The Influence of Failure Criteria in Geomechanical Wellbore Stability Modelling

Abstract

Wellbore instability may cause a significant operational and safety issue if not properly managed. Optimization strategy such as trajectory design based on the safe and stable drilling operation can be done through geomechanical approach to make sure the stability of the wellbore can be maintained. This requires detailed knowledge about the geomechanical parameters including the choice of the criteria to determine the onset of the rock failure. The MohrCoulomb failure criterion has been widely used and becomes a standard in petroleum geomechanics. Nevertheless, an appropriate selection of failure criteria plays an important factor in wellbore stability modelling. However, the decision as to which failure criterion is adequate for the problem requires a thorough analysis and investigation. This paper aims to compare several failure criteria commonly used in wellbore stability to examine their behaviour for a given problem. Traditionally, a linear-elastic solution is used to compute the near-wellbore stress through which the stability of the wellbore can be assessed upon inserting them into failure criteria. The piece of rocks around the wellbore will be subjected to a high near-wellbore stress concentration in response to the excavation. For a given hypothetical problem, the stress around the wellbore will be first computed and the onset at which the rock will fail will be obtained based on several failure criteria such as Mohr-Coulomb, Mogi-Coulomb, Modified Lade and Drucker-Prager. The chosen failure criteria are found to have produced a considerable range of variability in terms of predicting the onset of wellbore stability. The result also reveals the significant influence of failure criteria on the stability prediction of a wellbore. Triaxial failure criteria such as Mohr-Coulomb tends to give a more conservative value as compared to the true-triaxial failure criteria such as Mogi-Coulomb. This kind of comparison and assessment support the argument on the importance of selecting adequate failure criteria in wellbore stability modelling to predict the onset of rock shear failure.