Shear-enhanced compaction during non-linear viscous creep of porous calcite–quartz aggregates

In this paper, we extend the previous studies of semi-brittle flow of synthetic calcite–quartz aggregates to a range of temperatures and effective pressures where viscous creep occurs. Triaxial deformation experiments were performed on hot-pressed calcite–quartz aggregates containing 10, 20 and 30 wt% quartz at confining pressure of 300 MPa, pore pressures of 50–290 MPa, temperatures of 673–1073 K and strain rates of 3.0×10−5/s, 8.3×10−5/s and 3.0×10−4/s. Starting porosity varied from 5 to 9%. We made axial and volumetric strain measurements during the mechanical tests. Pore volume change was measured by monitoring the volume of pore fluid that flows out of or into the specimen at constant pore pressure. Yield stress increased with decreasing porosity and showed a dependence on effective pressure. Thus, the yield stress versus effective pressure can be described as a yield surface with negative slope that expands with decreasing porosity and increasing strain hardening, gradually approaching the envelope of strength at 10% strain, which has a positive slope. Creep of porous rock can be modeled to first order as an isolated equivalent void in an incompressible nonlinear viscous matrix. An incremental method is used to calculate the stress–strain curve of the porous material under a constant external strain rate. The numerical simulations reproduce general trends of the deformation behavior of the porous rock, such as the yield stress decreasing with increasing effective pressure and significant strain hardening at high effective pressure. The drop of yield stress with increasing porosity is modeled well, and so is the volumetric strain rate, which increases with increasing porosity.