Thermal Fracturing of Geothermal Wells and the Effects of Borehole Orientation

Abstract

An enhanced geothermal system (EGS) expands the potential of geothermal energy by enabling the exploitation of regions that lack conventional hydrothermal resources. The EGS subsurface system is created by engineering enhanced flow paths between injection and production wells. Hydraulic stimulation of existing fracture networks has been successfully achieved for unconventional geothermal resources. More recently proposed concepts increase the use of drilled wellbores in hard rock to connect the injection and production wells. The present work investigates the long-term thermal effects of deviated geothermal wellbores and studies how the cooling of the borehole wall results in thermally induced tensile fractures. The results show that induced fractures are created by a combination of in situ and thermal stresses, and that the extent to which thermally induced tensile wall fractures are created largely depends on how the wellbores are oriented with respect to the pre-existing stresses of the reservoir. If the system is not optimized with respect to in situ stresses, the risk of wellbore instability becomes severe within less than a year of production. In contrast, if the orientation of the wellbores is optimized, thermally induced instabilities can be completely excluded as potential risks for the operational lifetime of the system. Furthermore, our results show that the thermal failure process strongly depends on the temperature of the injected water but is only weakly affected by the injection rate.