Evaluating the exploitation and utilization prospects of hot dry rock in Bohai Bay Basin: Understanding based on numerical simulations for exploitation optimization

The Bohai Bay Basin, characterized by high terrestrial heat flow, formation temperatures average of 175℃ at a depth of 5000 m, and reservoirs with lithologies dominated by low-porosity and low-permeability metamorphic and igneous rocks, meets the requirements for the formation of hot dry rock (HDR) resources. Based on geothermal parameters such as terrestrial heat flow, rock thermal conductivity, and heat production rate, this study established a three-dimensional HDR exploitation model with thermo-hydro coupling using the COMSOL software. Furthermore, this study analyzed the effects of spacing between production and reinjection wells, reinjection and production rates, and well arrangement mode on the temperature evolution of the geothermal reservoir with exploitation time within 100 a. Accordingly, it selected the optimal scheme and estimated the potential HDR resources. Key findings are as follows: (1) Under certain reinjection and production rates, the rate of decline in production well water temperature over time is inversely proportional to injection and exploitation well spacing. (2) Under a certain well spacing, higher reinjection and production rates were associated with a quicker decrease in the water temperature of production wells and earlier thermal breakthroughs. (3) Regarding well arrangement mode, the HDR exploitation using two production wells and two reinjection wells yielded more heat and higher efficiency than that using one production well and one reinjection well providing other conditions remained unchanged. Based on these results, the optimal HDR exploitation scheme was determined, consisting of an exploitation duration of 50 a, a well spacing of 400 m, reinjection and production rates of 90 m³/h, and well arrangement mode comprising two production wells and two reinjection wells. Using this scheme, the average water temperature of the production wells will be 172℃, corresponding to total recoverable resources of 3.28×10¹⁹ J/a of the entire Bohai Bay Basin. For instance, in Renqiu City of Hebei Province, which falls within the delineated HDR favorable area, the heating demands of the entire city can be met using merely 157.75 km² of HDR favorable area if the optimal HDR exploitation scheme is adopted, as calculated based on a heat load index of residential buildings of 100 W/m². Therefore, the exploitation and utilization of HDR resources in the study area can enhance the energy supply and guarantee capabilities of North China, contributing to the creation of a sustainable, green, and low-carbon energy system.