Analysis of Time-Dependent Anisotropic Wellbore Stability Using Fully Coupled Hydro-Mechanical Model
Authors: Komatsubara T. , Furui K. , Sata T. , Tsusaka K. 
Published: January 2025

ABSTRACT:
A formation failure analysis was performed considering the anisotropic mechanical properties, time-dependent behavior, and failure process caused by mud invasion into the formation. To investigate the complex interactions between the mud pressure penetration and weak bedding planes, a fully coupled hydromechanical model was implemented and the crack tensor approach was used to consider the non-linear elastic properties of rock containing discontinuities. The numerical analysis results for the pressure transmission test showed that the hydromechanical model properly simulated the pore pressure changes experienced during the experiment and could be used to evaluate the permeability of the shale samples. In addition, the field case study results suggested that the effects of the pore fluid flow and the existence of bedding planes are significant for time-dependent wellbore instability phenomena: as the inclination angles of the bedding planes increased, the deformation of the borehole surface increased under the investigated stress conditions. The study results also suggest that mud invasion accelerates rock yielding and plastic deformation on the wellbore surface, leading to progressive wellbore failure over time in shale formations.

1. INTRODUCTION
Wellbore instability is one of the key factors that need to be considered in drilling engineering and requires technical measures to reduce drilling costs (Zeynali, 2012) and nonproductive time (Zhang, 2013). Most wellbore instability problems occur in shale formations (Steiger and Leung, 1992). These problems can be triggered by borehole failure owing to variations in strength properties along and across bedding planes (Tsusaka et al., 2021) and pore pressure changes (van Oort et al., 2019). Understanding the factors affecting wellbore instability is essential for improving drilling efficiency and safety.
Borehole failure occurs when the disturbed rock cannot support the load previously burdened by the removed rock, leading to a stress concentration around the wellbore (Abbasi et al., 2018). The magnitude and orientation of the increased stress region are affected by the anisotropic mechanical properties of rock containing discontinuities, whereas a change in the stress condition is often triggered by a pressure change caused by the penetration of drilling fluids into the formation (Asaka and Holt, 2023). Bedding planes are thought to be weaker than the intact rock and, thus, have a higher likelihood of shear failure (Shaver et al., 2020). Mud invasion of low-permeability shale formations induces effective stress changes (Albooyeh et al., 2018). Furthermore, when fluid pressure in the borehole penetrates the bedding planes, the apparent shear strength of the bedding planes is reduced because of the reduced normal stress. As the inclination of the well increases under a normal stress faulting regime, a high shear stress acts on the borehole surface and possibly causes formation failure (Yamamoto et al., 2004). Thus, designing the optimum mud weight and well trajectories is crucial for securing wellbore stability during drilling.