Constitutive Model Predictions for Drilling Anisotropic Shale: A Comparative Study

This experimental thesis delves into the study of Tournemire Shale, in particular it investigates failure mechanisms and wellbore integrity during drilling operations through a comparative approach using several constitutive models integrated in FLAC2D 8.1 (Fast Lagrangian Analysis of Continua, FDM). Shale formations are most often found as cap rocks in reservoirs, which makes their structural and hydraulic integrity vital, especially given their important role in the geological storage of CO2, H2 and CH4 in the current energy transition. The methodological approach found its basis on a validation check between the analytical solution of Kirsch and the first, simple but essential Isotropic Linear Elastic model under dry conditions implemented in the software. This step was crucial to maintain a rigorous framework for the following models. Mesh and boundary conditions for radial and tangential effective stresses were verified in sectors A (θ = 0°, along σH,max) and B (θ = 90°, along σh,min). The analysis was then extended to Transversely Isotropic Elastic (TIE), Isotropic Linear Elastic with Mohr-Coulomb (ILE-MC), Ubiquitous-Joint with weak-plane mechanics (UBI), and Transversely Isotropic Elastic with weak-plane mechanics (CANISO), incorporating dry, drained, and undrained conditions, together with variations in pore pressure pw and rotation of far-field stresses. In summary, the numerical simulations show consistent predictions of failure mechanisms in line with typical shale trends, with shear concentrating in the rock matrix and potential slip along weakness planes. Sector B is generally the most stressed; in some cases, local tensile zones are observed along the wall in sector B, a topic critically discussed in this work and deserving further investigation. Overall, the multi-model comparison clarifies load–response trends and highlights key factors for future research.