Wellbore Stability Analysis Using Stochastic 1D Geomechanics and Plane of Weakness (PoW)

Abstract

Abstract Wellbore stability models traditionally rely on inputs derived from rock physics models and empirical correlations, often iteratively adjusted using drilling observations and Mechanical Earth Models (MEM) predictions to obtain deterministic solutions. Given the inherent uncertainty in these parameters, scenarios with varying levels of uncertainty are commonly considered. Previously, assessing the impact of planes of weakness (PoW) on wellbore stability predictions in one-dimensional (1D) scenarios was challenging. Weak planes exhibit lower cohesion and friction angles compared to intact rock, affecting the stability of the wellbore. This paper introduces a stochastic approach that integrates PoW into existing workflows, allowing for the evaluation of inconsistent and non-intact rock masses’ influence on wellbore stability within the stable mud weight window. This approach can be employed across an interval, or just for a single depth, incorporating several sets of weakness planes. The method presented in this paper employs Monte Carlo simulation to provide deterministic solutions for Plane of Weakness failure and incorporate individual uncertainties associated with key variables of MEM. Outputs include standard deviations and means, enabling the calculation of probability logs for various mud-weight window limits (e.g., breakdown pressure, kick pressure, shear failure). Additionally, image log-identified fractures are considered as Planes of Weakness, providing insights into the behavior of inhomogeneous or non-intact rock masses during drilling when drilling parameters fall outside the stable mud weight window. This approach proves invaluable in addressing complex geomechanical environments characterized by uncertain subsurface pressures, stresses, rock properties, and the presence of PoW such as natural fractures. It aids in identifying significant sources of uncertainty, and guiding data acquisition, and rock-testing efforts. Furthermore, it enhances wellbore stability predictions in scenarios with limited calibration data. The paper showcases examples of its application in optimizing wellbore stability boundaries for proposed trajectories in the South of Iraq, highlighting its practical utility in real-world drilling scenarios.