Strategic approach to EGS working fluid selection: Performance assessment of water and CO2

Enhanced Geothermal Systems (EGS) offer a sustainable approach to clean energy production and play a critical role in achieving global net-zero emission goals. The selection of working fluids in EGS is crucial for optimizing energy recovery and ensuring sustainable energy production. While water has historically served as the primary working fluid in EGS, carbon dioxide (CO2) is emerging as an attractive alternative since contemporary research increasingly claims its superior performance under specific conditions. This study evaluates the relative performance of water and CO2 as EGS working fluids across various scenarios. Two operational approaches of mass rate control and pressure control were employed to understand performance behavior. Water consistently outperforms CO2 in mass rate-controlled cases. However, CO2 performs better under pressure-controlled conditions, particularly in reservoirs with high permeability. This modest improvement in performance requires managing substantially higher fluid throughput, which poses significant operational challenges. Furthermore, handling large volumes of CO2 demands advanced infrastructure and incorporates environmental risks, including increased chances of leakage issues and induced seismicity. The study highlights the influence of reservoir and operational parameters such as rock permeability, fracture spacing, and pressure drawdown on the efficiency of EGS working fluids. While CO2 is highly sensitive to these parameters and excels in specific conditions, water demonstrates reliable performance across a broader range of scenarios. These findings provide practical insights for selecting the appropriate working fluid and designing operational strategies. Consequently, the study contributes to the advancement of geothermal energy systems and supports sustainable development goals.