Simulation of Borehole Failure with Impact of Mud Permeation at Multilateral Junction

Abstract

Abstract Implemented into RFPA2D (Rock Failure Process Analysis), a coupled flow-stress-damage (FSD) model is presented to evaluate borehole failure with impact of mud permeation at multi-lateral junction. Specifically, the FSD model that incorporates the small-scale variability in both modulus and strength, is applied to examine the relation between borehole deformation mode and mud weight for boreholes subjected to both internal and biaxial compressive loadings. The multilateral junctions are treated as boreholes with different geometries including circular borehole, elliptic borehole, oval borehole and two adjacent circular boreholes at different positions. The progressive deformation processes of boreholes under incremental mud loading and constant biaxial compressive loading conditions are simulated and the associated failure modes are evaluated. These simulated borehole deformation processes indicate that failure modes are primarily controlled by ratios of the internal pressure (mud weight) to the external pressure in addition to the rock heterogeneity and the borehole geometry. By increasing the magnitude of this ratio, a whole spectrum of borehole deformation modes from unstable (low mud pressure), to stable (intermediate mud pressure), to hydrofracturing (high mud pressure) has been observed. This spectrum for deformation modes provides a basis to determine critical mud pressure to external pressure ratios under which deformation modes switch from unstable, to stable, to hydrofracturing. In this study, the critical ratio under which deformation modes switch from unstable to stable is used to define the lower bound of the safe mud weight window, while the critical ratio under which deformation modes switch from stable to hydrofracturing is used to define the upper bound of the safe mud weight window. Through a series of numerical simulations, the safe mud weight windows have been defined for boreholes at multilateral junction.