Simulation and optimization of enhanced geothermal systems using CO2 as a working fluid

Because of rising concerns about CO2 emissions from fossil fueled power plants, in recent years there has been strong emphasis on the development of safe and economical CCUS (Carbon Capture Utilization and Storage) technology. One such technology that shows some promise is EGS (Enhanced Geothermal System), where CO2 is used as a working fluid to extract heat from a geothermal reservoir. Permanent carbon sequestration is also achieved as a byproduct due to subsurface fluid losses throughout the life of the system. In this paper, numerical simulations of subsurface flow in EGS are conducted using the multi-phase flow solver TOUGH2 (Transport of Unsaturated Groundwater and Heat). An optimization code based on a multi-objective genetic algorithm is combined with TOUGH2 (designated as GA-TOUGH2) and modified for EGS application. Using GA-TOUGH2, the CO2 injection rate is optimized for both constant mass and constant pressure injection scenarios to manage the production temperature profile and to ensure that the heat extraction occurs for the entire life of the system thus allowing more efficient use of CO2. The results of this study show promise of EGS technology for consideration of deployment on a commercial scale.