Whole process analysis of geothermal exploitation and power generation from a depleted high-temperature gas reservoir by recycling CO2

The geothermal resource in depleted high-temperature gas fields is abundant, and CO2 is more suitable to exploit geothermal energy from these gas fields due to its high mobility and thermal physical properties. However, all the related mechanisms, operation processes, and economic analyses have not been comprehensively analyzed yet. To assess the technical and economic feasibility of this method of geothermal exploitation, a 120 °C depleted gas reservoir was selected to build geological and numerical models for analyzing its gas composition, temperature, and pressure during the whole process, including enhanced gas recovery, pressure build-up, and pure geothermal exploitation, based on existing wells. The results reveal that the CO2 injection during EGR and pressure build-up can affect the reservoir temperature, and the optimization analyses indicate the heat mining rate can be maintained about 10 MWth for 30 years. The thermodynamic cycle analyses show that a power of 132.7 kW can be obtained if the organic Rankine cycle system with R134a is adopted, and the cost of geothermal power generation is about 0.45 $/(kW∙h) when the CO2 price is 12 $/t. However, if the produced CO2 directly drives the turbine, the power can increase to 718.5 kW and the cost reduces to 0.1$/(kW∙h).