THE EFFECT OF FLUIDS ON THE GEOMETRIC AND MECHANICAL EVOLUTION OF FOLDS AT SMALL THRUST RAMPS: INITIAL RESULTS FROM 2D, COUPLED, FLUID-MECHANICAL MODELS
The software uses an elasticplastic constitutive relation, including a MohrCoulomb failure criterion and a non-associated flow rule, and coupled fluid flow, with bulk rock properties that approximate sandstone and shale, and the fluid properties of water. The influence of volumetric strains on the pore pressure is reflected in the fluid constitutive law. In turn, pore-pressure changes cause mechanical deformations to occur. The equations of motion use total stresses. However, effective stresses are used to determine whether the material has yielded The model is fully dynamic.
Results of initial models show that fluid flow occurs in 3 ways: 1) poro-elastic processes, i.e. fluid is forced out of areas of active compressional loading and into areas experiencing tension; 2) dilatant pumping into areas that are actively dilating during brittle-plastic simple shear, and; 3) where dilation may have evolved to zero, the active fluid pumping mechanism is stress-state softening, where pressure within the actively deforming shear zone is lower than the elastically loaded region outside.
The presence of fluids has distinct effects on the mechanical evolution on most models, the most obvious feature is the broad distribution of finite shear strain during wet-dilatant model runs, in contrast to focused zones of simple shear in dry runs.