Thermal-hydraulic-mechanical-chemical modeling and simulation of an enhanced geothermal system based on the framework of extended finite element methods - Embedded discrete fracture model

Abstract

The thermo-hydro-mechanical-chemical (THMC) coupling process plays a crucial role in Enhanced Geothermal System (EGS), particularly during long-term heat extraction. However, most models for fractured reservoirs are computationally expensive, with very dense grids required, especially near fractures, limiting their practical use in EGS research. The embedded discrete fracture model (EDFM) can construct fracture and matrix grids independently, significantly reducing computational complexity while maintaining high accuracy. Although it is suitable for efficiently evaluating the long-term performance of heat extraction in fractured reservoirs, there are few THMC-coupled models based on EDFM, and THMC coupling effects have often been overlooked in evaluations of the environmental benefits of EGS. To address this, a fully THMC-coupled model based on EDFM and the extended finite element method (XFEM) is proposed and verified. The evolution of heat production and environmental benefits of an EGS over 30 years are presented. The results show that the production temperature and electric power remain highly efficient in the first 5 years and decrease by 7.8% and 15.4% respectively over 15 years. Moreover, the total amount of extracted heat during this period is 6.52 × 1012 kJ, which is equivalent to saving 22.26 × 108 kg of coal consumption and reducing CO2 emissions by 5.83 Mt, SO2 emissions by 189.22 × 105 kg, and NOx emissions by 166.96 × 105 kg. These findings highlight the significant potential of geothermal energy for energy supply and environmental benefits. Additionally, the study investigates the effects of critical operating and physical parameters on the production performance of EGS, including injection temperature, injection flow rate, initial reservoir temperature, and initial reservoir permeability. This work serves as a reference for both the theoretical THMC coupled model and practical EGS engineering.