Controlling borehole geometry as a feasible strategy for optimization of heat extraction in geothermal systems

Abstract

Optimizing the heat extraction performance of geothermal systems is a long-standing issue in the study of geothermal energy. Besides the heat extraction in fractured hot rock, minimizing the heat loss during water flowback through boreholes is also critical for the system performance. Here, we conducted a series of experimental and numerical studies to understand the controlling factors of heat extraction rate and efficiency and to explore practical approaches for the optimization of heat extraction performance in a geothermal borehole. We performed water flow experiments to observe the heat extraction from neighboring hot granite and reproduced the heat extraction process using a three-dimensional water flow model. Our results show that the heat extraction rate first increases with a higher flow rate to the maximum value and then decreases with a further rise in flow rate. The heat extraction efficiency decreases constantly with a higher flow rate. To improve the heat extraction performance with both the heat extraction rate and efficiency approaching the maximum values, we scaled up the laboratory-scale borehole and found that the heat extraction performance is enhanced with a triangular zone of heat extraction and a reduced zone of low-temperature water along a field-scale borehole. We finally discovered that a proper control of borehole geometry, such as section diameters along a multi-section borehole and bending angle of borehole trajectory, is a feasible strategy to modulate the heat extraction performance in a geothermal borehole and to replenish the heat loss during water flowback.