How different parameters influence the efficiency of heat extraction using deep petrothermal multi-frac systems for central heating purposes

Abstract This study evaluates the thermal output characteristics of deep petrothermal multi-frac enhanced geothermal systems (multi-frac EGS) in the context of district heating for medium-sized cities. In contrast to conventional studies that focus on high-temperature reservoirs (well above 100 °C) for electricity generation, this work investigates a medium-temperature regime around 100 °C, with an emphasis on geothermal energy utilization for heating purposes. An analytical model is employed to calculate fracture outlet temperature and thermal power output, serving as the basis for assessing the system’s thermal extraction efficiency. A sensitivity analysis is subsequently conducted to examine the influence of key subsurface parameters of the underground heat exchanger — including injection flow rate, fracture spacing, fracture size, and initial rock temperature — on the evolution of thermal output over time. The results show that increasing flow rate enhances early thermal power but accelerates thermal depletion; moderately increasing fracture spacing helps delay thermal decline; and higher initial rock temperatures provide stronger output at the begin but also result in faster cooling. Among all parameters, fracture size adjustment is identified as the most effective strategy for improving long-term thermal sustainability. Based on this study, it is (1) recommended to integrate thermal storage technologies, (2) to proof if the use of heat pumps is necessary, and (3) to implement of multiple-doublet configuration to optimize key parameters such as fracture number and rock temperature, thereby improving system redundancy and reducing the risk of induced seismicity.