Study on the productivity and mechanism of physical field evolution of enhanced geothermal systems under different working fluid types and properties

The injected working fluid conditions directly affect the heat generation efficiency of a stimulated hot dry rock reservoir. Taking exploration well GR1 in the Gonghe Basin of Qinghai as the research object, a stochastic discrete fracture reservoir model was established around the main injection channel, and the productivity variation pattern of the enhanced geothermal system (EGS) and the spatiotemporal evolution mechanism of the reservoir fields were analyzed for CO2 and H2O working fluids. The interaction mechanism between the upper and lower rock formations and the reservoir during the heat mining process was discussed. This study obtained the following findings: (1) when the working fluid was CO2, after 20 years of heat recovery, the injection flow rate, output flow rate, and heat generation efficiency with a working fluid temperature of 60 °C reached 1.22 times, 1.18 times, and 1.92 times those with a working fluid temperature of 35 °C, respectively. The average subsidence and average geostress with the working fluid temperature of 60 °C were low, at only 90.61% and 95.96% of those with the working fluid temperature of 35 °C, respectively. However, high-temperature fluid injection increased flow loss. The changes in the various laws of H2O-EGS were similar to those of CO2-EGS. (2) When the working fluid temperature was 35 °C, after 20 years of heat recovery, the output flow rate and heat generation efficiency with the CO2-EGS reached 12.66 times and 1.28 times those with the H2O-EGS, respectively. However, the flow loss, average subsidence, and average geostress were higher with the CO2-EGS, reaching 6.58 times, 1.14 times, and 1.06 times those with the H2O-EGS, respectively. The patterns in these parameters observed at the other temperatures were similar to those observed at 35 °C. (3) The temperature decrease of the cushion layer was higher than that of the caprock, while the subsidence of the caprock was higher, and this phenomenon was more obvious when the working fluid temperature was lower. The conclusions obtained have important reference significance for the rational selection of working fluids.