Analysis of an enhanced closed-loop geothermal system

Abstract

Closed-loop geothermal systems are a novel heat extraction technique not requiring continuous fluid supply. Heat extraction efficiency of closed-loop systems is often limited due to the small heat exchange area between the circulating fluid in the wellbore and surrounding rock. An enhanced closed-loop geothermal system (ECLGS) is considered here to improve the heat extraction efficiency of closed-loop systems. The objective of ECLGS is to increase heat transfer from surrounding hot rocks to the wellbore by artificial fractures propped with high-thermal-conductivity proppants. To explore the mechanism controlling heat extraction through ECLGS, a systematic study was conducted. First, a three-dimensional hydrothermal coupled model was developed and validated, and the effects of several parameters were analyzed. It was found that highly-conductive fractures could enhance heat extraction through closed-loop systems considerably. As the flow velocity of the circulating fluid increases, the net power starts to decrease after reaching a peak value. An increase in fluid heat capacity would enhance heat extraction but without requiring more pumping power. The four-wing fracture configuration causes higher heat extraction improvement than the branched fracture. A dimensionless analysis was performed for ECLGS, and three dimensionless numbers integrating different involved parameters were proposed. The proposed dimensionless numbers have been verified numerically. The effect that each dimensionless number has on production temperature has been investigated. Via dimensionless numbers, it becomes feasible to comprehensively assess the impacts of involved parameters, thus making designing closed-loop systems more convenient. The possible ways to apply proposed dimensionless numbers to building a closed-loop system have been illustrated.