No abstract

No abstract

This work presents results from a series of triaxial compression tests on two quartz sands (differing principally in grain shape), at confining pressures high enough to cause grain breakage during shearing. Tests are performed inside an x-ray scanner, which allows specimens to be imaged non-destructively as they deform. Observation of the acquired images clearly shows different mechanisms of deformation, including shearing, dilation, compaction and grain breakage. These mechanisms are investigated quantitatively through 3D measurements of local porosity, as well as strain (obtained by 3D Digital Image Correlation), which is analysed in terms of volumetric and shear components. These tools allow the transition between macroscopically dilative (typically of a dense sand at low mean stress) and compactive behaviour to be investigated. The analysis reveals that at the high end of the confining pressure range studied (100 to 7000 kPa) the more rounded sand deforms with highly localised shear and volumetric strain-the porosity fields show a dilative band within which a compactive region (due to grain crushing) grows. The more angular material shows shear strain localisation, however its faster transition to compactive behaviour (due to a higher propensity for individual grains to crush) translates to much more distributed compactive volumetric strain.

No abstract

[1] Shear-enhanced compaction in shallow sandstone reservoirs has been investigated in laboratory experiments using high-pressure triaxial testing of poorly lithified sandstone and sand. We have studied the deformation mechanism involved during shear-enhanced compaction and controlling parameters for yield stress at varying confining pressures for sandstone/sand with different grain sizes, porosities, and packing. Experimental testing provides insights into the deformation mechanism during hydrostatic and axial compression of coarse-and fine-grained sands with different packing including (1) natural coarse-grained sandstone, (2) densely packed fine-grained sand, and (3) loosely packed fine-grained sand. Monitoring of deformation and ultrasonic velocity during deformation indicates porosity loss, compaction, and strain hardening for most of the samples. Visualization of deformation using pretest and posttest X-ray imaging and thin sections demonstrates localized deformation fabrics and grain damage. The results show grain rearrangement as the controlling deformation mechanism for material at low stress and high porosity, whereas for lower porosity and higher stress, grain fracturing controlled the deformation. The most pronounced localization of deformation was observed for the coarse-grained, low-porosity material. A Cam-Clay cap model was used to describe the porosity loss during compaction and shear-enhanced compaction, demonstrating large inelastic compaction with increasing confining pressure. Yield stress and end caps for poorly lithified sandstone are observed for various porosities and stress conditions and found to be lower than predicted using empirical relationships for sandstone.Citation: Skurtveit, E., A. Torabi, R. H. Gabrielsen, and M. D. Zoback (2013), Experimental investigation of deformation mechanisms during shear-enhanced compaction in poorly lithified sandstone and sand,