Anisotropic Wellbore Stability Model and Its Application for Drilling through Challenging Shale Gas Wells
Authors: Yan Gongrui , Karpfinger Florian , Prioul Romain , Tang Huiying , Jiang Yiming , Liu Chang 
Published: March 2015

This paper describes a new anisotropic wellbore stability model and its application in a challenging shale gas well in China. Wells drilled through laminated rocks like shales –particularly deviated wells such as laterals - are often less stable than comparable wells drilled into non-laminated rocks. This results in significant wellbore instability issues that can elevate drilling operation costs substantially. Traditional wellbore stability analysis considers the formation as a homogeneous isotropic material using isotropic elastic near wellbore stresses together with isotropic borehole failure criteria such as Mohr-Coulomb. In anisotropic rocks, for example in highly laminated shales and fractured rocks, those models are no longer valid as borehole failure can occur along weak planes.

We have developed such an anisotropic wellbore stability model in which the rock anisotropic elastic properties and anisotropic strength characteristic are incorporated. Near borehole induced stress is computed by Lekhnitskii-Amadei solution to account formation anisotropic behavior, and wellbore shear failure mechanism is modeled by a modified Plane-of-Weakness model, where the classical Plane-of-Weakness failure criterion is adopted to identify the onset of rock sliding along a weak plane; and the active shearing effect is adopted to judge whether the onset sliding at the borehole wall will develop and cause borehole instability. Arbitrary bedding/fracture orientations as well as arbitrary wellbore trajectory and in situ stress orientation is considered by this model. The model can predict the extent of failure region around the wellbore and then provide mud weight window for safe and effective drilling.

Additionally, this approach was programmed into a software system and applied to a field case study - wellbore stability analysis and mud weight computation for a slightly deviated pilot well drilling through laminated shales. The laminated shales are penetrated by high-angle natural fractures. Our modeling results shows the design borehole can be drilled without encounting severe instability issue, however, borehole damage due to either weak planes (laminated layers and fractures) sliding or rock matrix shear failure were predicted. The borehole damage mode and severity is largely dependent on borehole relative orientation with the weak planes, and the directions and magnitudes of the in situ three principal stresses. The computed borehole damage was compared to borehole image (FMI) and caliper measurement and gave a good agreement. Subsequently this model was applied to predict the mud weight window for an Extended Reach Drilling (ERD) in the field and provide wellbore stability analysis to drill high-deviated wells in the area. We showed that this developed model is effective to provide insight in understanding borehole failure mechanisms, and provide more reasonable mud weight window which can be used to effectively mitigate wellbore instabilities in anisotropic condition.