Impact of fracture roughness on geothermal energy exploitation

Abstract

Many factors can affect the temperature of recovered fluid, among which fracture roughness is a very important one, while most models with fracture networks failed to account for fracture roughness due to high computational cost and convergence issues. Therefore, some researchers developed a conjugate heat transfer model for fluid flowing through a single rough fracture, concluding that the outlet temperature from a rough fracture is lower than that from a smooth fracture with the same mechanical aperture and other boundary conditions. However, it is widely accepted that a rough fracture is of a much larger heating area, an increase of which can greatly improve the associated thermal performance. There must be a threshold point, before or after which fracture roughness could hinder the thermal performance. Hence, we conducted a systematic investigation to determine the heat transfer features of water (H2O) and carbon dioxide (CO2) flowing through rough rock fractures. It is found that H2O is better than CO2 as a working fluid when purely considering the capability of heat extraction from rough fractures, while the thermal performance of H2O is lower than that from an equivalent parallel plate model when the flow rate is less than a threshold value. Nevertheless, the threshold flow rate of CO2 is far lower than that of H2O, making rough fractures outperform smooth fractures in most situations.