Numerical simulation study of enhanced geothermal system modification and heat mining performance under cyclic injection

Abstract

Enhanced geothermal systems (EGS) involve complex thermal–hydraulic-mechanical (THM) coupling. The reservoir's permeability evolution and heat extraction characteristics under cyclic variable pressure injection conditions are unknown. In this paper, a THM model that considered the dynamic changes of rock matrix and fluid properties was developed based on the theory of local heat balance. The THM coupling model investigated the effects of the number of fractures, initial permeability of fractures (kf), maximum injection pressure (P), frequency (td), and working fluid on the heat recovery performance of the EGS under cyclic injection for 20 years. Finally, the sensitivity of each parameter was discussed. When the number of fractures increased from 100 to 200, the temperature of the reservoir decreased by 68.5 K, and the cumulative heat energy extracted increased by 6.6 × 1014 J. With the increase of the initial permeability of the fractures, the irregularities in the temperature and permeability fields of the reservoir were significantly enhanced. When the maximum injection pressure increased from 34 MPa to 40 MPa, the permeability ratio of the reservoir increased by about 13.45 %, and the cumulative extracted thermal energy increased by 3.5 × 1014 J. The frequency of cyclic injection increased from 5 d to 45 d, the production temperature decreased by 26.78 K, and the permeability ratio of the reservoir and the cumulative extracted thermal energy increased. The low-temperature range of CO2-EGS is significantly larger than that of H2O-EGS, and the reservoir temperature decreases faster. The permeability of CO2-EGS increases by 29.32 %, and the cumulative thermal energy extraction increases by 17.5 × 1014 J compared with H2O-EGS. Parameter sensitivities were found for cyclic injection. Factors affecting production temperature and reservoir permeability in that order: P > td > kf. Injection frequency had a slightly greater impression on cumulative extracted thermal energy than injection pressure.