Integrating Logged Borehole Breakouts With Hydraulic Fracturing to Constrain the State of Stress Across the Chelungpu Fault, Taiwan, Post-Chi-Chi Earthquake

The Taiwan Chelungpu-fault Drilling Project (TCDP) is located some 100 km north of the Chi-Chi earthquake (1999) epicenter. Scientific hole B intercepts the active fault at 1136 m. Borehole breakouts were logged between 930 and 1300 m, thus straddling the fault. The vertical stress (σv) within this depth range, as estimated from density logs, varies between 22 and 30 MPa. Within the same depth range, two pairs of hydraulic fracturing tests, one above and one below the fault, were conducted by Hung et al. (2007), yielding least horizontal stresses (σh) between 16 and 24 MPa. That left the largest horizontal stress (σH) still unknown. A recent method of constraining σH has been developed that makes use of the logged breakout span at the borehole wall (Vernik and Zoback, 1992; Song and Haimson, 1997; Haimson and Chang, 2002). It is based on the reasonable assumption that the points of intersection between breakout and borehole wall cross section represent the boundary between stable rock on the outside of the breakout and failed rock on the inside. Equating the correct strength criterion to the state of stress at the points of intersection, one obtains an equation in terms of the three in situ principal stresses and the breakout location on the borehole perimeter. The Mohr-Coulomb criterion, which neglects the effect of the intermediate principal stress, was found inadequate for the highly differential principal stresses at the borehole wall (Vernik and Zoback, 1992). A true triaxial strength criterion was determined experimentally at the University of Wisconsin by subjecting rectangular prismatic specimens of the extracted core to a general state of stress (σ1 > σ2 > σ3): τoct = 5.66[0.5(σ1 + σ3)]0.49 (where τoct is the octahedral shear stress). At the point of breakout-borehole intersection this criterion is assumed to be represented by the local state of stress σθ > σz > σr (in polar coordinates), which is related to the in situ principal stresses σH, σh, and σv by the well- known Kirsch solution. By substituting the known σh, σv and the angle of the intersection at each breakout depth, a non-linear equation is obtained in terms of one unknown: σH. Solving the equation using Newton's numerical method yielded estimates of σH varying between 48 and 56 MPa (SE-NW trending based on breakout azimuths), indicating that the stress condition presently prevailing along the Chelungpu Fault favors strike-slip faulting, bordering on thrust faulting.