Mohammad Abdideh - Department of Petroleum Engineering, Omidiyeh Branch, Islamic Azad University, Omidiyeh, Iran
Mehdi Navadeh Tayyebi - Department of Petroleum Engineering, Omidiyeh Branch, Islamic Azad University, Omidiyeh, Iran

Wellbore instability has always been one of the challenging issues in the drilling industry, and it could cause a delay in the drilling program, leading to an increase in the cost of the drilling projects. This study utilized data from seven wells to investigate and model directional wells’ stability in a shale formation during drilling in one of the largest oilfields in the southwest of Iran. In this study, two mthods, i.e. (۱) mechanical earth model (MEM) and (۲) quantitative risk assessment (QRA) are applied to investigate and model directional wells’ stability in shale formation. Herein, a wellbore with full suite log data and compressional and shear wave slowness was selected to construct the mechanical earth model (MEM). Appropriate equations are provided to estimate the field’s static geomechanical data, and laboratory data were used for validation (i.e. core). The minimum mud weight required at azimuth and different angles of the directional well was calculated using numerical and analytical analysis of the wellbore stability using the Mohr-Coulomb failure criterion. The purpose of the quantitative risk assessment (QRA) phase is to investigate the impact of the uncertainty of key parameters (i.e. input variables of the minimum mud weight equation based on the Mohr-Coulomb failure criterion) and their sensitivity to an increase in success rate and a decrease in failure. In the QRA phase, the Monte Carlo simulation method is used, and the results are displayed on a Tornado diagram. The results of the Hoek-Brown and Mogi-Coulomb failure criteria propose that the sensitivity of the mud density obtained by the above methods to the uncertainty is low. The results maintain that the prediction of the minimum mud weight required for the stability of the investigated wellbore is strongly dependent on changes in the maximum horizontal stress (σ_H) parameter and minimum horizontal stress. Moreover, the internal friction angle and rock adhesion coefficient have the least impact on determining the minimum mud weight needed for wellbore stabilization.

Wellbore Stability, Mechanical Earth Model, Minimum and Maximum Horizontal Stress, Quantitative Risk Assessment, Monte Carlo Method, Tornado Diagram