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

A discrete element model for cemented granular material is described w hich combines simple mechanisms of granular deformation, intergranular and intragranular microcracking, and pore channel fluid flow. Althoug h the microstructural mechanics are simulated with very simple and ide alized models, the dominant physical processes appear to be captured w ith sufficient completeness that complex macroscopic behavior may be i nvestigated, including non-linear inelastic deformation, creation and coalescence of microcracks into localized damage zones and shear bands , and stress-induced permeability alteration and anisotropy. Simulatio n results compare well with experimental observations, providing insig ht into the physical mechanisms which may control inelastic material b ehavior and stress-induced permeability anisotropy in weakly-cemented geological materials. The magnitude of stress-induced permeability red uction is related to the amount and strength of intergranular cementat ion. At low stress levels fluid permeability is reduced due to compres sion of intergranular flow channels. For near-hydrostatic loading perm eability continues to decrease as the material compacts. At increasing deviatoric stress levels, however, compression-induced permeability r eduction is counteracted by enlargement of additional flow channels du e to shear and tensile damage to the intergranular bonds and compressi on-induced intragranular microcracking. The material yields in a dilat ant manner. Because these stress-induced microcracks have preferred or ientation parallel to the maximum load direction, permeability of the rock becomes anisotropic at the macroscopic level.