Geothermal energy exploitation from depleted high-temperature gas reservoirs by recycling CO2: The superiority and existing problems

CO2 can be used as an alternative injectant to exploit geothermal energy from depleted high-temperature gas reservoirs due to its high mobility and unique thermal properties. However, there has been a lack of systematic analysis on the heat mining mechanism and performance of CO2, as well as the problems that may occur during geothermal energy exploitation at specific gas reservoir conditions. In this paper, a base numerical simulation model of a typical depleted high-temperature gas reservoir was established to simulate the geothermal energy exploitation processes via recycling CO2 and water, with a view to investigate whether and/or at which conditions CO2 is more suitable than water for geothermal energy exploitation. The problems that may occur during the CO2-based geothermal energy exploitation were also analyzed along with proposed feasible solutions. The results indicate that, for a depleted low-permeability gas reservoir with dimensions of 1000 m × 500 m × 50 m and temperature of 150 °C using a single injection-production well group for 40 years of operation, the heat mining rate of CO2 can be up to 3.8 MW at a circulation flow rate of 18 kg s−1 due to its high mobility along with the flow path in the gas reservoir, while the heat mining rate of water is only about 2 MW due to limitations on the injectivity and mobility. The reservoir physical property and injection-production scheme have some effects on the heat mining rate, but CO2 always has better performance than water at most reservoir and operation conditions, even under a high water saturation. The main problems for CO2 circulation are wellbore corrosion and salt precipitation that can occur when the reservoir has high water saturation and high salinity, in which serious salt precipitation can reduce formation permeability and result in a decline of CO2 heat mining rate (e.g. up to 24% reduction). It is proposed to apply a low-salinity water slug before CO2 injection to reduce the damage caused by salt precipitation. For high-permeability gas reservoirs with high water saturation and high salinity, the superiority of CO2 as a heat transmission fluid becomes obscure and water injection is recommended