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Abstract This paper presents a unified approach through which the influence of the elastic and strength anisotropy on wellbore instability can be thoroughly examined. The stresses at the wellbore wall are first calculated using the Lekhnitskii-Amadei solution, which accounts for elastic anisotropy. Then, shear failure is treated by combining the Mogi-Coulomb criterion for intact rock, with the Jaeger plane of weakness concept. The developed model accounts for all three principal stresses in predicting the onset of shear failure. The results of the specific case investigated show that rock elastic anisotropy induce higher stress concentrations. The difference, compared with the stresses found using the isotropic elastic model, could reach as high as 25% for the highest degree of anisotropy that might be expected for rocks of practical interest. The strengthening effect of the intermediate stress, as reflected in the Mogi-Coulomb criterion, reduces the required mud weight density by approximately 1.0 pounds-per-gallon (ppg). Furthermore, it is demonstrated that the risk posed by bedding slippage, for a wellbore with an inclination between 15° and 50° from the vertical, is masked when an isotropic elastic stress model is used. In contrast, the fully anisotropic model shows that an extra mud weight of approximately 4.5 ppg would be required, in order to avoid bedding plane slippage for the case under investigation. Although these results apply for a particular choice of strength properties and elastic properties, they give an indication of the implications of fully accounting for anisotropy and the effect of the intermediate stress when doing borehole stability analysis. [ABSTRACT FROM AUTHOR]

A major component of the Mohr-Coulomb (M-C) criterion is that the failure of the material is independent of the intermediate principal stress σ 2 along the potential planes of sliding. To account for the effect of σ 2, a linear relationship between the octahedral shear stress τ oct and the mean stress σ m,2 on the plane of failure (called the Mogi-Coulomb criterion) is widely used to represent rock failure in polyaxial compression (PXC) in practice. In this research, a linear relationship between the equivalent stress σ e (or τ oct ) and major principal stress σ 1 (or minor principal stress σ 3 ) is proposed, which has a similar expression as the Mogi-Coulomb criterion. The relationship between the proposed failure criteria and M-C criterion is like the relationship between the Tresca and Von-Mises criteria. The material parameters can be related to the M-C strength parameters, and can be easily obtained from conventional triaxial tests. The proposed criteria are numerically calibrated against triaxial compression (TXC) and PXC data sets using three different methods. Sensitivity of the range of TXC stress data employed for fitting on the prediction results in PXC are further discussed. Comparisons of two proposed criteria with the Mogi-Coulomb criterion for rocks in PXC demonstrate that one new criterion with convex failure envelope gives the best performance for seven rock types and provides an acceptable modelling for the rest of rock types. Another new criterion and the Mogi-Coulomb criterion result in large prediction error for three (or two) rock types though they can give best performance for some rock types. [ABSTRACT FROM AUTHOR]

The failure criterion is a very important aspect of the wellbore stability analysis. But the Mohr–Coulomb criterion, a most commonly used failure criterion, is usually too conservative, due to the influence of the intermediate principal stress is not taken into account. Furthermore, the safe mud weight (MW) predicted by the conventional method is usually too high, due to the safe MW is predicted to yield no shear failure around the borehole. To solve the above problems, a semi-analytic model of wellbore stability analysis was proposed based on the stress distribution model, failure criteria and the breakout width model. The Mogi–Coulomb criterion, a new true-triaxial failure criterion, is involved in this semi-analytic model to determine the shear failure. The computational models of the lowest and highest safe MW are derived for different failure mode. The numerical method and flowchart of this semi-analytic model is provided, and a computer program is also developed in Matlab software. The impacts of the failure criteria, stress regimes and breakout width had been systematically analyzed. The optimal well path had been optimized for different stress regimes. The results showed that the lowest safe MW predicted by Mogi-Coulomb criterion is always lower than the Mohr-Coulomb criterion. The most stable drilling direction is along the minimum horizontal stress for the NF and NF–SS stress regimes, while it’s along the maximum horizontal stress in the SS, SS–RF and RF stress regimes. If the breakout width model is used, the lowest safe MW is always lower than the traditional method. The optimal well path and the wellbore stability are different between traditional and new methods. The most stable path is always the horizontal and deviated well for the traditional method, instead, it’s always the vertical well for this new method, it’s more consistent with the actual situation. These results also illustrated that an acceptable breakout is usually allowed to occur in the actual borehole, and the borehole still has enough arc to support the wellbore wall and to prevent collapse accident. Finally, both of the traditional and new methods are applied in the field case studies. The results showed that the lowest safe MW predicted by the integration of the breakout width model and Mogi–Coulomb criterion is closest to the real MW, instead, the lowest safe MW predicted by the traditional methods has exceeded the actual MW. It also illustrated the traditional method is too conservative in predicting wellbore stability, while this new method is more accurate for the integration of the breakout width model and MG–C criterion. This research can provide support to optimize the well path and MW in the drilling practices. [ABSTRACT FROM AUTHOR]