深部EGS系统中裂隙岩体热-水-力(THM)耦合机理研究

Enhanced geothermal systems (EGS) is a new type of geothermal power technology, that is heat extraction from the artificial reservoir established in the low permeable hot rock underground 3-10km. EGS technologies depends on the development of research on coupled thermal-hydrologic-mechanical (THM) processes of hot fractured rock masses. At present, the numerical methods used in the study of fractured rock masses fall into two categories: the implicit (or equivalent continuum) approach, in which the influence of fractures is treated equivalently, and the rock masses containing fractures is considered as one homogeneous medium, neglecting the exact position and direction of fractures; the other one is the explicit (or discrete) approach, in which the fractures are simulated discretely by one special element, demanding enormous time, patience, and energy, and the topological structures of elements might be abnormal evidently, shown distortedly, which would bring unfavorable error for the calculation. Based on the composite element method, extending the "filled model", applying the iterative algorithm, the coupled THM composite element model of hot fractured rock masses is established in this project, with considering the effect of normal stress and shear stress on the deformation and permeability of rock fractures, the effect of thermal on the physical and mechanical characteristics of rock masses.The proposed coupled model is aimed at reflecting the characteristics of fractures actually, such as permeability, deformation, and heat conduction, with simple preprocess work. Laboratory rock triaxial test with high temperature and high pressure is carried out, and the proposed numerical model can be further modified and improved on the basis of experimental results. Finally the numerical experiments and parameters back analysis of EGS system are carried out. This project is intended to reveal the coupled THM mechanism of hot fractured rock masses in deep EGS systems, and provide a theoretical foundation and scientific basis for the EGS technologies development.

增强地热系统(EGS)是指从地下3-10km低渗透性高温岩体中建立人工热储并获取热能的系统。EGS系统开发依赖于裂隙岩体热-水-力(THM)耦合模型研究水平的提高。目前针对裂隙岩体的数值模型主要是等效模拟和离散模拟两大类，前者将裂隙特性均化在岩体中，细节模拟过于简化，后者裂隙的前处理工作繁琐复杂，网格易畸形，导致计算误差大或结果不收敛。本申请项目拟将基于复合单元法，拓展"充填模型"，考虑压剪联合作用对裂隙变形和渗透特性的影响，以及温度对岩体物理力学特性的影响，采用交叉迭代算法建立高温裂隙岩体THM耦合的复合单元模型，其前处理简便快捷，且可真实反映裂隙的渗透、变形和导热特性；进行实验室高温高压岩体三轴试验，依据试验对耦合模型进行修正与完善；最后针对EGS系统进行数值试验和参数反演。本项目旨在深入揭示深部EGS系统中裂隙岩体THM耦合演化机理，为EGS系统开发提供理论基础和科学依据。

传统能源日渐枯竭，“可再生”清洁的增强型地热能日益受到重视，如何科学地进行增强地热系统(EGS)的开发依赖于裂隙岩体热-流-固(THM)耦合机理的研究水平。本项目基于复合单元法，结合“充填模型”，考虑应力对裂隙变形和渗透特性的影响，考虑温度对裂隙中流体运动粘度的影响，以及温度对岩体物理力学特性的影响，分别采用虚功原理和变分原理，推导相应的岩块子单元和裂隙片段的控制方程，联合其他复合单元或有限单元形成研究域的整体控制方程。若某复合单元内将不含裂隙，则此复合单元自动回归成常规有限单元，可有效实现有限单元与复合单元算法的统一。采用交叉迭代算法建立裂隙岩体THM耦合的复合单元算法。该算法可真实反映裂隙与相邻岩块之间的流量、热能量交换规律、以及流体与相邻岩块间渗流-传热过程，同时其前处理简便快捷，网格剖分不受限制。首先视研究域为均匀介质建立有限元计算网格，忽略裂隙和孔隙的具体位置和方位；然后应用前处理程序，可依据裂隙的真实信息自动将其离散在单元内，生成含裂隙和孔隙的复合单元的计算网格信息。通过与传统有限元算法、已有近似解析解、以及相应的实验数据进行相比较，验证本项目建立的复合单元耦合算法的可靠性与有效性。依据本项目建立的裂隙岩体THM耦合算法可对深部EGS系统进行全程数值仿真，可直观、准确的反映人工热储中裂隙的变化特征、开发过程中热能的迁移规律，为EGS系统的“智能化”开发奠定理论依据。