ABSTRACT: In engineering practice, a linear poroelasticity stress model in combination with a rock strength criterion is commonly used to determine a minimum mud weight for stable well drilling. Rock strength criterion therefore plays a key role in minimum mud weight prediction. There are a variety of rock strength criteria available in literature. It is well known that all those criteria fall into two categories: intermediate principal stress dependent (ó2-dependent) criteria and intermediate principal stress independent (ó2-independent) criteria. To identify if a specific rock failure is ó2-dependent or ó2-independent, polyaxial (true triaxial) rock strength test is essential. Similarly, to study the effect of rock strength criteria on wellbore stability and minimum drilling mud weight prediction, polyaxial rock strength test data are most useful. In this paper, we present a systematic approach to quantify the effect of three most commonly used rock strength criteria on minimum drilling mud weight prediction using polyaxial rock strength test data for Yuubari shale and Dunham dolomite.
In oil/gas well drilling, to maintain a stable borehole without inducing near borehole shear failure, a minimum mud weight should be used. Applying a mud weight significant larger than the minimum value reduces rate of penetration and potentially induces formation damage and thus is not desirable. Many investigations have been made to develop theories/models for determining a lower mud weight that is feasible for efficient drilling. Zheng [1] proposed that mud weight can be reduced by controlling borehole breakout size. Bradford and Cook [2] found that a mud weight calculated using a linear poroelastoplasticity model is less conservative than the one calculated from a commonly used linear poroelasticity model. In engineering practice, a linear poroelasticity stress model such as the ones by Bradley [3] and Aadnoy & Chenevert [4] in combination with a rock strength criterion such as the Mohr-Coulomb criterion [5] is commonly used to determine the minimum mud weight. Rock strength criterion therefore plays a key role in minimum mud weight recommendation. McLean and Addis [6] compared different versions of the Drucker-Prager criteria [7] and the Mohr-Coulomb criterion and assessed their effects on minimum mud weight recommendations. It is found that those strength criteria can give extreme differences in predicted minimum mud weights. Ewy [8] introduced a modified Lade criterion [9] to compare with the Mohr-Coulomb and Drucker-Prager criteria. Ewy [8] concludes that the modified Lade criterion predicts minimum mud weight values that are less conservative than those predicted using the Mohr-Coulomb criterion and yet are not as optimistic as those predicted by the Drucker-Prager criterion. With similar arguments, a modified Wiebols and Cook criterion [10] was introduced by Zhou et al. [11].