Numerical Simulation for Enhanced Geothermal System Considering Thermal-Induced Fracture Permeability Variations Based on EDFM-DPDK

Abstract

For the economic development of hot dry rock (HDR), volume hydraulic fracturing has been successfully applied to artificially construct enhanced geothermal systems (EGS) and improve heat extraction efficiency. However, existing numerical simulation methods for EGS fail to adequately characterize the multi-scale media (i.e., matrix, natural fractures, and large-scale fractures) within the stimulated reservoir volume (SRV). This limitation leads to unreasonable predictions of both reservoir temperature distribution in the reservoir and EGS production performance. Given that the dual-porosity dual-permeability model (DPDK) implicitly characterizes natural fractures and the matrix, while the embedded discrete fracture model (EDFM) explicitly represents large-scale fractures, and both models exhibit high computational efficiency, this study develops a novel hybrid fracture model by integrating these two approaches to characterize multi-scale media. Additionally, an analytical method is employed to account for the effect of thermal stress on the permeability of fractures across different scales. Furthermore, a corresponding thermal-hydraulic coupling numerical simulation method is established based on the proposed hybrid fracture model and applied to the simulation of field-scale EGS. Numerical simulation results demonstrate that the hybrid fracture model effectively captures the temperature distribution across multi-scale media, confirming that fractures act as the dominant channel for heat extraction and the matrix serves as the primary heat storage zone. After considering the thermal effects, fracture permeability increases within the range of the low-temperature swept front, leading to an accelerating breakthrough of the low-temperature front, a faster decline in the production temperature and a lower final production temperature, while the injection pressure decreases significantly. The results and insights presented in this study provide significant theoretical support for advancing numerical simulation techniques and deepening the understanding of EGS production performance.