Numerical simulation study of closed-loop geothermal system well pattern optimisation and production potential

•A closed-loop mining for HDR using the grid-shaped horizontal wellbore is proposed.•The influence of wellbore placement on heat extraction performance is evaluated.•The multiple-injection and one-production mode has the highest heat production rate.•The power generation is not positively correlated with the number of grids.•Multilayer layout can further optimize heat extraction. Previous numerical simulation studies on U-shaped closed-loop geothermal systems (UCLGS) have been based on single horizontal well, but the reservoir temperature influence range of single well is limited and cannot effectively utilize deep geothermal resources. There are few studies on the well pattern design of UCLGS, especially concerning the shape, number, and arrangement of well. Therefore, we proposed a grid-shaped horizontal well heat exchange system of multi-layer and multi-branch to achieve optimal heat transfer and fluid circulation effect. The effects of the well layout location, injection-production ratio, number of grids, and well length on the heat extraction performance were investigated by numerical simulations after establishing a three-dimensional geothermal mining model. We explored the temperature response characteristics of the reservoir. Finally, the actual production characteristics and potential of the grid-shaped horizontal well heat extraction system were characterised using production indicators, such as production temperature, heat production rate, accumulated thermal energy, and generated energy. The results showed that the diagonal well layout is more effective for heat extraction. Multiple injection wells and one production well mode is the optimal design scheme under the conditions of this study. In addition, when using the diagonal well layout, the system’s heat extraction capacity is positively correlated with the number of grids. However, when using the horizontal well layout, it is no longer proportional to the number of grids once the injection rate exceeds 3 kg/s. Increasing the well length can improve heat production efficiency, but it is limited by the total thermal energy storage capacity of the reservoir. When a single-layer well layout cannot meet the heat extraction requirements, a multi-layer well layout can be used to enhance the heat extraction capacity and reduce the interference of the well network system with the reservoir. Therefore, this well design promotes the efficient development of deep geothermal resources.