Comprehensive in-situ stress estimation in a fractured geothermal reservoir in Pohang, South Korea using drilling data, hydraulic stimulations, and induced seismicity

Abstract

A comprehensive in situ stress estimation is carried out in the fractured reservoir at the enhanced geothermal system development site in Pohang, South Korea. Various types of stress indicators were collected from the hydraulic stimulation data, drilling records, lost circulation records, well logs, seismic events, and the stress constraints from previously proposed stress models. The comprehensive comparison of the collected stress constraints was performed, and resulted in the possible stress magnitude range and corresponding limits on the range of possible friction coefficients. The resulting stress ratio is S_V: S_hmin: S_Hmax = 1 : 0.92–0.94 : 1.42–1.66 with the azimuth of S_Hmax in N101°E –N110°E range, based on the compilation of both direct and indirect stress information. The results also suggest a friction coefficient range of 0.35–0.38 that can best explain the involved stress constraints. The stress model suggested in this study can explain the characteristics of the M_w 5.5 Pohang earthquake in November 2017 in terms of reproducing the slip rake of mainshock and the slip tendency of the corresponding fault. Therefore, the result can be used for clarifying the causal mechanism of the Pohang earthquake, providing an insight for fault stability analysis or possible geo-energy application in the southeastern part of the Korean Peninsula. Comprehensive in-situ stress estimation method suggested in this study integrating extensive direct and indirect stress indicators can improve the credibility of the in-situ stress model at a fractured reservoir.