Numerical analysis and economic evaluation of pinnate horizontal well for supercritical CO2-based enhanced geothermal system

Abstract

Supercritical carbon dioxide (scCO2) is considered an alternative heat carrier for enhanced geothermal system due to its superior thermophysical property. Extensive investigations have been conducted to improve the heat extraction performance of scCO2-based EGS. However, few investigations have focused on the response mechanism of thermal reservoir for the scCO2-based EGS with fully coupled thermal-hydraulic-mechanical (THM) method. The evaluation differences between THM and TH coupling model are still not clear for the scCO2-based EGS. To further improve the heat extraction performance of scCO2-based EGS, a pinnate horizontal well (PHW) EGS is proposed in present work. Three-dimensional (3D) model with fully coupled thermal-hydraulic-mechanical (THM) is established considering the in-situ stress, which is used to evaluate the geothermal energy utilization and economic performance of PHW EGS. The effects of injection-production parameters, reservoir properties and fracture parameters on the heat extraction performance of PHW are probed. The results revealed that reservoir permeability shows a descending tendency while fracture aperture experiences a significant increase, when the effects of in-situ stress are taken into account. If the in-situ stress is ignored, the production temperature and maximum useful work will be overestimated by about 13 K and 16.4%, respectively. PHW EGS is characterized by the low pump consumption, high system efficiency and net revenue. Compared with doublet EGS, pump power is decreased by 58.1%, the system efficiency is boosted by 2.6 times, and the net revenue is increased by 35.3% at the terminal stage. It has been proved that the heat extraction performance is evidently affected by the injection flow rate and temperature of working fluid, initial reservoir permeability, initial aperture and maximum closure. Appropriate operation strategy can effectively improve the performance of PHW EGS. In addition, the optimized average electricity price, accumulative income and net revenue of PHW EGS can reach to 0.251 CNY/(kW·h), 9.27 × 108 CNY and 6.65 × 108 CNY after 40 years of operation, respectively.