页岩超临界CO_2压裂起裂压力与裂缝形态试验研究
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摘要
我国页岩气储层普遍黏土含量高,且多富集于缺水地区,超临界CO_2(SC-CO_2)因具有低黏度、低表面张力及对储层无伤害的特性,有望成为一种新型无水压裂方法。采用页岩露头与砂岩开展了真三轴SC-CO_2与水力压裂对比试验,结合工业CT扫描分析裂缝形态,并研究了温度对起裂压力的影响。结果表明:SC-CO_2压裂页岩时较水力压裂的起裂压力低约50.9%,压裂砂岩时起裂压力低约57.1%;相比水力压裂,SC-CO_2压裂升压过程中,由于CO_2的压缩性,增压速率较慢,由于页岩本身的层理特征,页岩SC-CO_2压裂有多次起裂的现象,更易形成复杂的裂缝;随着温度的升高,SC-CO_2压裂起裂压力呈下降趋势;CT断面扫描显示超临界CO_2压裂页岩时更容易形成多条网络化裂缝,达到类似体积压裂的效果。
The clay content of shale gas reservoirs in China is high,and most shale reservoirs are located in water-deficient areas.The SC-CO_2 fracturing is expected to be a new non-aqueous fracturing method because of its characteristics of low viscosity and low surface tension.In this paper,the comparison tests of SC-CO_2 fracturing and hydraulic fracturing in shales and sandstones were carried out under triaxial conditions.The fracture morphology was investigated by CT scanning and the effect of temperature on fracturing was studied.The results show that the initiation pressure of SC-CO_2 fracturing in shale is about 50.9% lower than that of hydraulic fracturing,and the initiation pressure of SC-CO_2 fracturing in sand is about 57.1% lower than that of hydraulic fracturing.Compared to hydraulic fracturing,the pressurization rate of SC-CO_2 fracturing is slow due to the compressibility of CO_2.Furthermore,the SC-CO_2 fracturing in shale has repeated cracking phenomenon,and it is easier to induce complex cracks.With the increase of temperature,the initiation pressure by SC-CO_2 fracturing decreases.The CT scanning shows that SC-CO_2 fracturing in shale is easier to form multiple network cracks and achieves the similar effect of stimulated reservoir volume.
The clay content of shale gas reservoirs in China is high,and most shale reservoirs are located in water-deficient areas.The SC-CO_2 fracturing is expected to be a new non-aqueous fracturing method because of its characteristics of low viscosity and low surface tension.In this paper,the comparison tests of SC-CO_2 fracturing and hydraulic fracturing in shales and sandstones were carried out under triaxial conditions.The fracture morphology was investigated by CT scanning and the effect of temperature on fracturing was studied.The results show that the initiation pressure of SC-CO_2 fracturing in shale is about 50.9% lower than that of hydraulic fracturing,and the initiation pressure of SC-CO_2 fracturing in sand is about 57.1% lower than that of hydraulic fracturing.Compared to hydraulic fracturing,the pressurization rate of SC-CO_2 fracturing is slow due to the compressibility of CO_2.Furthermore,the SC-CO_2 fracturing in shale has repeated cracking phenomenon,and it is easier to induce complex cracks.With the increase of temperature,the initiation pressure by SC-CO_2 fracturing decreases.The CT scanning shows that SC-CO_2 fracturing in shale is easier to form multiple network cracks and achieves the similar effect of stimulated reservoir volume.
引文
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[2]JIN X,SHAH S,SHENG M.Hydraulic fracturing model based on nonlinear fracture mechanics:Theory and simulation[A].Spe Technical Conference and Exhibition[C].2012.
[3]JIN X,SHAH S,JIN X,et al.Fracture propagation direction and its application in hydraulic fracturing[J].Society of Petroleum Engineers,2013.
[4]王玉满,董大忠,李建忠,等.四川南下志留统龙马溪组页岩气储层特背景[J].石油学报,2012,33(4):551-561.WANG Yuman,DONG Dazhong,LI Jianzhong,et al.Reservoir characteristics of shale gas in Longmaxi Formation of the Lower Silurian,southern Sichuan[J].Acta Petrolei Sinica,2012,33(4):551-561.
[5]TOUZEL P.Managing environmental and social risks in China[J].Society of Petroleum Engineers,2012.
[6]SLUTZ J A,ANDERSON J A,BRODERICK R,et al.Key shale gas water management strategies:An economic assessment[A].International Conference on Health,Safety and Environment in Oil and Gas Exploration and Production[C].Society of Petroleum Engineers,2012.
[7]LIU F,ELLETT K,XIAO Y,et al.Assessing the feasibility of CO2,storage in the New Albany Shale(Devonian-Mississippian)with potential enhanced gas recovery using reservoir simulation[J].International Journal of Greenhouse Gas Control,2013,17(17):111-126.
[8]HAIMSON,B C,ZHAO Zhongliang.Effect of borehole size and pressurization rate on hydraulic fracturing breakdown pressure[A].Rock Mechanics as a Multidisciplinary Science:Proc 32nd US Symposium Norman[C].1991:191-199.
[9]ITO T.Effect of pore pressure gradient on fracture initiation in fluid saturated porous media[J].Rock Engineering Fracture Mechanics,2008,75(7):1753-1762.
[10]SCHMITT D R,ZOBACK M D.Infiltration effects in the tensile rupture of thin walled cylinders of glass and granite:Implications for the hydraulic fracturing breakdown equation[J].International Journal of Rock Mechanics&Mining Science&Geomechanics Abstracts,1993,30(3):289-303.
[11]XIE H,LI X,FANG Z,et al.Carbon geological utilization and storage in China:Current status and perspectives[J].Acta Geotechnica,2014,9(1):7-27.
[12]陈立强,田守嶒,李根生,等.超临界CO2压裂起裂压力模型与参数敏感性研究[J].岩土力学,2015,36(S2):125-131.CHEN Liqiang,TIAN Shouceng,LI Gensheng,et al.Initiation pressure models for supercritical CO2fracturing and sensitivity analysis[J].Rock and Soil Mechanics,2015,36(S2):125-131.
[13]ISHIDA T,AOYAGI K,NIWA T,et al.Acoustic emission monitoring of hydraulic fracturing laboratory experiment with supercritical and liquid CO2[J].Geophysical Research Letters,2012,39(16):440-453.
[14]侯冰,陈勉,程万,等.页岩气储层变排量压裂的造缝机制[J].岩土工程学报,2014,36(11):2149-2152.HOU Bing,CHEN Mian,CHENG Wan,et al.Fracturing mechanism of shale gas reservoir with variable pumprates[J].Chinese Journal of Geotechnical Engineering,2014,36(11):2149-2152.
[15]衡帅,杨春和,曾义金,等.页岩水力压裂裂缝形态的试验研究[J].岩土工程学报,2014,36(7):1243-1251.HENG Shuai,YANG Chunhe,ZENG Yijin,et al.Experimental study on hydraulic fracture geometry of shale[J].Chinese Journal of Geotechnical Engineering,2014,36(7):1243-1251.
[16]FOUILLAC C,SANJUAN B,GENTIER S,et al.Could sequestration of CO2be combined with the development of enhanced geothermal systems[A].Third Annual Conference on Carbon Capture and Sequestration[C].Alexandria,2004.
[17]靳遵龙,李德雨,刘东来,等.超临界CO2体系黏度分子动力学研究[J].郑州大学学报(理学版),2015,47(2):101-103.JIN Zunlong,LI Deyu,LIU Donglai,et al.Molecular dynamics simulation of viscosity in supercritical CO2[J].Journal of Zhengzhou University(Natural Science Edition),2015,47(2):101-103.
[18]李维特,黄保海,毕仲波.热应力理论及应[M].北京:中国电力出版社,2004:59-67.
[19]郝振良,马捷,王明育.热应力作用下的有效压力对多孔介质渗透系数的影响[J].水动力学研究与进展,2003,18(6):792-796.HAO Zhenliang,MA Jie,WANG Mingyu.Influence of effective stress on the coefficient of permeability of porous medium under thermal stress[J].Journal of Hydrodynamics,2003,18(6):792-796.