Development of a New Geomechanical Constitutive Model for Soft, Highly Compressible Clays in the Overpressured Shale Sequences of the Offshore Niger Delta

Soft, highly compressible overpressured shale sequences in the offshore Niger Delta overburden exhibit inelastic deformation behaviour under drilling-induced stress perturbations that standard linear elastic and Mohr-Coulomb geomechanical models cannot adequately represent, leading to systematic prediction errors in wellbore stability, casing design, and subsidence calculations. This study develops a new geomechanical constitutive model for soft, highly compressible clays in the overpressured shale sequences of the offshore Niger Delta. The model framework extends the Modified Cam-Clay plasticity model by incorporating a strain-softening post-yield function calibrated to triaxial compression test data on offshore Niger Delta shale samples, and a secondary creep model based on the Singh-Mitchell formulation to represent time-dependent shear deformation during the extended periods of drill string rotation and formation exposure during deepwater well construction. Laboratory triaxial testing programme results from 24 core samples from three deepwater wells provide the constitutive parameter dataset for model calibration, including yield surface parameters, compression index, swelling index, and secondary creep constants. The constitutive model is implemented as a user-defined material subroutine in Abaqus finite element software. Wellbore stability predictions using the new model are compared against caliper log measurements from two deepwater calibration wells, demonstrating a 41 percent improvement in borehole enlargement volume prediction accuracy compared to linear elastic modelling. Applications to casing design and formation compaction prediction are presented. Keywords: geomechanical constitutive model, soft clay, overpressured shale, Niger Delta offshore, wellbore stability.