Drilling underbalanced is often used to prevent formation damage, avoid lost circulation, and increase rate of penetration. However, it is also risky and may lead to wellbore collapse due to lack of positive support provided by the hydrostratic wellbore fluid column. Hence, the application of underbalanced drilling (UBD) should be evaluated thoroughly through the use of in-situ stresses and rock mechanical properties to estimate under what hydraulic drilling conditions the wellbore is stable.
This paper presents a mechanical wellbore stability analysis for two depleted Iranian fields, named herein as field A and B. Depleted Iranian fractured carbonate fields are suffering from severe wellbore stability problems and lost circulation during overbalanced drilling conditions. The application of UBD in these fields with a pressure less than formation pore pressure brought on new wellbore stability problems like risk of shear failure and collapse of borehole wall. Using good geomechanical model description matching field characteristics in conjunction with rock failure criteria in some cases may lead to a good prediction for avoiding wellbore stability problems and choosing the optimum mud weight window. By analyzing cores, log and triaxial rock mechanical data, an elastoplastic model combined with a finite-explicit code was used in the wellbore stability analysis to estimatimate an optimum Equivalent Circulating Density (ECD) for these fields. Compared to some actual field data it was observed that using an elastoplastic constitutive model would be sufficient to analyze mechanical wellbore stability in these fields.