Sensitivity Analysis and Comparison of Safe Mud Weight Using Three Criteria in a Vertical Well

Instability of borehole has been one of the most important issues in the drilling industry during years. The Mohr-Coulomb failure criterion is the most frequent criterion used to analyze the stability of wellbores. This criterion ignores the strengthening effect of intermediate stress, so it can’t be used to predict the mud weight window properly. During years, lots of criteria that concern the influence of intermediate stress have been developed. In this paper wellbore stability analysis using Mohr-Coulomb, Mogi-Coulomb and Modified Lade failure Criteria is studied and a comparison between results based on these criteria were done. Results showed that the Mohr-Coulomb failure criteria is more conservative and overestimates the pressure in predicting the safe mud weight window. The Modified Lade failure criterion predicted the critical mud weight closer to the reality and less conservative than Mohr-Coulomb criterion and less unconservative than the Mogi-Coulomb failure criterion. A Sensitivity analysis was done to evaluate the effect of some factors on the predicting the safe mud weight window using the aforementioned criteria. Results showed that in the majority of cases, the maximum horizontal stress had the most effect on the internal wellbore pressure and the Poisson’s ratio using all the criteria had the least effect

Keywords:

wellbore stability , Mohr-coulomb , Mogi-Coulomb , Modified Lade , Sensitivity Analysis