In-situ rock stress is one of the key parameters for safe and economical design of underground rock structures and well stability related to oil, gas, EGS (Enhanced Geothermal System) and CCS (Carbon Capture and Storage) projects. The first part of this paper deals with the overall characteristics of the rock stress in shallow crustal region in Korea based on the in-situ stress measurement data obtained by hydraulic fracturing stress measurement for the last 14 years. The horizontal stress orientations were determined by the hydraulic fractures and the observations on borehole breakouts and DIFs (Drilling Induced Fractures) through acoustic televiewer and high resolution optical imaging techniques.
For the depth less than 720 m, the horizontal stress ratio (K) had a tendency to decrease and stabilize with depth, however, for some areas, the excessive horizontal stress with the horizontal stress ratio (K) greater than 5.0 was observed. The average orientation of the maximum horizontal stress (SH) was ENE ~:WSW direction, which matched well with the direction inferred from the geological, geographical and geophysical approaches. Nonetheless, predominant orientation of the maximum horizontal stress was not obvious and scattered stress orientations were observed over the test regions.
The second part of this paper introduces the observed progressive brittle failure around underground openings and boreholes at less than 200 m depth. These observations found in several locations in Korea, imply that the stability of underground rock structures does not necessarily depend on the depth but on the ratio of strength to far field stress.