The effect of <i>in situ</i> stress variation on heat extraction from enhanced geothermal systems (EGS): An experimental study

Enhanced geothermal systems (EGS) are susceptible to the formation of dominant flow paths and the occurrence of “thermal short-circuiting” during long-term heat extraction due to the combined effects of the heterogeneity in flow path conductivity and the evolution of the triaxial stress. To investigate the impact of stress evolution during the cyclic injection–production process on production well capacity and heat extraction, this study established a large-scale multi-well injection–production experimental model based on naturally fractured granite. By varying triaxial stress loading under cyclic injection conditions, phenomena of thermal short-circuiting and flow path shifts, which are consistent with those observed in the hot dry rock demonstration projects, were replicated. Throughout the experiment, real-time measurements of production temperature, flow rate, and production pressure differences were collected. Additionally, single-fracture contact tests using pressure-sensitive paper were conducted to quantify fracture surface contact and stress variations under different stress conditions. The experimental results demonstrate significant differences in production capacity between injection and production wells under low triaxial stress, and as the triaxial stress increases, the production capacity of different injection-production wells shifts, with production temperature and flow rate showing asymmetric variation. Based on the stress loading results of single fractures, it was observed that an increase in stress led to an expansion of contact area and a reduction in aperture, thereby suppressing the initial flow paths. Furthermore, pore pressure diffusion induced by cyclic injection reduced local effective stress, reactivating fractures in a subcritical state. This reactivation of fractures was identified as the main cause of the shift in dominant flow paths and the loss of heat transfer fluid between injection and production wells. This study reveals the significant influence of triaxial stress variation during cyclic injection–production on the thermal response, providing new experimental evidence and technical support for improving the heat extraction efficiency of EGS.