Numerical evaluation of thermal performance in an enhanced geothermal system by using impure CO2 as a working fluid

The utilization of impure CO2 as a working fluid in enhanced geothermal systems (EGS) has the potential to reduce the cost of CO2 purification and sequestration. However, the flow behavior and thermal performance of impure CO2 in high-temperature reservoirs are not well understood, which limits its potential for practical applications. This study investigates the migration behavior of impure CO2 in reservoirs and its thermal performance at various impurity levels via numerical simulations based on geological models of the Gonghe Basin. The findings revealed that the injection of impure CO2 changes the compositional distribution of the gas-water phase, causing N2 and O2 to accumulate at the reservoir top due to buoyancy while reducing the dissolved CO2 concentration in the aqueous phase as impurity levels rise. Water displacement drives three heat extraction phases, with durations unaffected by CO2 purity. With increasing impurity levels, the heat production rate and energy generated slightly increase, whereas the heat extraction efficiency is negatively correlated. Generally, the thermal performance of impure CO2 is comparable to pure CO2 but still inferior to water. Unlike that of water, the injection pressure of impure and pure CO2 gradually decreases with increasing heat production, leading to improved heat extraction efficiency. The injection rate significantly influences heat production, followed by reservoir permeability and injection temperature, the average heat production rate varies by 35.9%, 7.3%, and 1.1%, respectively. To enhance CO2 sequestration, low injection temperatures, higher injection rates, and lower reservoir permeability are recommended.