STRESS PERTURBATIONS ASSOCIATED WITH ACTIVE FAULTS PENETRATED BY BOREHOLES - POSSIBLE EVIDENCE FOR NEAR-COMPLETE STRESS DROP AND A NEW TECHNIQUE FOR STRESS MAGNITUDE MEASUREMENT

Detailed studies of stress-induced wellbore breakouts in wells drilled through active faults reveal stress field discontinuities that are ap parently associated with recent fault movements. These discontinuities are expressed as localized rotations in wellbore breakout orientation in the vicinity of the fault penetrated by the borehole. This phenome non is observed in a variety of tectonic environments and rock types. Utilizing cases where relatively complete knowledge of the horizontal principal stresses is available from in situ measurements, we use thre e-dimensional dislocation modeling to demonstrate that these discontin uities can be explained as the superposition of a reference stress sta te and a perturbation caused by movement on preexisting faults. Case s tudies from normal, strike-slip and reverse faulting stress states ind icate that nearly complete stress drop is required to match the observ ed breakout orientation anomalies. Hydraulic fracturing data independe ntly confirm the occurrence of near-complete stress drop on some fault s penetrated by drilling. Modeling of the observed interactions betwee n breakouts and fractures can also be used to obtain information about the magnitude of in situ stress.