Numerical evaluation of heat extraction performance in enhanced geothermal system considering rough-walled fractures

Abstract

Fracture surface roughness has a significant effect on fluid flow and heat transfer characteristics in fractured rocks. Constructing the rough-walled fracture networks in the numerical modeling of Enhanced Geothermal System (EGS) can reflect the geothermal reservoir more reasonably. This study focuses on the role of rough-walled fractures in the modeling of EGS compared with parallel plates. A total of 144 geothermal reservoir models based on the discrete fracture network (DFN) method are modeled in consideration of different fracture orientations, model sizes, and degrees of aperture heterogeneity. Thermal recovery performance of EGS is numerically evaluated with thermal-hydraulic-mechanical (THM) coupling method. Numerical results indicate that the peak value of heat extraction rate in the parallel-plate model appears earlier and larger, the total heat extraction is always higher, and the duration of heat recovery process is shorter. Two ratio indices related to the duration and total energy of heat extraction process are proposed to quantitatively evaluate the differences between the models of parallel-plates and rough-walled fractures. The results show that the rough-walled effect is mainly affected by the mechanical aperture and roughness distribution of the fracture network and the fracture orientation is of great significance to the thermal recovery performance of EGS.