A hybrid method for constraining all three principal in situ stresses and their directions around vertical boreholes at great depths is described. It involves hydraulic fracturing tests for estimating the minimum horizontal stress and its direction. The vertical stress is computed from the weight of the overlying strata. In order to estimate the maximum horizontal stress additional field and laboratory efforts are employed. Geophysical logging using such tools as the Borehole Televiewer or the Formation Micro Imager captures oriented images of borehole breakouts, from which breakout span as it varies with depth is obtained. Laboratory tests of core samples in a polyaxial cell render the true triaxial strength criterion of the rock. Using the condition of limit equilibrium between the local state of stress at the edges of breakout-borehole wall intersections and the strength criterion, a non linear equation emerges from which the maximum horizontal principal stress is derived, thus completing the estimation of the prevailing state of stress. Two field case histories are described in which the hybrid method was used: the KTB, Germany scientific ultra deep hole, and the Taiwan Chelungpu Fault Drilling Project (TCDP).
Hydraulic fracturing (HF) is the most common method of estimating the state of in situ stress around vertical holes at great depths. In such holes HF typically induces vertical fractures. Correct analysis of pressure vs. time records and of any of the available fracture delineation logging techniques leads to reliable estimations of the least horizontal stress σh and its direction (Haimson & Fairhurst, 1970, Haimson & Cornet, 2003). However, there is considerable controversy regarding the accuracy of the maximum horizontal stress σH as derived from the classical interpretation of HF recorded data, especially where tests are conducted at great depths (Rutqvist et al, 2000, Ito et al, 1997).
