Research on thermal recovery efficiency of dry hot rock based on a fully coupled thermal-hydro-mechanical model- Taking the GR1 well in Gonghe, Qinghai as an example

It is widely believed that developing deep dry hot rock resources is crucial for solving China's energy security issue and achieving its "dual carbon" goals. However, this type of energy is located in a complex geological environment with high temperature and pressure, making it challenging to extract. Therefore, it's essential to establish a thermal-hydro-mechanical coupling model that considers various physical factors to study the interaction and impact of enhanced geothermal systems (dry hot rock). This paper presents a fully coupled thermal-hydro-mechanical model, which takes into account reservoir deformation caused by fluid pressure and temperature changes, reservoir porosity and permeability, fault fracture opening that's affected by reservoir deformation, fluid properties controlled by temperature (density, viscosity, etc.), and heat conduction caused by fluid flow. The study aims to determine the effects of reservoir permeability and fault type on the development process of geothermal resources (dry hot rocks) under two typical production modes: constant flow rate and constant pressure. The research also discusses the applicability of four evaluation indicators. The results indicate that the reservoir permeability increases in the early stage of water injection in the water injection area, but decreases in the later stage. On the other hand, the reservoir permeability decreases in the pumping area due to the decrease in fluid pressure and temperature. The fault opening increases in the early stage due to the increase of fluid pressure, and decreases in the later stage due to the decrease of fluid pressure and temperature. In the constant pressure production mode, increasing the permeability of thermal reservoirs can significantly improve the efficiency of geothermal extraction. However, barrier-type faults can seriously reduce the efficiency of geothermal extraction. Under the constant flow production mode, a smaller permeability will bring higher intra-domain flow velocity, while under the condition of blocking faults, the inter-domain flow velocity is larger, and the impact of the two is not significant. Finally, the research shows that the average temperature of production wells, output heat power, heat recovery rate, and cumulative heat recovery are all applicable to constant pressure production conditions. However, for constant flow production, output heat power and heat recovery rate are more applicable.