Numerical modelling of the cooling effect in geothermal reservoirs induced by injection of CO2 and cooled geothermal water

The utilization of geothermal energy can reduce CO2 emissions into the atmosphere. The reinjection of cooled return water from a geothermal field by a closed loop system is an important strategy for maintaining the reservoir pressure and prolonging the depletion of the geothermal reservoir by avoiding problems, e.g., water level drawdown, ground subsidence, and thermal pollution. However, the drawdown of water injectivity affected by physical and chemical clogging may occur in sandstone aquifers, and the reservoir temperature may be strongly affected by the reinjection of large amounts of cooled geothermal water, thus resulting in early thermal breakthrough at production wells and a decrease in production efficiency. In addition to the injection of cooled geothermal water, the injection of CO2 can be used to maintain the reservoir pressure and increase the injectivity of the reservoir by enhancing water–rock interactions. However, the thermal breakthrough and cooling effect of the geothermal reservoir may become complex when both CO2 and cooled geothermal water are injected into aquifers. In this paper, a simplified small-scale multilayered geological model is established based on a low-medium geothermal reservoir in Binhai district, Tianjin. The ECO2N module of the TOUGH2MP simulator is used to numerically simulate temperature and pressure responses in the geothermal reservoir while considering different treatment strategies (e.g., injection rates, temperatures, well locations, etc.). The simulation results show that a high injection pressure of CO2 greatly shortens the CO2 and thermal breakthrough at the production well. A much lower CO2 injection pressure is helpful for prolonging hot water production by maintaining the reservoir pressure and eliminating the cooling effect surrounding the production wells. Both pilot-scale and commercial-scale cooled water reinjection rates are considered. When the water production rate is low (2 kg/s), the temperature decrease at the production well is negligible at a distance of 500 m between two wells. However, when both the production and reinjection rates of cooled return water are increased to 100 m3/h, the temperature decrease in the production well exceeds 10 °C after 50 years of operation.