  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.
