Role of pore attribute in the localized deformation of granular rocks: A numerical study

Abstract

Practical attempts are made for a better understanding of the deformation mechanisms among porous siliciclastic, carbonate and volcanic rocks, in which the localized compaction has commonly been detected, with emphasis on the effect of micro-pore and size difference between grains and macro-pores. Based on a distinct element model comprised of multiple identical elements, the variation in relative macro-pore size is obtained through a building-block approach to the grain or macro-pore element, while the micro-pores are randomly extracted from the grain interior. The micro-cracking activity, energy budget and associated grain crushing are recorded to reveal the localization process under contractional regimes across the brittle-ductile transition. Numerical results suggest that the formation of shear bands is a universal feature at low confining pressures, and the macro-pore is an essential prerequisite for the compaction localization at high confining pressures. The reduction in either macro-porosity or relative macro-pore size contributes to a distributed cataclasis, and the abundance of micro-pore promotes the inception and propagation of compaction bands. Generally, macro-pore structure dominates the rupture morphology, on which the micro-porosity exerts a limited effect unless it represents a high portion of total porosity. Within the tested relative macro-pore size range that corresponds to ~2.65 to 0.25 times the grain size, the grain-scale fracture always favors the periphery of macro-pores, and the localized compaction derives from the competition of tensile and shear failures on both interior and boundary of a grain, which is heavily dependent on pore attributes such as the relative size and abundance of macro-pores. Typically, the micro-cracking activity shows a gradual growth before peak stress during shear localization, whereas the compaction localization is characterized by intermittent stress drops that related to surges in the micro-cracking activity. In addition, the influences of pore attributes on strength and deformation parameters are also discussed.