Comprehensive in-situ stress estimation for a fractured geothermal reservoir from drilling, hydraulic stimulations, and induced seismicity

In November 2017, a Mw 5.5 earthquake occurred in vicinity of the geothermal development site in Pohang, South Korea. The Korean government-appointed investigation commission concluded that the earthquake was affected by a series of hydraulic stimulations conducted at the geothermal development site. In spite of its critical importance, the previously suggested stress models for the Pohang geothermal site had large discrepancies, based on limited number and types of stress-indicating data and each different stress estimation approach. In this study, a comprehensive in-situ stress estimation was conducted for the target depth of the enhanced geothermal system development site in Pohang, South Korea, based on variety of direct and indirect in-situ stress indicators collected from drilling, logging, hydraulic stimulations, and induced seismicity data. The stress magnitudes and orientations are suggested as well as the possible range of friction coefficient of the dominant fault structures at the site. The stress model of this study well explains the characteristics of the Pohang earthquake in terms of the slip direction and the slip tendency of the mainshock fault, and can be used for various studies clarifying the causal mechanism of the Pohang earthquake, thus providing an insight for fault stability analysis and geo-energy development applications in the southeastern part of the Korean Peninsula.