This paper summarizes the results of an integrated program of laboratory measurements and numerical analyses that have been performed to understand the mechanisms of instability associated with drilling of high angle wells through unlithified mudstones/very hard clays. The laboratory tests use Resedimented Boston Blue Clay (RBBC) consolidated to confining pressures up to 10 MPa as an analog test material. Cavity deformations are measured in a novel Thick-Walled Cylinder (TWC) apparatus with independent control of the vertical stress and radial pressures acting on the inner and outer walls of the specimen. Numerical simulations of these experiments are obtained using finite element analyses incorporating the MIT-E3 effective stress soil model, with parameters directly calibrated from high-pressure element tests. Comparisons with the deformations measured in undrained TWC tests provide an initial validation of predictive capabilities for vertical wellbores.