Abstract
Wellbore stability is one of the biggest challenges for drilling engineers. The main question is how to keep the wellbore stable to assure a safe drilling operation. Optimizing borehole pressure, hence the equivalent circulating density (ECD) within the limit between collapse pressure and fracture pressure is important to avoid borehole failure. In this study, the drilling induced wellbore failure in two wells in one of the North Sea fields was examined. Four-arm caliper log was used for observation of wellbore failure where density log, sonic log and leak-off test were used to estimate elastic parameters and the magnitude of the three principal in-situ stresses. The typical problems encountered in both wells have included washout and breakout sections. Due to lack of data such as: laboratory test measurements and shear wave in one of the wells, empirical formulas were used. An analytical model was constructed in order to optimize the mud weight requirements to drill through instable zones, and mud weight ranges were identified to minimize wellbore instability. The key parameters were imported from Excel sheet including elastic properties, in-situ stresses and rock strength. The collapse pressure, fracture pressure and mud weight, which can be used safely while drilling, were calculated. As the appropriate mud weight range was estimated ECD has been predicted for different assumed pressure losses.