 and there is a little difference. The spectralresolution of 4 cm for the solid product and of 88cm- for standard calcium carbonate calcium sulfatemay probably result in the difference above400035003000Wavenumbers/cmFig 3. FT-IR spectrum of calcium sulfate2. 2 ThermodynamicsIt is important to calculate the thermodynamic paameters to know the possibility of the reaction. In thispaper, the thermodynamics for the reaction of CHCasO4 was investigated. The thermodynamic data arelisted in Table 2350030002500200015001000500according to the data in Table 2, the values ofWavenumbers/ cmGibbs free energy at different temperatures were ob-Fig. 1. FT-IR spectrum of solid products for the reaction of CH- tained The results are shown in table 3CasOn.It is obvious that the reaction can proceed spontaOn the basis of experimental results, it can be con- neously according to the negative values of Gibbs freecluded that the reaction of CH -CaSO4 has taken place energy. Theto produce H2S, CaCO3 and HO as the main productcreases with中國(guó)煤化工e, This impliesThe reaction pathway is as followsthat the inC MH Favorable to theCH4+ CasO4 H2s+ H2O+ CaCO3(1) reaction of CH4-CaSO41200Science in China ser D Earth sciencesTable 2 The thermodynamic data for the reaction of CH-CaSo.1SpAHm o/kJ mol- AG/kJmol-Sa/Jmol-·Kb×0374.8150.72188,014.1517.99CaSo(s)-1434.1113218570.207CacO (s)1128.8092920.6205.8-237699129.1614.492.022Cp, m=a+bT+cr.Table 3 Gibbs free energy for the reaction of CHe-Casomperature/K298.15523.15573.15623.15673.1AG/k. mo1-269645277-4958853.934-58288Temperature/K723.15773.15823.15873.15923.15AGn/kmol-62.622-66.90571.106-75.195-79.1382.3 KineticsAccording to reaction theory, kinetic equation isusually writtend xE/RT5205405Temperature/CConsider constant heating rateand takeFig 4. The conversion vs temperature for CHa-CasO4 readtlog of both sides of eq.(2)to yieldI can be obtained(see Table 4). From Table 4. it candxβbe concluded that the reaction order is zero. ThisIn AEmeans that reaction rate is independent of the pressof CHa and the weight of CaSO4. The additional ex-On the basis of the conversions at different tem- periments were carried out to determine the reactionperatures, the linear regression coefficients of lines of order. By using the same weight of CaSO4, the pressures of CHa were changed. The reaction conversionIndT(1-x)VS T can be determined for dif- was almost identical. During the other experiments,the pressure of Cha was fixed and the weight offerent reaction orders n. When the linear regression CaSeOa was changed. no change in reaction conver-oefficient is the closest to l, the corresponding value sion has been found. Therefore. the reaction order isof n can be taken as the reaction orderproved to be zero. The reaction rate becomes onlyThe reaction conversions were 1.45%, 2.55%0, 3.7%, function of temperature6. 42% and 10.99%0 at the temperatures of 550, 600,650, 670 and 700 respectively. The relationshipTable 4 The linear regression coefficients for the reactionof CHy-CaSO4between reaction conversion and temperature is shown Rea0.8in Fig. 4Rcoefficientsnaon80.98030.9797Taking different reaction order n from 0 to 1. the中國(guó)煤化工CNMHGlinear regression coefficients of IndxβThe kinetic parameters can be calculated using thekinedel with zero reaction ordoStudy of TSR system and natural gas destruction入2/=l-AREx(4) elementary steps as follows(E+2RT) RTCaSO4 Ca+ SO4The linear regression of in xB vs I in eq, (4)CHa+SO4 CO2+H2S + 2OH(6)Ca+CO2+ 20H CaCO3+HO (7will give a straight line with a regression coefficient of0.996(see Fig. 5). The apparent activation energy EIt is hard to detect the reaction products at the temand apparent frequency factor A can be determined bperature of less than 400 in lab. When the simulationthe slope and intercept of regressed line(E= 152.919 temperature ranges from 550 to 700, the reactionkJ/mol and A=1.162X10s-)can take place obviously. These phenomena indicatedthat the chemical bond in calcium sulfate is very dif180ficult to be broken. Eq.(5)is a rate-controlled step atthe low temperatures. When the simulation temperaform SOa. which undergoes the redox-reaction withCH4 to produce H2s and CO2. The reaction conver160sions increase with the increasing te15.5cause methane is the most stable organic reactant5.0TSR process, the conversion is not high even at high14.5temperatures. Therefore, eq. (6) becomes arate-controlled step at high temperatures. Because eq1101120125130(7)can proceed at room temperature, it is not a(1/T)×103rate-controlled stepFig. 5. The curve of -In xp vs i for the reaction of CH2.5 DiscussionCasoA minimum temperature range of 100-200 forThe reaction rate constant k and the time needed to TSR process has been proposed in geologic environ-achieve 50% conversion for the CH-CaSO4 reaction ments,6, 13-15. Methane and solid calcium sulfate areat geological temperatures were calculated using equa- the most stable of all possible reactive compounds inTSR process. Therefore, the reaction of CHa-CaSO4tions =k and k= Aexp(E/RT). The results are has a high activation energy and no products can bedtlisted in Table 5. The time of 1.44 million years was found below 400 in lab. When the simulation tem-needed to achieve 50% conversion at the temperature peratures range from 550 to 700, the reaction canof 200.Therefore, the CHa-CaSO4 reaction is a very proceed to produce HS, Caco3 and ho as the mainslow process at geological settingsproducts. According to the literature of the simulationexperiments on hydrocarbon formation, the vitriniteTable 5 The values of k and time needed to achieve 50%0reflectance of source rocks is 2.5%0-3.5% at the tem-conversion for the reaction of CH -CaSO4Temperature/KRate constant k/s-ITime/aperature of 550-700. The temperature of 5503.12×10-219.90×10700 in lab corresponds to the temperature of 180373.151.52×10184.82×1010423.154.50×10-15142×10200 in geology settings[16] So, the reaction of CH473.154.55×101.44×10°CasA can c中國(guó)煤化工ervous2. 4 Tentative study of reaction mechanismI CNMHGtion energy oftor the 'Isk process has been reThe reaction of CH4-CasO4 may consist of some ported. The large range is probably caused by the dif1202Science in China ser d earth sciencesferent hydrocarbon-sulfate mixtures. The activation2001,40(1):143-175energy in this paper is 152.919 kJ/mol. The researches 3. Machel, H G, Krouse, H.R., Sassen, R, Products and distinon the kinetics of tsR process in geologic settings areuishing criteria of bacterial and thermochemical sulfate reduction,not well understood. Tridinger, Goldhaber andApplied Geochemistry, 1995, 10(4): 373-3894. Worden, R. H, Smalley, P. C, HS-producing reactions in deepOrr5. 17,18 indicated that TSR process could be per-carbonate gas reservoirs: Khuff Formation, Abu Dhabi, Chemicalformed in lab only at temperatures in excess of 175Geology,1996,133(1):157-171and that geological reaction rate could be measured5. Heydari, E, The role of burial diagenesis in hydrocarbon destruconly above 250. Goldhaber and Orr also determinedtion and His accumulation, Upper Jurassic Smackover FormationBlack Creek Field, Mississippi, American Association Petroleumthat the typical sour gas reservoirs may form in a fewGeology Bulletin, 1997, 81(1): 26-45million years. In this work, the time of 1. 44 million 6. Machel, H G, Gas souring by thermochemical sulfate reductionyears was needed to achieve 50% conversion at theat 140C: discussion, American Association Petroleum Geologytemperature of 200. This result can be comparedBulletin,1998,82(6):1870-1873with those reported in precious work.7. Wang Yigang, Dou lirong, Wen Yingchu et al, Origin of HS inTriassic Feixianguan Formation gas pools, Northeasten Sichuan3 Conclusionsasin,China, Geochimica(in Chinese), 2002, 31(6): 517-5248. Krouse, H.R. Viau, C.A., Eliuk, L S. Chemical and isotopic()According to the experimental results, mainevidence of thermo chemical sulfate reduction by light hydrocarproducts of CHa-CaSO4 reaction consist of hydrogenbon gases in deep carbonate reservoirs, Nature, 1988, 333(6171)sulfide calcium carbonate and water. thermodynamic9. Manzano. B. K. Fowler. M. G. Machal. H. G. The influence ofresult shows that the reaction occurs spontaneouslythermochemical sulfate reduction on hydrocarbon composition inand the increasing temperature is favorable. From theNisku reservoir, Brazeau River area, Alberta, Canada, Organickinetic calculation, it is found that CH4-CaSO4Geochemistry, 1997, 27(8): 50tion is the zero order reaction with the activation en-10. Wen Lu, Liang Wanxue, Zhang Zhenggang et al., The Infraredergy of 152.919 kJ/mol. When extrapolated to the geo-Spectroscopy of Minerals (in Chinese), Chongqing: ChongqingUniversity Press, 1989, 55-6logical temperature of 200, the time needed to 11. J.A. Dean, Langes Handbook of Chemistry (in Chinese), Beijingachieve 50%o conversion is 1.44 million yearsScience Press. 1991.9-1-9-6712. Li Shuyuan, The Application of Chemical Kinetics to Estimate2)The temperature of 550-700 in lab corre-in the Basin (in Chinese), Dongyingsponds to the temperature of 180-200 in geologytroleum University Press, 2000, 89-90settings. 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