MICROMECHANICS OF STRESS-INDUCED PERMEABILITY ANISOTROPY AND DAMAGE IN SEDIMENTARY-ROCK

A discrete element model for cemented granular material is described which combines simple mechanisms of granular deformation, intergranular and intragranular microcracking, and pore channel fluid flow. Although the microstructural mechanics are simulated with very simple and idealized models, the dominant physical processes appear to be captured with sufficient completeness that complex macroscopic behavior may be investigated, including non-linear inelastic deformation, creation and coalescence of microcracks into localized damage zones and shear bands, and stress-induced permeability alteration and anisotropy. Simulation results compare well with experimental observations, providing insight into the physical mechanisms which may control inelastic material behavior and stress-induced permeability anisotropy in weakly-cemented geological materials. The magnitude of stress-induced permeability reduction is related to the amount and strength of intergranular cementation. At low stress levels fluid permeability is reduced due to compression of intergranular flow channels. For near-hydrostatic loading permeability continues to decrease as the material compacts. At increasing deviatoric stress levels, however, compression-induced permeability reduction is counteracted by enlargement of additional flow channels due to shear and tensile damage to the intergranular bonds and compression-induced intragranular microcracking. The material yields in a dilatant manner. Because these stress-induced microcracks have preferred orientation parallel to the maximum load direction, permeability of the rock becomes anisotropic at the macroscopic level.