Effect of wellbore layout and varying flow rate on fluid flow and heat transfer of deep geothermal mining system

Abstract

Optimal wellbore layout and fluid flow supplying scheme are crucial in thermal breakthrough alleviation and performance improvement for geothermal energy recovery. This study aims to evaluate the effect of wellbore layout and fluid flow injection scheme on heat extraction performance and determine the optimal scheme for a sCO2-based deep geothermal mining system. A three-dimensional numerical model is established and the fluid flow and heat transfer characteristics for six types of wellbore layouts is comparatively analyzed. Then the system performance under increasing and decreasing type of varying flow injection scheme is evaluated. The results show that the wellbore layout has great impact on the sufficient degree of heat exchange between the cold working fluid and the hot rock. The triplet triangle layout is superior to the other kinds of wellbore layouts in heat transfer and gives moderate pressure loss. Quintuplet and doublet diagonal layouts give moderate performance, while doublet center straight-line has slightly poorer heat transfer performance than these two layouts. The triplet center straight-line and triplet diagonal layouts provide the two worst wellbore layouts with obvious dead zone visualized in system temperature contour. Besides, the injection flow rate needs to be supplied into the reservoir cautiously. Too large mass flow rate makes too fast heat extraction, and the thermal breakthrough time is shortened and temperature drops dramatically. Moreover, the increasing type of varying flow rate scheme can extend thermal breakthrough time by 50% and significantly raise reservoir outlet temperature, while the decreasing type of varying flow rate scheme deteriorate the system heat transfer performance due to excessive heat extraction in early operation period. Overall, the triplet triangle layout combined with increasing type of varying flow rate scheme can improve the heat extraction performance, enabling a more stable and sustainable geothermal mining system.