Abstract
In certain shale formations, time-dependent deformation causes the annulus between the borehole and casing to close naturally, offering the potential for a self-sealing barrier without annulus cement in petroleum well. In this study, the mechanical behavior of Pierre II shale is investigated as a candidate material for such shale barriers. Laboratory tests are combined with numerical modeling to evaluate the long-term performance of the annular seal. A downscaled shale barrier experiment is conducted at drained condition using a hollow cylinder shale specimen and a central casing, subjected to externally applied isotropic stress under controlled pore pressure. Strain gauges attached to the casing allowed continuous monitoring of stress evolution after contact of the borehole with the casing. To characterize the time-dependent behavior of the shale, stress relaxation tests are performed on companion specimens at zero volumetric strain. A constitutive model incorporating both viscoelastic (Burgers-type) and viscoplastic (Bingham-type) components is implemented in a finite difference simulation, which can be calibrated using laboratory data. The simulation for the shale barrier experiment with the calibrated parameters captures the main trend of the casing stress evolution. Due to the axisymmetric assumption of the model, the non-uniform stress response measured in the experiment cannot be explained. Nevertheless, the simulation provides insights into the continuing reduction in annulus permeability driven by increasing mean stress over a long period after casing contact. The study demonstrates a viable workflow for predicting the mechanical response of the shale barrier and supports the use of casing stress measurements to track barrier development. The integrated experimental-modeling approach is promising for assessing the feasibility of natural shale barriers in field applications.