Fracture conductivity management to improve heat extraction in enhanced geothermal systems

Abstract

The efficiency of an enhanced geothermal reservoir (EGS) depends on the effective circulation of the working fluid through the geothermal formation. To remove the thermal shortcut that leads to thermal short-circuiting in the EGS, we present a method of fracture hydraulic conductivity management and present some practical ways to implement it in the field. The core idea is to correlate the flow resistance in different flow channels with temperature, in order to make uniform heat extraction through each flow path. The presented technique is an adaptive and reversible system and is trying to engineer the fracture system for effective delay of the early thermal breakthrough in EGS. Results indicate that the proposed fracture conductivity tuning technique (FCTT) can help to avoid flow and thermal shortcuts between the wells and maintain high heat extraction rates. The autonomous management of fracture conductivity may increase cumulative heat extraction by more than 214 %, and reduce an extra 1.06 – 3.65 Mt of greenhouse gas emission through electricity generation after 50 years of EGS development, which is highly considerable for EGS development. Besides, we found that it could be still beneficial to apply such methods solely to the injection wells, which can also bring a heat extraction improvement of 201.9 %. Furthermore, we look at the situations when the special conductivity tuning agents are placed in the middle between the wells, it is still very effective to control the fluid flow in the reservoir and enhance heat extraction. By proposing the tunable fracture conductivity, we are trying to revisit the old problem of thermal short-circuiting and provide new insight into the efficient EGS operation.