裂隙岩石渗流与传热的多尺度模拟及其在地热发电中的应用

The mechanism and characteristics of porous flow and heat transfer in fractured rock are systematically and thoroughly investigated in this study by employing a multiscale numerical modeling approach, with a focus of the connection to the real world problems of geothermal power generation. In the mesoscale simulation, the discrete element method is adopted to model the deformation in solid as well as the formation, enlargement, connection and closure behaviors of microfractures, while the lattice Boltzmann method is adopted to model the fluid flow in pores and fractures; the combination of these two methods provides a fully coupled simulation of solid deformation, fluid flow as well as heat conduction and convection. Based on the analysis and integration of the mesoscale modeling results, quantitative relations of parameters like permeability and thermal dispersion in fractured rock as functions of pressure, deviatoric stress and temperature are determined, and then the macroscale constitutive relations of the porous flow and heat transfer in fractured rock are derived, which form the basis for constructing finite element models to simulate the macroscale hydro-thermal behaviors of fractured rocks. In terms of multiscale modeling of the porous flow and heat transfer processes between injection and production wells in geothermal power plants, the effects of injection pressure, injection rate and natural convection on the geothermal reservoir temperature and its serving life will be thoroughly investigated, and the optimal distribution of the injection and production wells as well as the technological strategy for wisely and economically extracting deep geothermal resources will be accordingly explored.

本项目采用多尺度数值模拟的方法系统深入地研究裂隙岩石渗流与传热的机理和特征，并联系地热发电的实际应用问题作重点研究。细观模拟用离散元方法模拟固体变形和微裂隙形成、扩展、连通、闭合的行为，用格子波尔兹曼方法模拟流体在固体孔隙和裂隙中的流动；两者结合能够实现对固体变形、流体流动以及热传导和对流的完全耦合模拟。通过分析、概括细观模拟结果，确定裂隙岩石渗透率、热弥散系数等参数随压力、偏应力、温度变化的定量关系，进而构建裂隙岩石渗流和传热的宏观本构关系，并以此为基础建立宏观模拟裂隙岩石渗流与传热过程的有限元模型。基于对实际地热电站注水井与采热井之间渗流与传热过程的多尺度数值模拟，本项目将重点研究注水压力、速度以及自然对流对地热田温度变化和地热电站使用寿命的影响，探索注水井与采热井的优化布局以及经济合理地开采深部地热能的技术方案。

本项目采用多尺度数值模拟的方法系统深入地研究裂隙岩石渗流与传热的机理和特征，并联系地热发电的实际应用问题作重点研究。细观模拟用离散元方法模拟固体变形和微裂隙形成、扩展、连通、闭合的行为，用格子波尔兹曼方法模拟流体在固体孔隙和裂隙中的流动；两者结合能够实现对固体变形、流体流动以及热传导和对流的完全耦合模拟。宏观模拟以有限元模型为基础，系统模拟了地热田中注水井与采热井之间的渗流与传热过程，深入研究了注水压力、速度对地热田温度变化和地热电站使用寿命的影响，并相应地探索了注水井与采热井的优化布局以及经济合理地开采深部地热能的技术方案。针对增强型地热系统商业开发面临的实际挑战，提出了井阵式增强型地热系统的概念。井阵式增强型地热系统以注水井和采热井的共享为特征，能够有效地降低钻井成本并实现地热水流量最大化。在深入讨论增强型地热系统两维模拟条件的基础上，本研究对井阵式增强型地热系统的长期运行进行了两维有限元模拟。数值模拟结果的精度不但由网格、单元阶次、时间步长独立性的系统检验所定量表征，而且由本研究导出的两维孔隙流动解析解所验证。本研究表明两维模拟不但能够以低计算成本高精度地求解很多增强型地热系统问题，而且是定量估计三维数值模拟误差的有力工具。本研究得到的井阵式增强型地热系统的数值结果可以作为检验未来增强型地热系统模型的基准。