增强地热系统中多孔热储层内传热传质的分形分析

增强地热系统可能是将来地热能利用的主要发展方向。在增强地热系统中，压裂干热岩以形成人工热储层和如何最大化地提取人工热储层热能是其中的关键技术，研究人工热储层这类多孔介质的裂隙网络及其传热传质特性，对提高增强地热系统的采热效率和延长寿命至关重要。本项目结合分形理论，对人工热储层裂隙网络结构进行描述和研究分析，通过理论推导和数值模拟实验对其进行优化，以期给压裂过程提供理论依据和技术指导；建立裂隙-支撑剂双重多孔介质模型，系统分析裂隙和支撑剂形状、大小、分布等对传热传质特性的影响；探讨在增强地热系统中采用硅凝胶防止流动"短路"的方法，延长流体驻留储热层时间，加强流固换热，以提高采热效率；并拟研究分析不同采热工质的优劣。

Enhanced geothermal system (EGS) has the potential to become a major means for future utilization of geothermal energy. There may exist two key steps: how to fracture the subsurface hot dry rock to form an artificial geothermal reservoir, and how to maximize heat extraction from the reservoir. The fractured porous network of and the involved heat and mass transfer inside the reservoir are identified to be critically important to the heat extraction efficiency and the life-time of the EGS. By virtue of fractal theory, we propose to conduct a systematic study on the fractured porous network of the artificial geothermal reservoir. We will describe the structure of porous heat reservoir and seek for an optimized structure via theoretical derivation and numerical simulation. Results obtained are deemed to provide theoretical basis and technical support for practical EGS enginneering. We will establish a dual-component fractal model for the porous system consisting of reservoir and proppant additives, and analyze the impacts of the proppant shape, size, and distribution on the heat and mass transfer process. We will also explore an emerging technique, using silica gel to prevent short-circuiting of fluid flow and thus to increase the residence time of heat transmission fluid in the reservoir for better heat extraction efficiency. Finally, we will analyze and compare advantages and disadvantages of EGSs using different heat transmission fluids (such as water and supercritical CO2)。