Integrated In Situ Stress Estimation by Hydraulic Fracturing, Borehole Observations and Numerical Analysis at the EXP-1 Borehole in Pohang, Korea

It is desirable to combine the stress measurement data produced by different methods to obtain a more reliable estimation of in situ stress. We present a regional case study of integrated in situ stress estimation by hydraulic fracturing, observations of borehole breakouts and drilling-induced fractures, and numerical modeling of a 1 km-deep borehole (EXP-1) in Pohang, South Korea. Prior to measuring the stress, World Stress Map (WSM) and modern field data in the Korean Peninsula are used to construct a best estimate stress model in this area. Then, new stress data from hydraulic fracturing and borehole observations is added to determine magnitude and orientation of horizontal stresses. Minimum horizontal principal stress is estimated from the shut-in pressure of the hydraulic fracturing measurement at a depth of about 700 m. The horizontal stress ratios (S _Hmax/S _hmin) derived from hydraulic fracturing, borehole breakout, and drilling-induced fractures are 1.4, 1.2, and 1.1–1.4, respectively, and the average orientations of the maximum horizontal stresses derived by field methods are N138°E, N122°E, and N136°E, respectively. The results of hydraulic fracturing and borehole observations are integrated with a result of numerical modeling to produce a final rock stress model. The results of the integration give in situ stress ratios of 1.3/1.0/0.8 (S _Hmax/S _V/S _hmin) with an average azimuth of S _Hmax in the orientation range of N130°E–N136°E. It is found that the orientation of S _Hmax is deviated by more than 40° clockwise compared to directions reported for the WSM in southeastern Korean peninsula.