基于离散裂缝模型的CO2增强型地热系统THM耦合数值模拟

Numerical simulation for heat extraction of CO2-EGS with thermal-hydraulic-mechanical coupling method based on discrete fracture models

增强型地热系统(EGS)是实现干热岩地热能资源开发利用的关键技术。以二氧化碳为载热工质的新型增强型地热系统(CO2-EGS)具有节能、高效及温室气体减排的多重优势。应用分形几何理论构建考虑裂缝长度、密度和连通性不同参数组合的离散裂缝模型,基于局部非等温热平衡理论建立CO2在裂缝系统内渗流-传热-应力多物理场耦合数学模型,并进行有限元数值求解。结果表明热储裂缝特征参数、空间拓扑结构和注采压力是影响CO2-EGS运行寿命及采热效率的关键因素;高穿透系数裂隙系统匹配高注采压差可获得较高采热效率,而低穿透系数裂隙系统采热效率随注采压差升高而降低;裂隙静态特征参数与注采井压差的合理匹配是实现CO2-EGS采热效率最大化的关键因素。

Enhanced geothermal system(EGS)is an important technology for the development and utilization of geothermal energy in hot dry rocks.Novel EGS using CO2(CO2-EGS)has the advantages of energy saving,high efficiency and greenhouse gas emission reduction.In this study,based on the fractal geometry theory,discrete fracture models with different fracture length,intensity and connectivity were established.Considering the local thermal non-equilibrium principle,a numerical approach for CO2-EGS to simulate the thermal-hydraulic-mechanical(THM)coupling processes was presented and solved by the finite element method.The results show that the characteristic parameters of thermal reservoir fractures,spatial topological structure and injection-production pressure gradient play critical roles in the heat extraction efficiency and service life of the CO2-EGS process.Furthermore,CO2-EGS with a high connectivity between injector and producer can be highly efficient in heat extraction under high pressure gradient,but a poor connectivity can have a rapid decline in heat extraction with the pressure gradient increasing.Reasonable matching between the static characteristic parameters of fractures and the injection-production pressure difference is the key factor for maximizing the heat production efficiency of the CO2-EGS process.