Effects of lateral-well geometries on multilateral-well EGS performance based on a thermal-hydraulic-mechanical coupling model

Abstract

Our previous study (Song et al., 2018) presented a novel enhanced geothermal system (EGS) with multilateral wells to exploit geothermal energy, and illustrated that the multilateral-well EGS had a better heat extraction performance than conventional double-well EGS. However, to further improve the heat extraction performance of the multilateral-well EGS, the effects of lateral-well geometries on multilateral-well performance should be studied and the geometrical parameters should be optimized. In this paper, we use a thermal-hydraulic-mechanical (THM) coupling model with a complex discrete fracture network (DFN) to investigate the influences of lateral-well geometrical parameters on the multilateral-well EGS performance. The lateral-well geometrical parameters include the lateral-well length, number, spacing and diameter. The results indicate that the longer lateral well and larger well spacing are beneficial for enhancing the multilateral-well EGS performance. In terms of better heat extraction performance and lower pressure impedance, the 400 m well spacing is the optimizing value under the fracture density of 0.067 m−1. The lateral-well number has a complicated effect on the heat extraction process. A large number of lateral wells can improve swept volume and reduce pressure impedance, but simultaneously connect more fractures and accelerate thermal breakthrough. Therefore, the lateral-well number should be optimized according to the specific DFN and under the DFN in this paper, 6 lateral wells are the best for the multilateral-well EGS performance. When there is a rotation angle between injection and production lateral wells, it can postpone the thermal breakthrough and be beneficial for heat extraction. The influence of the lateral-well diameter on the EGS performance can be ignored. The results of this study provide good guidance for the lateral-well design of the multilateral-well EGS.