Stability of Infill Wellbores in Dissimilar Layers of Depleted and Confining Rock

This paper presents the stability analysis of infill wellbores when drilled toward and through a depleted rock layer confined in between two mechanically dissimilar formations. The analysis is based on an analytical solution for the nonuniform distribution of the in-situ stress change within each rock formation due to an axisymmetric volume of pore fluid pressure change within the middle layer. The solution is derived using the Hankel transform of the stress equilibrium equations and by adopting poroelastic constitutive relations to describe the mechanical behavior of fluid-saturated rock. The obtained expressions for change in the in-situ stress tensor are next applied as boundary conditions of the infill wellbore stability problem to find the wellbore wall stress, and subsequently, the post-depletion values of the wellbore shear and tensile failure gradients. Results indicate that aside from the contrast in mechanical properties between the depleted and confining rock layers, the distance from the center point of depleted volume, as well as the geographical azimuth of the point of interest along the well trajectory, are among the key factors that determine the wellbore mud weight window.