Numerical simulation study of intermittent heat extraction from hot dry rock using horizontal well based on thermal compensation

Hot Dry Rock (HDR) is an important renewable and clean energy source. In this study, we investigated the feasibility of intermittent heat extraction in UCLGS (U-shaped Closed-Loop Geothermal Systems) for the devel-opment of HDR. We explored the optimal timing of well shut-in and well opening for intermittent heat extraction. The reservoir temperature response characteristics and ground temperature recovery law during the production process were analysed in detail. On this basis, we explored the factors affecting the duration of intermittent heat extraction cycle. Finally, the performance and production characteristics of continuous and intermittent heat extraction were compared and evaluated. The results showed that the optimal timing of well shut-in was judged by the slope of the reservoir temperature drop curve around-1 x 10-8. The durations of the heat-extraction and stop-injection periods were positively correlated with the injection rate and reservoir tem-perature, and negatively correlated with the injection temperature. When the injection rate, the injection tem-perature and reservoir temperature were 1 kg/s, 60 degrees C and 236 degrees C, respectively, the temperature dropped to 205.18 degrees C in 12 years of heat extraction and rose to 227 degrees C in 5 years of thermal compensation, which was 96.2% of the original temperature and can provide energy sustainably. Compared to continuous heat extraction, intermittent heat extraction under this study conditions can extend the operational life by 10 years, increase the average heat production rate by 3.95 KW, and increase the accumulated heat by 34.6%. The findings of this study contribute to a better understanding of the potential of intermittent heat extraction for the development of HDR.