增强型地热系统热流固耦合模型及数值模拟

A thermo-hydro-mechanical coupling model for numerical simulation of enhanced geothermal systems

增强型地热系统(EGS)利用水力压裂地下高温岩体形成人工热储,通过载热流体循环提取干热岩(HDR)所存储的地热能,其开采过程包含渗流、热能交换及岩体介质变形,为典型的热流固(THM)耦合问题。将裂隙岩体视作基于离散裂隙网络和基质岩体的双重介质,给出THM耦合的数学模型,基于商业有限元软件COMSOL Mutilphysics进行二次开发,实现裂隙岩体温度场、渗流场和应力场的全耦合求解。利用一个已知解析解的算例验证耦合模型和全耦合求解方法的正确性。最后利用随机生成的二维裂隙网络模型模拟EGS的运行过程,分析干热岩储层内渗流、温度、应力及变形的分布规律。计算结果表明,储层内的贯通裂隙构成主要导水区,水的对流传热作用明显;高压水注入和温度变化导致岩体裂隙发生位移,改变储层的传输特性,进而影响地热能的提取;考虑THM耦合作用对于研究增强型地热系统的的利用效率和运行规律非常有必要。

In the enhanced geothermal system （ EGS） , an artificial fracture network can be formed in the geothermal reservoir by hydraulic fracturing for heat transmission fluids to be circulated in the reservoir to extract heat from hot dry rocks （ HDR） , which involves complex thermo-hydro-mechanical （ THM） coupling processes in the fractured rock matrix. Therefore, THM coupling analysis is of great importance for studying the performance of the EGS. A THM coupling model was presented in this paper, in which the fractured rock was regarded as a dual medium （discrete fracture networks and matrix rock）, and a fully-coupling finite element analysis was implemented in a commercial software, COMSOL multiphysics, for analysis of the temperature, pressure and stress distributions in the fractured rock system during geothermal recovery. The coupling model and the numerical approach were verified in comparison with a proven analytical solution method. The model was used to simulate the EGS process with a 2D randomly generated fracture network to study the characteristics of flow, heat transfer and mechanical behaviors in a HDR reservoir. The results show that the connected fractures in the reservoir form the main flow pathways for fluid circulation, in which heat convection is essential for heat transfer. High pressure water injection and fracture extension induced by temperature variation can increase the conductivity of the fractured rock matrix, thus affecting the heat extraction. It is important to consider the THM coupling effects for studying the efficiency and performance of the EGS process.