Designing multi-well layout for enhanced geothermal system to better exploit hot dry rock geothermal energy

• Thermal–hydraulic processes of EGSs with various multi-well layouts are simulated.
• Simulation results reveal well layout effects on EGS heat extraction performance.
• Basic principles for the design of multi-well layout are proposed.
• The HDR heat recovery factor is discussed.

Heat extraction mode, e.g. well layout or arrangement of wells, of enhanced or engineered geothermal system (EGS) is crucial to its performance and directly affects its commercial viability. Assuming the subsurface target hot dry rock (HDR) has been well-fractured and the created heat reservoir can be treated as a homogeneous porous medium, we numerically simulate the long-term heat extraction process of EGSs of various well layouts, including the standard doublet well layout, two triplet well layouts, and a quintuplet well layout. The simulation results enable a detailed analysis on the effects of well layout on EGS heat extraction performance. We find simply deploying more production wells does not necessarily improve the EGS heat extraction performance; an EGS with triplet well layout can perform better than an EGS with a quintuplet well layout or worse than an EGS with the standard doublet well layout. One more finding is an EGS with the injection well positioned close to the edge of the reservoir gets more thermal compensation from the un-fractured rocks surrounding the reservoir during heat extraction. Further, we deduce an optimized EGS well layout must ensure enough long major flow path and less preferential flow in the reservoir, and the injection well is located close to the edge of the reservoir. We then design a quartuplet well layout accordingly. Results from an additional simulation with respect to the quartuplet well EGS indicate its enhanced heat extraction performance, corroborating the success of design. Last, we discuss about the hot dry rock (HDR) heat recovery factor based on numerous simulated cases and estimate the amount of HDR geothermal resource that can be converted into electricity by EGS.