| Popis: |
Knowledge of the magitude and direction of the three principal in-situ stresses is critical for performing wellbore stability analysis in order to optimize the mud weight program for drilling. In vertical wells, breakouts tend to occur parallel to the minimum horizontal stress direction, and the sonic fast shear azimuth (FSA) coincides with the direction of maximum horizontal stress. However, for arbitrary well deviations, breakouts and the FSA are not oriented 90 degrees to each other. Breakout orientations in a deviated well depend on well trajectory as well as in-situ stress magnitudes and orientations, which can vary from the scale of the regional to the well scale. We present a case study from western offshore, India, where breakouts associated with different well orientations were used to determine the in-situ stress regime and minimum horizontal stress direction. In addition, the stress-induced sonic fast shear azimuth at different well orientations was determined and analyzed using dispersion plots. The horizontal stress anisotropy was confirmed with crossover behavior on the fast shear - slow shear profile. Because the sonic fast shear azimuth and the breakout direction are complementary, careful evaluation of change in their orientation with well deviation improved inferred stress directions. The study concludes that a strike-slip stress regime (σH > σv > σh) exists, which is different from the previously determined stress regime. We further demonstrate that once stress state and stress directions are determined, wellbore stability analysis using a mechanical earth model (MEM) can be performed more accurately. In this study, a post-drill MEM that comprises rock mechanical properties, stress tensors, and pore pressure was used to investigate the causes of overgauged holes and drilling problems that occurred in the deviated wells in this region. |
