Thermal Decline and Management in Energy Generation from Fractures in Super Hot Rock Formations

Abstract

Geothermal wells drilled into super-hot rock formations have the potential to generate significant levels of thermal power in closed loop geothermal systems. In addition, a properly designed system of wells can meet target electric power demands even after accounting for conversion losses. The most efficient means of working fluid enthalpy gain is through a network of fractures connecting a pair of injector (cold fluid) and riser (hot fluid) wells, thereby maximising fluid-to-formation contact area. The motivation behind many projects is usually based on the rough estimates of thermal power due to the difference between the well inlet and undisturbed geothermal temperatures. This ignores however the fact that in the vicinity of the fractures, the geothermal temperature declines with time. In addition, the problem is exacerbated when the cold fronts from adjacent fractures coalesce. Larger spacing between fractures to mitigate this effect comes at the expense of fewer fractures in the available space. This study investigates both decline mechanisms and design optimisation strategies to combat them in the aforementioned scenarios. Since the thermal transients cannot be correctly estimated with a pseudo-transient approach, a fully transient model taking into account heat transfer in both the fracture and formation is necessary. Semi-analytical solutions of the energy equation are developed, and it is shown how thermal decline can be managed with a mass flow rate schedule. The results indicate that fracture geometry and resource temperature are by far the most significant contributors to efficient energy generation. Larger spacing between fractures helps arrest thermal decline, but this effect tends to diminish once the spacing is substantially greater than the thermal front progression, or if high mass flow rates cause early coalescing of the cold fronts from adjacent fractures. It is hoped that the results from this study will prove useful in the design of many enhanced and closed loop geothermal systems.