AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Geomechanical Modeling of Stresses Adjacent to Salt Bodies: Poro-Elasto-Plasticity and Coupled Overpressures
(1) Bureau of Economic Geology, University of Texas, Austin, TX.
We predict how stresses and pore pressures evolve in the sediments bounding a salt body using coupled geomechanical models. We show that salt relaxation alters the stress state in the wall rocks and can induce pore pressure perturbations that extend kilometers away into the sediments. The time scale of dissipation of these perturbations is on the order of millions of years, suggesting that pore-pressure anomalies should commonly be present in mudstones near salt systems. Because previous models have not coupled changes in the stress field to changes in the pore-pressure field, they are unable to predict the interdependence between pore pressure and stress. However, accurate estimation of both stresses and pore pressures is critical to well-bore design. We employ a poro-elastoplastic soil model (Modified Cam Clay) and study how different drainage conditions affect the changes in strain, stress and pore pressure. We show that in drained systems, stress perturbations can generate low least principal stresses and a small drilling window (the difference between least principal stress and pore pressure) beneath salt. In contrast, in undrained systems, underpressures can lead to a relatively large drilling window, coupled with a significant decrease in pore pressure. These results may provide insight into pressure perturbations that have been encountered in deepwater drilling near salt. Coupled poromechanical models such as the ones discussed have the potential to illuminate how deformation occurs and predict stress and pore pressures in salt systems around the world.