Wellbore Stability Assessment of an Anisotropic Shale Formation in the North Sea

Abstract

The mechanical stability of wellbores is governed by formation stresses, the local pore pressure and strength properties of the rock. These formation and rock properties are often associated with uncertainty that in turn affects the confidence of collapse and fracture prognoses. We use formation stress and well-specific rock strength data to perform a mechanical wellbore stability analysis for a deep, anisotropic shale formation in the North Sea. Uncertainty in the available data and stress prognoses is accounted for and reflected in the safe mud weight window for the formation. We base the wellbore stability analysis on predictions of shear and tensile failure at the borehole wall for different wellbore trajectories through the shale formation above the reservoir. We utilize in-situ stress and pore pressure prognoses in combination with results from rock mechanical tests of 11 oriented core specimens from the actual shale formation, including unconfined and triaxial tests with different bedding plane orientations relative to the specimen axis. We propagate uncertainty in both rock strength and anisotropy, and formation properties to generate operational mud weight windows in terms of fracture and collapse pressures using a Monte Carlo approach. The formation stress prognosis for the actual shale formation suggested the largest in-situ stress to be the vertical overburden, while the two horizontal stresses are of nearly equal magnitude. The wellbore stability analysis shows a relatively minor sensitivity in the predicted mud weight window to the azimuthal direction of the trajectory and improved stability for near-vertical compared to highly inclined trajectories. The observation is in line with established field practice of limiting the inclination angle drilled through the shale. Due to the relatively low values of cohesion in the shale, the formation collapse pressure is sensitive to the internal friction angle of the shale which in turn varies with bedding plane orientation. A safe mud weight window for varying trajectories through a deep shale formation in the North Sea has been presented. The study demonstrates how uncertainty in formation and rock properties can be accounted for to determine the confidence level of the mud weight window. The resulting operational pressure window is essential for avoiding drilling problems and optimizing casing setting depths.