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Mechanisms of deformation of porous rocks at the pore scale
Deformation mechanisms at the grain scale are responsible for producing large strains in porous rocks. They include cataclastic flow, dislocation creep, dynamic recrystallization, diffusive mass transfer, and grain boundary sliding, among others. In this paper, we focus on two dominant grain-scale mechanisms resulting from purely mechanical, isothermal loading: crystal plasticity and micro-fracturing. We examine the contributions of each mechanism to the overall behavior at a scale larger than the grains but smaller than the specimen, which is commonly referred to as the mesoscale. Crystal plasticity is assumed to occur as dislocations along the many crystallographic slip planes, whereas micro-fracturing entails slip and frictional sliding on microcracks. It is observed that under combined shear and tensile loading, micro-fracturing generates a softer response compared to crystal plasticity alone, which is attributed to slip weakening where the shear stress drops to a residual level determined by the frictional strength. For compressive loading, however, micro-fracturing produces a stiffer response than crystal plasticity due to the presence of frictional resistance on the slip surface. Behaviors under tensile, compressive, and shear loading invariably show that porosity plays a critical role in the initiation of the deformation mechanisms. Both crystal plasticity and micro-fracturing are observed to initiate at the peripheries of the pores, consistent with results of experimental studies.Author(s):
Martin Tjioe
Stanford University
United States
Ronaldo Borja
Stanford University
United States