Numerical simulation of micro-cracking and energy budget in porous rocks under contractional regimes across the brittle-ductile transition

Abstract

A distinct element model is developed to simulate the progressive localization of deformation that evolves from shear to compaction bands with respect to confining pressure in porous rocks. The numerical samples with nominal porosities ranging from 0 to 14% in two-dimension, are established by synthesizing breakable grains and compressible macro-pores. The hydrostatic and unconfined tests are conducted to address the two endmembers of deformation behavior, and a broad range of confining pressures are adopted in confined tests so that the whole transition can be observed. The micro-cracking activity and associated energy components are synchronously tracked, to investigate the pure cataclastic deformation under the effects of porosity and confining pressure. Numerical results confirm that the incidental compaction of macro-pore space, originating from grain fragmentation, accounts for the major distinction in failure mode between different porosity samples. Generally, the rise in confining pressure promotes the shear cracking and intra-granular failure, and the increase in porosity facilitates the relative abundances of tensile cracking and inter-granular failure. Shear localization dominates the rupture pattern at low confining pressure regardless of porosity. At high confining pressure, the nonporous sample is characterized by ductile behavior, along with a declined fraction of energy release; whereas the porous samples are featured with macro-pore collapses that relate to surges in both micro-cracking activity and energy release, some of which would eventually coalesce and develop into a compaction band if the conditions (e.g., porosity and contractional displacement) permit.