Successful mechanical earth model construction and wellbore stability analysis using elastic and plasticity solutions, a case study

Abstract

Geomechanics issues and stability of wells are essential parts of the drilling operation. If wellbore stability is not analyzed comprehensively, instability problems or failures can occur in the wellbore leading to many drilling difficulties. Many parameters such as mud weight, well trajectory, in-situ stresses, etc., could affect the wellbore instability. Wellbore stability analysis can be implemented using an elastic or a plastic solution. This paper uses both solutions to analyze wellbore stability for a well called Well-A in an Iranian southwest oil field. First, a mechanical earth model (MEM) was established and calibrated through the required information. Afterward, regarding the linear elastic solution, four rock failure criteria, including Mohr–Coulomb, Mogi–Coulomb, Hoek–Brown, and Modified Lade, were employed to predict the safe mud weight windows. In the section on plasticity, the stability analysis was executed analytically using an elastic/ideally plastic model. Then the numerical solutions using a finite difference code were applied to determine the optimum mud weight and evaluate the results of analytical computations. From the obtained results, it was found that the Mogi–Coulomb criterion provides the most reliable safe mud weight window for an elastic analysis. Numerical solution results sufficiently confirmed the authenticity of analytic elastic and elastic/perfectly plastic solutions. In addition, the numerical method was capable of determining the proper optimum mud weight using Mohr–Coulomb combined with plasticity. According to the depths of Well-A experiencing wellbore instabilities and associated geomechanical problems, the results of both numerical and linear elastic solutions demonstrated an advantageous agreement with the actual observations. Consequently, the study elucidates that in order to achieve superior drilling operations, the approaches used to analyze wellbore stability could be competently employed to determine the possibility of wellbore instabilities and design an appropriate mud weight concerning Well-A or subsequent similar wells located in the present oil field.