In-situ stress is critical for wellbore stability, selection of injection and production well location as well as performance of hydraulic stimulation for Enhanced Geothermal System (EGS). This study introduces the integrated in situ stress estimation method and result applied to the Pohang EGS research site, which is the first of its kind in Korea.
The in-situ stress estimation analysis was conducted at an exploration borehole, EXP-1, which is about 1 km in depth. Three hydraulic fracturing stress measurement tests were conducted at around 700m depth. Several borehole breakouts and drilling-induced fractures were observed at similar depth from the borehole televiewer. The analysis combining the hydraulic fracturing results and borehole observation was carried out to make an integrated in-situ stress model at EXP-1 hole. The rock mechanical property data are based on the laboratory experiments on rock core samples extracted from EXP-1.
Instead of simply averaging the principal stress and orientation data from independent measurement methods, we took an average after transforming the measured in situ stress data to an identical axis. A few examples with synthetic data shows that this method can be more accurate since it truly treat the in situ stress as 2nd order tensor. The suggested in-situ stress model is also compared with results from previous in situ stress measurements in the region in order to investigate the appropriateness of the suggested model.
In-situ stress measurement and its application have been regarded as critical factors in the geomechanics field. Especially for Enhanced Geothermal System (EGS), in-situ stress estimation and stress model composition is essential for wellbore stability, selection of injection and production well location as well as performance of hydraulic stimulation.