A Numerical Study of Factors Affecting the Efficiency of Closed-Loop Geothermal Wells

Geothermal energy has a promising prospect of becoming a feasible source of clean energy. However, conventional open-loop geothermal systems need continuous water circulation, which limits their application in dry areas. On the other hand, long-term fluid injection into the reservoir may cause issues such as induced seismicity. To overcome these drawbacks, closed-loop geothermal systems (CLGS) are proposed as they are not requiring any water withdrawal. However, the mechanisms controlling the efficiency of closed-loop systems are not well-analyzed. Therefore, a coupled three-dimensional model is established using finite element methods to explore factors affecting production efficiency in a closed-loop geothermal well (CLGW). The model is validated against the experimental results available in the literature. Using this model, the effect of several factors on the harvested heat are investigated. These factors include thermal conductivity of cement, circulation velocity of the fluid, and the heat capacity of the circulating fluid. The results indicate that increasing the cement thermal conductivity enhances produced fluid temperature as well as the thermal power. As the circulation velocity of the working fluid increases, the temperature of produced fluid decreases but thermal power increases asymptotically. Using circulating fluid with high heat capacity would result in a higher thermal power up to a limit. Results and conclusions obtained in this paper may provide an insight to design closed-loop systems.