Sensitivity analysis on geomechanical properties of the formation affecting the wellbore stability during underbalanced drilling in one of the southwest fields of Iran

Underbalanced drilling has been considered to reduce damage to the formation, decrease costs and increase drilling speed. Although underbalanced drilling has many advantages, it requires accurate modeling and fundamental study before drilling operations due to the risks of lower drilling mud pressure than formation pore pressure. Therefore, in this study, by using numerical analysis and Mohr-Coulomb analytical method, the wellbore stability of the studied well in underbalanced drilling was investigated. Then, the sensitivity of geomechanical parameters, in-situ stresses, and mud pressure on the wellbore radial strain was analyzed in order to present their effect of them on the wellbore stability. According to the results, the range of safe and stable mud weight window for underbalanced drilling of the formation was 21.24 to 34.7 MPa. In underbalanced drilling of the studied well, the sensitivity factor of all geomechanical parameters revealed that mud pressure and maximum horizontal in-situ stress have the greatest effect, while Poisson's ratio and friction angle have the least effect on the wellbore radial strain. Additionally, according to the sensitivity factor among all geomechanical parameters, the well pressure and maximum horizontal stress with the values of 6.43 and 5.33 respectively have the greatest effect on radial strain during underbalanced drilling.