| Determination of the appropriate drilling fluid density by rock failure analysis is an essential step to control wellbore instability. To determine wellbore failure stresses, the rock strength has to be known, an appropriate constitutive model has to be selected, and finally an accurate rock failure criterion must be chosen. Linear-poro elastic modeling of the mechanical wellbore failure is the most common approach to investigate wellbore instability. Numerous failure criteria have been used for the rock failure analysis but there is not any commonly accepted agreement of which failure criteria to use. Rock failure criteria differ in two main characteristics. First, there are linear and non-linear forms of failure criteria. The second characteristic is how different rock failure criteria consider the effect of the intermediate principal stress. Quantitative comparison has been previously studied on some of the failure criteria but few evaluations for the selection of the failure criteria are based on typical petroleum related situations. In this thesis, the thirteen most common rock failure criteria are compared and analyzed. The rock failure criteria were evaluated for three lithologies. A statistical analysis was performed to determine the similarities and differences among the failure criteria. Field case evaluation of failure criteria was conducted for three different offshore wells. According to the results of the statistical analysis, Tresca, Von Mises, and Inscribed Drucker-Prager represent the higher bounds of results for the minimum required mud weight for all cases. Although Circumscribed Drucker-Prager usually predicts the lower bound for the minimum required mud weight, its results are in the middle range for hard formations. Modified Lade, Modified Wiebols-Cook and Mogi-Coulomb give similar results for the three cases studied so these failure criteria may be used interchangeably without changing the results. The results of field cases show that Mogi-Coulomb, Modified Lade, and Modified Wiebols-Cook criteria mainly predict minimum required mud weight which is close to the field mud weight used to successfully drill the borehole. |
