Simulating the impact of complex fracture networks on the heat extraction performance of hot-dry rock masses

Abstract

The complex network of fractures formed by randomly distributed natural fractures in hot-dry rocks (HDRs) complicates the heat transfer regularity of injected fluid. On the basis of the fracture network, exploring the characteristics of the fluid flow and heat transfer as influenced by different parameters helps enable efficient resource extraction and effectively promotes the construction of diversified energy utilization structures. Accordingly, accounting for the effect of the thermal shock on the evolution of the permeability of the rock matrix, a thermo-hydro-mechanical (THM) coupling model is developed to analyze the influences of fracture network characteristics on the heat extraction performance of HDRs. In addition, a large-scale injection and production physical simulation experiment is performed using a newly developed, in-house, large-scale true triaxial experimental system. The corresponding numerical model is established and validated. The good agreement between the numerical and experimental results verifies the reliability and accuracy of the proposed THM model. Subsequently, a two-dimensional model is established under complex fracture network conditions, taking, as a research object, the natural fracture characteristics of HDR in the Qinghai Gonghe Basin in combination with the regional geological information. The effects of different parameters, including the production well location, rock matrix permeability, injection rate, initial fracture width, and number of fractures, on the production temperature and heat extraction performance are systematically analyzed. The results indicate that an increase in the number of fractures, the distance between the injection well and the production well, or the width of the initial fractures leads to an improved heat extraction performance. The number of fractures increased from 11 horizontal fractures and 22 high-angle fractures to 35 horizontal fractures and 70 high-angle fractures, with a 20% increase in heat extraction rate. While the influence of the rock matrix permeability is not highly significant, it cannot be ignored. It is crucial to select an injection rate that is neither too low nor too high, taking into consideration economic factors.