Heterogeneous In Situ Stress due to the Presence of Weak Natural Fractures and Faults

In this paper, two field examples of hydraulic fracturing stress measurements are reported, in which the measured stress magnitudes scatter quite significantly. The stress measurements were conducted in vertical boreholes at two different locations in South Korea. The boreholes are 300 and 400 m deep, respectively, both penetrating granites. Several isolated intervals of intact rocks in the boreholes were vertically fractured by injecting water. The magnitudes of the minimum horizontal principal stress (S_hmin) were determined from shut-in pressures. The magnitudes of the maximum horizontal principal stress (S_Hmax) were estimated based on the Hubbert and Willis (1957) equation using tensile strengths determined from either hollow cylinder tests or Brazilian tests, in which pressurization-rate effects on tensile strength were taken into account. The stress states in both locations are in reverse faulting stress regimes, in which vertical stress (S_v) is the least principal stress. The magnitudes of S_Hmax are generally within the stress range limited by frictional coefficients of 0.6 and 1.0 of faults optimally oriented for slip. However, S_Hmax magnitudes do not increase consistently with depth, but rather scatter quite significantly. It is observed that near the depths of relatively low stress, natural fractures and faults with wide apertures (possibly weak friction) are abundant, and near those of relatively high stress, those wide discontinuities are scarce. In particular, the wide fractures and faults are oriented predominantly in the directions of high slip tendency. If excessive stress is exerted, those weak fractures and faults would slip to release the stress, which would reduce the stress magnitudes to the values that can only be sustained by the discontinuities. This observation suggests that stress magnitudes are constrained quite locally by such weak, high slip-tendency discontinuities. Although the remote stress field might be uniformly applied, the stress state in the rock seems inherently heterogeneous because of the heterogeneous distribution of weak natural fractures and faults.