Wellbore Stability Analysis for Trouble Free Drilling

Abstract

This study systematically investigates the mechanical instability of a wellbore as a function of rock mechanical properties and in-situ earth stresses, and validates the predicted failures with post-drilling problematic wells. Results from this study will help to avoid wellbore instability related issues. Extended Reach Wells (ERD) were drilled in field X through shale formations inter-bedded by unconsolidated sandstone formations and carbonate formations which experienced severe wellbore instabilities. Rock mechanics lab tests were performed on preserved core samples and all the available well logs were obtained and subsequently analyzed thoroughly to develop the rock mechanics properties profile. Daily drilling reports were analyzed and this drilling history was used to evaluate the best failure criterion for modeling purposes. Accordingly, Mogi-coulomb was found to be the most appropriate failure criterion that matches the drilling history failures with predicted failures and can be used for more accurate future wellbore problem predictions. An industrial geomechanical software was used to build a Mechanical Earth Model (MEM). The software output was calibrated and validated with the observed failed wellbore condition in selected drilled wells. Subsequently, the root causes of different wellbore instability problems were identified. After preliminary validation of the MEM, the safe mud weight window for drilling future wells was predicted with respect to each formation by providing plots for different inclinations and azimuths. The safe mud weight windows for drilling in any inclination, azimuth and measured depth for Aruma, Rumaila, Ahmadi, Safaniya, Biyadh and Buwaib were found and reported. Furthermore, the safest mud weight window that works for the entire section at any inclination, azimuth and measured depth was discovered and reported for the fisrt time. Results from this study indicate that the predicted mud weight window profile could be adopted in future drilling operations for safe problem-free drilling in problematic formations.