Micromechanics of high pressure compaction in granular quartz aggregates

Abstract

The mechanical behaviour of porous sandstones is generally modelled using concepts from granular mechanics, often overlooking the effect of cementation. To probe the key differences between sand and sandstone mechanics, we performed triaxial deformation experiments on Ottawa quartz sand at $5-40$ MPa effective confining pressure. At $5$ MPa, the samples are able to dilate. At higher confinement, the aggregates show continuous compaction, displaying strain hardening. The stress-strain behaviour is non-linear and the exact onset of inelastic compaction could not be determined accurately. Measured P-wave velocities show the development of anisotropy. With increasing axial strain, the along-axis velocities tend to increase, while velocities perpendicular to the compression axis tend to decrease (at low pressure) or remain constant (at high pressure). In samples deformed under elevated pressure conditions, acoustic emission event locations are diffuse. Microstructural investigations show an increase in grain chipping and crushing with increasing confining pressure, but no evidence of localised compaction could be observed. The nature of the pore-fluid, either decane or water, does not significantly influence the mechanical behaviour at strain rates of $10^{-6}$ to $10^{-4}$ $s^{-1}$. Grain angularity and grain-size distribution also did not significantly change the mechanical behaviour. We infer that our observations indicate that the lack of cementation introduces additional degrees of freedom for grains to slide, rotate, and reorganise at the sample-scale, precluding the existence and sustainability of stress concentrations beyond the grain-scale. This results in progressive compaction and hardening, and lack of compaction localisation.