In-Situ Stress Determination in Anisotropic Formation Underground with the Help of Wellbore Breakout Observations

The in-situ stress information underground is of crucial importance for drilling and reservoir development. As the cost of direct stress measurements is high and only limited information can be obtained, the application of wellbore imaging devices made it possible to interpret the local stress state more economical. Breakout azimuths revealed by well logs are widely used to determine principal stress directions in traditional method, however strength anisotropic related to bedding planes may greatly affect breakout azimuth, width and depth, which can result in misleading for in-situ stress interpretations. Based on weak-plane borehole stability model and Mogi-Coulomb matrix strength criterion, we analyzed the relationship between in-situ stress and breakout geometry (azimuth, width and depth) both in isotropic and anisotropic formation. The results show that the azimuth of breakout is controlled by weak plane’s azimuth, while steep dipping bedding plane widen the breakout and make it extending into rocks. For specific strata with determined property, breakout geometry will change regularly with the increase of the principal stress ratio. A breakout accident observed in well WFSD-2 help to illustrate the method and the magnitude of the maximum horizontal principal stress is determined.