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: Uncoupled Models
(1) Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX.
We have explored and compared four approaches to geomechanical modeling of stresses adjacent to salt bodies. These approaches are distinguished by their use of elastic or elastoplastic rheology for the sediments surrounding the salt, and by the way that they treat fluid pressures in the modeling. In all cases the fluid pressures are assumed (hydrostatic), rather than calculated during the modeling; thus fluid pressures and stresses are uncoupled. The four approaches are: (1) simulate total stresses in an elastic medium, and then subtract an assumed pore pressure after the calculations are complete, (2) simulate effective stresses in an elastic medium by using the assumed pore pressure during calculations, (3) simulate total stresses in an elastoplastic medium, either ignoring pore pressure or approximating its effects by decreasing the internal friction angle, and (4) simulate effective stresses in an elastoplastic medium by using the assumed pore pressure during calculations. We evaluate these approaches by comparing stresses generated by stress relaxation of a salt sphere. In all cases, relaxation causes the salt sphere to shorten vertically and expand laterally, producing extensional strains above and below the sphere, and shortening against the sphere flanks; mean stress is dropped above and below the salt sphere, and minimum principal stress is lowered everywhere around the salt sphere. Elastic models of sediments may induce unrealistically large shear stress and unrealistically low minimum principal stress at salt boundaries. In contrast, elastoplastic models of sediments, placing an upper limit on shear stresses due to plastic yield of sediments, can predict smaller stress perturbations and better simulate stresses around salt than elastic models. Our model of an irregular salt sheet shows that mean stress within the salt converges to the far-field vertical stress and that minimum principal stress is lowest where the salt is thick. These results and comparisons provide insights into stresses around salt bodies, and give interpreters a basis to evaluate and compare stress predictions.