Constraining horizontal stress orientations for geothermal exploration in the inclined Sisjön borehole, Southwest Sweden

About half of all heating in Sweden comes from district heating, which makes it the most common form of heating in Sweden. The Otaniemi deep drilling project by St1 in Finland was a game changer, as it launched the idea to feed district heating systems with fossil-free, sustainable geothermal heat from great depths. Göteborg Energi AB has been conducting an exploration drilling program over the last three years targeting major Precambrian deformation zones in radiogenic heat-producing granites. The in-situ temperatures of these granites are boosted by radioactive gamma-ray decay from heat producing elements (K, U, Th). The objective is to develop an engineered geothermal system at 5-7 km depth where the groundwater temperatures reach 120°C to feed the local district heating system. Knowledge of the state of stress is central for this development because it provides insights for the understanding bedrock stability, induced seismicity and fluid flow patterns.

A first 1 km deep sub-vertical borehole in Högsbo was fully cored in 2021, and a second 1 km long inclined borehole in Sisjön was completed in 2022, less than 5 km south of the first borehole. Downhole logging was conducted in both boreholes to constrain downhole temperature, natural gamma radiation, fracture occurrence and orientation of in situ stress.

An acoustic borehole televiewer was used to map fracture occurrence and their geometry, as well as to investigate if stress-induced failure has occurred in the wellbore. For vertical boreholes, drilled parallel with a principal (vertical) stress, borehole breakouts and drilling-induced tensile fractures reveal the orientation of minimum- and maximum horizontal stress, respectively, if the tangential stress concentration generated by the borehole overcomes the compressional and tensile strength of the rock mass, respectively. Here, the objective is to investigate stress-induced failure in an inclined borehole, that is not aligned with a principal stress.

We analysed and processed acoustic borehole images from the Sisjön borehole and mapped fractures occurrence and stress induced features (borehole breakouts and drilling-induced tensile fractures). We have detected some stress-induced features at about 400-500 m depth, but most stress-induced features occur below 800 m. Preliminary results of drilling-induced tensile fractures suggest an orientation of maximum horizontal stress of 145±10°N along the inclined borehole. Corresponding mean orientation from borehole breakouts is slightly lower (132±21°N). More detailed studies are required to confirm the preliminary observations, and to account for the borehole inclination.