Geomechanical modeling of pore pressure in evolving salt systems

Abstract

Abstract We predict pore pressure and stress in an evolving salt basin (salt rising as a diapir and developing into a sheet) using a transient, large-strain, evolutionary geomechanical model. This model simulates mudrocks with a poro-elastoplastic material and couples sedimentation with salt deformation and porous-fluid flow. We show that pore pressures near salt are higher than predicted by assuming vertical uniaxial deformation. During salt-sheet emplacement, subsalt pore pressures equal the weight of salt, resulting in low effective stresses and very low sediment strength. We find that a dissipation zone develops parallel to the base of salt, allowing excess pore pressure to decrease with time by lateral drainage. In addition, we show that welding of the source layer and pedestal subsidence affect pore pressure in the deeper parts of the basin. We discuss how changes in both pore pressure and least principal stress lead to a narrow drilling window subsalt. We translate our stress field into an equivalent P-wave velocity field and find very low seismic velocities below salt. Finally, we compare our geomechanical pore-pressure prediction with that of porosity–effective stress workflows. We demonstrate the role of mean total stress in pressure prediction. In addition, we show that the prediction accuracy of porosity-based workflows depends on the relative level of shear (q/σ′m) in their calibration dataset, and how this ratio compares to the field (q/σ′m)field. Overall, our transient evolutionary model provides an estimate of the full stress tensor and pore pressure over time, and can help identify potential hazardous areas below salt. Our study illustrates the relative contribution of stress-tensor invariants to pore pressure and errors resulting from their omission. Furthermore, it advances our fundamental understanding of the interaction between fluid pressure, stress, and deformation in salt basins.