Insight into the investigation of heat extraction performance affected by natural fractures in enhanced geothermal system (EGS) with THM multiphysical field model

Geothermal resources have important value for its development and utilization. In this work, a thermal-hydraulic-mechanical (THM) coupled numerical model is developed to simulate the heat extraction process in an enhanced geothermal system (EGS). In particular, the matrix permeability enhancement is related to thermal unloading, and the fracture conductivity is determined by the normal effective stress state. Then, a theoretical solution and a field measurement are selected to validate the accuracy/practicality of the THM coupled model. Afterwards, two simulation cases are designed to discuss the effect of natural fractures on the heat extraction performance. Firstly, the EGS contains only one natural fracture. The heat exchange and the temperature distribution in the natural fracture and rock matrix are greatly affected by the location of the natural fracture. Secondly, the EGS contains multiple natural fractures. The results indicate that more natural fractures improve the performance of heat exchange in middle natural fracture and generate a larger cooling area in the rock matrix. With an increase in the fracture number in the EGS, the influence of the included angle between the hydraulic fracture and natural fracture on the production temperature becomes more significant, and the heat extraction efficiency is greatly improved.