热应力影响下干热岩水压致裂数值模拟
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摘要
干热岩水压致裂过程中低温诱导热应力与注入水压共同影响裂缝的萌生与扩展。首先通过THM耦合分析了低温压裂液注入过程中注入水压与热应力的相互作用及其对裂缝萌生的影响,随后建立描述岩石细观结构的THMD耦合模型对热应力影响下高温岩石水压致裂过程进行初探。结果表明:低温压裂液注入高温岩石产生的热应力包括岩石自身温度梯度形成的热应力与岩石颗粒非均匀膨胀导致的热应力,并在井筒周围呈现为拉应力。高注入压力将抑制热应力导致的多裂缝萌生,井筒附近热应力的存在对注入压力也具有削弱作用。基岩温度升高,裂缝萌生阶段更多裂缝在井筒附近起裂,缝网沿最大地应力方向的扩展速度减慢,但改造规模增加,同时多裂缝的存在也使得裂缝延伸压力增加。
Crack initiation and propagation in hydraulic fracturing process of hot dry rocks is affected by thermal stress induced by the low temperature of injected cryogenic fracturing fluid as well as its injected water pressure. Firstly, the interaction between injected water pressure and thermal stress during the injection of cryogenic fracturing fluid and its effect on crack initiation are studied by THM coupling analysis. Subsequently, a THM-D coupling simulation considering the meso-structure of rock is conducted to investigate the hydraulic fracturing process of hot dry rocks under different thermal conditions. The results show that the thermal stress which is generated by both temperature gradient in the rock itself and the non-uniform expansion of rock particles, appears as tensile stress around the wellbore. The high injection pressure will prohibit the initiation of multiple cracks induced by thermal stress and the existence of thermal stress around the wellbore will weaken the injection pressure. As the rise of rock temperature, more cracks emerge near the wellbore at the crack initiation phase and the propagation velocity of fracture network along the maximum geo-stress direction decreases while the reconstruction scale enlarges. Simultaneously, the generated multiple cracks also increase the crack extension pressure.
Crack initiation and propagation in hydraulic fracturing process of hot dry rocks is affected by thermal stress induced by the low temperature of injected cryogenic fracturing fluid as well as its injected water pressure. Firstly, the interaction between injected water pressure and thermal stress during the injection of cryogenic fracturing fluid and its effect on crack initiation are studied by THM coupling analysis. Subsequently, a THM-D coupling simulation considering the meso-structure of rock is conducted to investigate the hydraulic fracturing process of hot dry rocks under different thermal conditions. The results show that the thermal stress which is generated by both temperature gradient in the rock itself and the non-uniform expansion of rock particles, appears as tensile stress around the wellbore. The high injection pressure will prohibit the initiation of multiple cracks induced by thermal stress and the existence of thermal stress around the wellbore will weaken the injection pressure. As the rise of rock temperature, more cracks emerge near the wellbore at the crack initiation phase and the propagation velocity of fracture network along the maximum geo-stress direction decreases while the reconstruction scale enlarges. Simultaneously, the generated multiple cracks also increase the crack extension pressure.
引文
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[3]ZHANG Y J,GUO L L,LI Z W,et al.Electricity generation and heating potential from enhanced geothermal system in Songliao Basin,China:different reservoir stimulation strategies for tight rock and naturally fractured formations[J].Energy,2015,93:1860-1885.
[4]SUN Z X,ZHANG X,XüY,et al.Numerical simulation of the heat extraction in EGS with thermal-hydraulicmechanical coupling method based on discrete fractures model[J].Energy,2017,120:20-33.
[5]QU Z Q,ZHANG W,GUO T K.Influence of different fracture morphology on heat mining performance of enhanced geothermal systems based on COMSOL[J].International Journal of Hydrogen Energy,2017,42:18263-18278.
[6]RUTQVIST J,JEANNE P,DOBSON P F,et al.The northwest geysers EGS demonstration project,CaliforniaPart2:modeling and interpretation[J].Geothermics,2015,63:120-138.
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[8]赵阳升,万志军,张渊,等.岩石热破裂与渗透性相关规律的试验研究[J].岩石力学与工程学报,2010,29(10):1970-1976.ZHAO Yang-sheng,WAN Zhi-jun,ZHANG Yuan,et al.Experimental study of related laws of rock thermal cracking and permeability[J].Chinese Journal of Rock Mechanics and Engineering,2010,29(10):1970-1976.
[9]TOMAC I,GUTIERREZ M.Micro-mechanics of hydrothermo-mechanical fracture propagation in granite[R].Minneapolis:[s.n.],2014.
[10]郭亮亮.增强型地热系统水力压裂和储层损伤演化的试验及模型研究[D].长春:吉林大学,2016.GUO Liang-liang.Test and model research of hydraulic fracturing and reservoir damage evolution in enhanced geothermal system[D].Changchun:Jilin University,2016.
[11]ZHOU C B,WAN Z J,ZHANG Y,et al.Experimental study on hydraulic fracturing of granite under thermal shock[J].Geothermics,2018,71:146-155.
[12]门晓溪,唐春安,马天辉.水压致裂作用下岩体参数对裂纹扩展影响的数值模拟[J].东北大学学报(自然科学版),2013,34(5):700-703.MEN Xiao-xi,TANG Chun-an,MA Tian-hui.Numerical Simulation on Influence of rockmass parameters on fracture propagation during hydraulic fracturing[J].Journal of Northeastern University(Natural Science),2013,34(5):700-703.
[13]李连崇,李根,孟庆民,等.砂砾岩水力压裂裂缝扩展规律的数值模拟分析[J].岩土力学,2013,34(5):1501-1507.LI Lian-chong,LI Gen,MENG Qing-min,et al.Numerical simulation of propagation of hydraulic fractures in glutenite formation[J].Rock and Soil Mechanics,2013,34(5):1501-1507.
[14]李连崇.岩石破裂过程THM-D耦合数值模型及其应用研究[D].沈阳:东北大学,2006.LI Lian-chong.Investigation on numerical model of coupled thermo-hydro-mechanical-damage(THM-D)for rock failure process and associated application[D].Shenyang:Northeastern University,2006.
[15]朱万成,魏晨慧,田军,等.岩石损伤过程中的热-流-力耦合模型及其应用初探[J].岩土力学,2009,30(12):3851-3857.ZHU Wan-cheng,WEI Chen-hui,TIAN Jun,et al.Coupled thermal-hydraulic-mechanical model during rock damage and its preliminary application[J].Rock and Soil Mechanics,2009,30(12):3851-3857.
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[19]GHASSEMI A,ZHANG Q.A transient fictitious stress boundary element method for poro-thermo-elastic media[J].Engineering Analysis with Boundary Elements,2004,28(11):1363-1373.