Borehole stability analysis considering spatial variability and poroelastoplasticity

Typically, stability analysis of oil-producing wells is carried out considering that both the hydraulic and mechanical parameters of the rock mass are deterministic. When analyzing borehole stability, two failure mechanisms are generally considered, namely, failure due to either tensile or compressive (shear) stresses. These mechanisms are produced, respectively, by either too high or too low drilling fluid pressure. Mechanical and hydraulic properties of rock masses, and in particular sedimentary rock masses, may show a considerable degree of spatial variability. This paper focuses on the evaluation of the limits for the internal pressure associated with a target probability of failure, taking into account both the spatial variability of hydraulic and mechanical properties, and the simple variability of the initial pore pressure and in-situ stresses. The analysis is performed with a finite element program that incorporates coupled fluid-mechanical effects and elastoplastic behavior of the rock. In this way, the proposed borehole stability analysis can be set into a reliability-based framework described in the paper. Examples are shown, using stochastic data from the literature, and conclusions are drawn regarding the effect of spatial variability on the borehole stability.