随机裂隙网络储层与井筒热流耦合数值模拟
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摘要
增强型地热系统(Enhanced Geothermal System,EGS)的渗流传热通道主要由注采井井筒与随机裂隙网络储层两大部分组成,过去对裂隙网络储层进行热流耦合模拟研究大多数都忽视了井筒壁的换热,导致模拟结果的准确性欠佳。为了更准确地评价EGS出力、寿命等性能指标,开展了井筒—随机裂隙网络储层热流耦合数值模拟研究,基于商业有限元软件COMSOL Multiphysics对井筒与储层的渗流场、温度场进行耦合求解,分析了影响EGS采出温度与热开采速率的各项要素。研究结果表明:(1)注、采井的开孔长度(L_0)对EGS的产能与寿命具有重要影响,400 m为最佳开孔长度,其EGS具有最佳出力与寿命;(2)在井筒壁上加保温材料可以有效提高开采初、前期的采出温度,减少热损失,提高开采速率;(3)随着开采的进行,注入井周围出现明显的低温区,并沿裂隙通道向采出井方向推移,这将导致系统达到开采寿命而衰竭,应停止开采一段时间后再进行热能开采;(4)裂隙渗透率、裂隙宽度等参数对开采速率的影响都呈现正相关性,参数值增大会提高开采速率、缩短开采年限。结论认为,井筒壁的换热对于EGS出力与寿命的综合评价具有重要的意义,考虑井筒壁热损失的井筒—热储耦合模拟能够实现对EGS的完整性评价。
The seepage and heat transfer channel of enhanced geothermal system(EGS) is mainly composed of the wellbore of injection and production wells and the random fracture network reservoirs. The previous thermal–hydraulic coupling simulation studies only focused on fracture network reservoirs, but ignored the heat transfer of wellbore wall, so the simulation results are less accurate. For more accurate evaluation of the performance indexes of EGS(e.g. output and life), the thermal–hydraulic coupling of wellbore and random fracture network reservoir was numerically simulated in this paper. Then, based on the commercial finite element software COMSOL Multiphysics, the coupling solution of seepage field and temperature field of wellbore and reservoir was conducted, and the factors affecting the recovery temperature and thermal mining rate of EGS were analyzed. And the following research results were obtained. First,the opening length(L_0) of injection/production wells has an important effect on the productivity and life of EGS. The optimum opening length is 400 m, and its corresponding EGS has the optimum output and life. Second, installing thermal insulation materials on the wellbore wall can effectively increase the recovery temperature at the beginning and early stage of mining, reduce the heat loss and improve the mining rate. Third, as the mining goes, obvious low temperature zones occur around the injection well and advance to the production well along the fracture channel, and consequently the system will reach the recovery life and become exhausted. In this case, the thermal energy recovery shall be continued after stopping for a period of time. Fourth, the influence of fracture permeability and thickness on the thermal mining rate is in positive correlation. The increase of parameter values will lead to the increase of thermal mining rate but the reduction of mining life. It is concluded that the heat transfer of wellbore wall is of great significance to the comprehensive evaluation on EGS output and life, and the integrity evaluation on EGS can be realized by virtue of the wellbore–geothermal reservoir coupling simulation considering wellbore wall heat transfer.
The seepage and heat transfer channel of enhanced geothermal system(EGS) is mainly composed of the wellbore of injection and production wells and the random fracture network reservoirs. The previous thermal–hydraulic coupling simulation studies only focused on fracture network reservoirs, but ignored the heat transfer of wellbore wall, so the simulation results are less accurate. For more accurate evaluation of the performance indexes of EGS(e.g. output and life), the thermal–hydraulic coupling of wellbore and random fracture network reservoir was numerically simulated in this paper. Then, based on the commercial finite element software COMSOL Multiphysics, the coupling solution of seepage field and temperature field of wellbore and reservoir was conducted, and the factors affecting the recovery temperature and thermal mining rate of EGS were analyzed. And the following research results were obtained. First,the opening length(L_0) of injection/production wells has an important effect on the productivity and life of EGS. The optimum opening length is 400 m, and its corresponding EGS has the optimum output and life. Second, installing thermal insulation materials on the wellbore wall can effectively increase the recovery temperature at the beginning and early stage of mining, reduce the heat loss and improve the mining rate. Third, as the mining goes, obvious low temperature zones occur around the injection well and advance to the production well along the fracture channel, and consequently the system will reach the recovery life and become exhausted. In this case, the thermal energy recovery shall be continued after stopping for a period of time. Fourth, the influence of fracture permeability and thickness on the thermal mining rate is in positive correlation. The increase of parameter values will lead to the increase of thermal mining rate but the reduction of mining life. It is concluded that the heat transfer of wellbore wall is of great significance to the comprehensive evaluation on EGS output and life, and the integrity evaluation on EGS can be realized by virtue of the wellbore–geothermal reservoir coupling simulation considering wellbore wall heat transfer.
引文
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[8]张树光,李志建,徐义洪,张传成.裂隙岩体流-热耦合传热的三维数值模拟分析[J].岩土力学,2011,32(8):2507-2511.Zhang Shuguang,Li Zhijian,Xu Yihong&Zhang Chuancheng.Three-dimensional numerical simulation and analysis of fluid-heat coupling heat-transfer in fractured rock mass[J].Rock and Soil Mechanics,2011,32(8):2507-2511.
[9]陈必光,宋二祥,程晓辉.二维裂隙岩体渗流传热的离散裂隙网络模型数值计算方法[J].岩石力学与工程学报,2014,33(1):43-51.Chen Biguang,Song Erxiang&Cheng Xiaohui.A numerical method for discrete fracture network model for flow and heat transfer in two-dimensional fractured rocks[J].Chinese Journal of Rock Mechanics and Engineering,2014,33(1):43-51.
[10]Xu Chaoshui,Dowd PA&Tian Zhaofeng.A simplified coupled hydro-thermal model for enhanced geothermal systems[J].Applied Energy,2015,140(2):135-145.
[11]Rutqvist J.Status of the TOUGH-FLAC simulator and recent application related to coupled fluid flow and crustal deformations[J].Computers&Geosciences,2011,37(6):739-750.
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[13]Zhao Yangsheng,Feng Zijun,Feng Zengchao,Yang Dong&Liang Weiguo.THM(thermo-hydro-mechanical)coupled mathematical model of fractured media and numerical simulation of a3D enhanced geothermal system at 573 K and buried depth 6000-7000 m[J].Energy,2015,82(3):193-205.
[14]Sun Zhixue,Zhang Xu,Xu Yi,Yao Jun,Wang Hanxuan,LüShuhuan,et al.Numerical simulation of the heat extraction in EGSwith thermal-hydraulic-mechanical coupling method based on discrete fractures model[J].Energy,2017,120(2):20-33.
[15]Kolditz O.Modelling flow and heat transfer in fractured rocks:Conceptual model of a 3D deterministic fracture network[J].Geothermics,1995,24(3):451-470.
[16]Juanes R,Samper J&Molinero J.A general and efficient formulation of fractures and boundary conditions in the finite element method[J].International Journal for Numerical Methods in Engineering,2002,54(12):1751-1774.
[17]赵春兰,殷慧敏,王兵,范翔宇,吴昊.基于结构方程与蒙特卡洛方法的钻井现场作业风险评价[J].天然气工业,2019,39(2):84-93.Zhao Chunlan,Yin Huimin,Wang Bing,Fan Xiangyu&Wu Hao.Risk assessment of drilling site operation based on the structural equation and Monte Carlo method[J].Natural Gas Industry,2019,39(2):84-93.
[18]Jiang Fangming,Luo Liang&Chen Jiliang.A novel three-dimensional transient model for subsurface heat exchange in enhanced geothermal system[J].Internatinal Communication in Heat and Mass Transfer,2013,41(11):57-62.
[19]Zeng Yuchao,Su Zheng&Wu Nengyou.Numerical simulation of heat production potential from hot dry rock by water circulating through two horizontal wells at Desert Peak geothermal field[J].Energy,2013,63(12):92-107.
[20]曹文炅,黄文博,蒋方明.地下热流固耦合对EGS热开采的影响[J].新能源进展,2015,3(6):444-451.Cao Wenjiong,Huang Wenbo&Jiang Fangming.The thermal-hydraulic-mechanical coupling effects on heat extraction of enhanced geothermal systems[J].Advances in New and Renewable Energy,2015,3(6):444-451.
[21]陈小凡,唐潮,杜志敏,汤连东,魏嘉宝,马旭.基于有限体积方法的页岩气多段压裂水平井数值模拟[J].天然气工业,2018,38(12):77-86.Chen Xiaofan,Tang Chao,Du Zhimin,Tang Liandong,Wei Jiabao&Ma Xu.Numerical simulation on multi-stage fractured horizontal wells in shale gas reservoirs based on the finite volume method[J].Natural Gas Industry,2018,38(12):77-86.
[22]王昌龙.考虑流体损失的增强型地热系统(EGS)数值模拟研究[D].合肥:中国科学技术大学,2017.Wang Changlong.Numerical simulation study of enhanced geothermal systems(EGS)considering fluid losses[D].Hefei:University of Science and Technology of China,2017.
[23]王维勇,黄尚瑶.地热基础理论研究[M].北京:地质出版社,1982.Wang Weiyong&Huang Shangyao.Geothermal basic theory research[M].Beijing:Geological Publishing House,1982.
[24]Dershowitz WS&Einstein HH.Characterizing rock joint geometry with joint system models[J].Rock Mechanics and Rock Engineering,1988,21(1):21-51.
[2]陈必光.地热对井裂隙岩体中渗流传热过程数值模拟方法研究[D].北京:清华大学,2014.Chen Biguang.Study on numerical methods for coupled fluid flow and heat transfer in fractured rocks of doublet system[D].Beijing:Tsinghua University,2014.
[3]王晓星,吴能友,苏正,曾玉超.增强型地热系统开发技术研究进展[J].地球物理学进展,2012,27(1):15-22.Wang Xiaoxing,Wu Nengyou,Su Zheng&Zeng Yuchao.Progress of the enhanced geothermal systems(EGS)development technology[J].Progress in Geophysics,2012,27(1):15-22.
[4]Hayashi K,Willis-Richards J,Hopkirk RJ&Niibori Y.Numerical models of HDR geothermal reservoirs--a review of current thinking and progress[J].Geothermics,1999,28(4):507-518.
[5]赵阳升,万志军,康建荣.高温岩体地热开发导论[M].北京:科学出版社,2004.Zhao Yangsheng,Wan Zhijun&Kang Jianrong.Introduction to geothermal exploitation of high temperature rock mass[M].Beijing:Science Press,2004.
[6]高诚,计秉玉,张汝生,牛骏,张乐.流体力学数值模拟方法在增强型地热系统的应用分析[J].流体动力学,2017,5(2):47-55.Gao Cheng,Ji Bingyu,Zhang Rusheng,Niu Jun&Zhang Le.The application study of fluid dynamic numerical simulation methods on enhanced geothermal system[J].International Journal of Fluid Dynamics,2017,5(2):47-55.
[7]孙致学,徐轶,吕抒桓,徐杨,孙强,蔡明玉,等.增强型地热系统热流固耦合模型及数值模拟[J].中国石油大学学报(自然科学版),2016,40(6):109-116.Sun Zhixue,Xu Yi,LüShuhuan,Xu Yang,Sun Qiang,Cai Mingyu,et al.A thermo-hydro-mechanical coupling model for numerical simulation of enhanced geothermal systems[J].Journal of China University of Petroleum(Edition of Natural Science),2016,40(6):109-116.
[8]张树光,李志建,徐义洪,张传成.裂隙岩体流-热耦合传热的三维数值模拟分析[J].岩土力学,2011,32(8):2507-2511.Zhang Shuguang,Li Zhijian,Xu Yihong&Zhang Chuancheng.Three-dimensional numerical simulation and analysis of fluid-heat coupling heat-transfer in fractured rock mass[J].Rock and Soil Mechanics,2011,32(8):2507-2511.
[9]陈必光,宋二祥,程晓辉.二维裂隙岩体渗流传热的离散裂隙网络模型数值计算方法[J].岩石力学与工程学报,2014,33(1):43-51.Chen Biguang,Song Erxiang&Cheng Xiaohui.A numerical method for discrete fracture network model for flow and heat transfer in two-dimensional fractured rocks[J].Chinese Journal of Rock Mechanics and Engineering,2014,33(1):43-51.
[10]Xu Chaoshui,Dowd PA&Tian Zhaofeng.A simplified coupled hydro-thermal model for enhanced geothermal systems[J].Applied Energy,2015,140(2):135-145.
[11]Rutqvist J.Status of the TOUGH-FLAC simulator and recent application related to coupled fluid flow and crustal deformations[J].Computers&Geosciences,2011,37(6):739-750.
[12]赵阳升,王瑞凤,胡耀青,万志军,谢耀社.高温岩体地热开发的块裂介质固流热耦合三维数值模拟[J].岩石力学与工程学报,2002,21(12):1751-1755.Zhao Yangsheng,Wang Ruifeng,Hu Yaoqing,Wan Zhijun&Xie Yaoshe.3D numerical simulation for coupled THM of rock matrix-fractured media in heat extraction in HDR[J].Chinese Journal of Rock Mechanics and Engineering,2002,21(12):1751-1755.
[13]Zhao Yangsheng,Feng Zijun,Feng Zengchao,Yang Dong&Liang Weiguo.THM(thermo-hydro-mechanical)coupled mathematical model of fractured media and numerical simulation of a3D enhanced geothermal system at 573 K and buried depth 6000-7000 m[J].Energy,2015,82(3):193-205.
[14]Sun Zhixue,Zhang Xu,Xu Yi,Yao Jun,Wang Hanxuan,LüShuhuan,et al.Numerical simulation of the heat extraction in EGSwith thermal-hydraulic-mechanical coupling method based on discrete fractures model[J].Energy,2017,120(2):20-33.
[15]Kolditz O.Modelling flow and heat transfer in fractured rocks:Conceptual model of a 3D deterministic fracture network[J].Geothermics,1995,24(3):451-470.
[16]Juanes R,Samper J&Molinero J.A general and efficient formulation of fractures and boundary conditions in the finite element method[J].International Journal for Numerical Methods in Engineering,2002,54(12):1751-1774.
[17]赵春兰,殷慧敏,王兵,范翔宇,吴昊.基于结构方程与蒙特卡洛方法的钻井现场作业风险评价[J].天然气工业,2019,39(2):84-93.Zhao Chunlan,Yin Huimin,Wang Bing,Fan Xiangyu&Wu Hao.Risk assessment of drilling site operation based on the structural equation and Monte Carlo method[J].Natural Gas Industry,2019,39(2):84-93.
[18]Jiang Fangming,Luo Liang&Chen Jiliang.A novel three-dimensional transient model for subsurface heat exchange in enhanced geothermal system[J].Internatinal Communication in Heat and Mass Transfer,2013,41(11):57-62.
[19]Zeng Yuchao,Su Zheng&Wu Nengyou.Numerical simulation of heat production potential from hot dry rock by water circulating through two horizontal wells at Desert Peak geothermal field[J].Energy,2013,63(12):92-107.
[20]曹文炅,黄文博,蒋方明.地下热流固耦合对EGS热开采的影响[J].新能源进展,2015,3(6):444-451.Cao Wenjiong,Huang Wenbo&Jiang Fangming.The thermal-hydraulic-mechanical coupling effects on heat extraction of enhanced geothermal systems[J].Advances in New and Renewable Energy,2015,3(6):444-451.
[21]陈小凡,唐潮,杜志敏,汤连东,魏嘉宝,马旭.基于有限体积方法的页岩气多段压裂水平井数值模拟[J].天然气工业,2018,38(12):77-86.Chen Xiaofan,Tang Chao,Du Zhimin,Tang Liandong,Wei Jiabao&Ma Xu.Numerical simulation on multi-stage fractured horizontal wells in shale gas reservoirs based on the finite volume method[J].Natural Gas Industry,2018,38(12):77-86.
[22]王昌龙.考虑流体损失的增强型地热系统(EGS)数值模拟研究[D].合肥:中国科学技术大学,2017.Wang Changlong.Numerical simulation study of enhanced geothermal systems(EGS)considering fluid losses[D].Hefei:University of Science and Technology of China,2017.
[23]王维勇,黄尚瑶.地热基础理论研究[M].北京:地质出版社,1982.Wang Weiyong&Huang Shangyao.Geothermal basic theory research[M].Beijing:Geological Publishing House,1982.
[24]Dershowitz WS&Einstein HH.Characterizing rock joint geometry with joint system models[J].Rock Mechanics and Rock Engineering,1988,21(1):21-51.