Experimental investigation of deformation mechanisms during shear‐enhanced compaction in poorly lithified sandstone and sand

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,

From mechanical modeling to seismic imaging of faults: A synthetic workflow to study the impact of faults on seismic

“…They showed that cemented sandstone typically exhibit velocities that increase with strain. Skurtveit et al (2013) showed the same impact of strain on seismic velocities by investigating deformation mechanisms and their impact on ultrasonic velocity during shear-enhanced compaction of poorly lithified, cm-scale sandstone samples (volumetric strains lower than 5%).…”

“…Porosity changes are unlikely to vary linearly with strain, however data from Skurtveit et al, 2013, Figure 5) and from Sigernes (2004) show that porosity changes fluctuate less than velocity changes. Although simplistic, a linear relation can be considered as an acceptable approximation.…”

“…These studies also show high variability of properties within the fault zones (Table 2). , and Skurtveit et al (2013). Similar workflows have previously been applied to faults.…”

Seismic Imaging of Salt-influenced Compressional Folds

“…Therefore, based on the few available studies about the impact of compaction on seismic properties (e.g., Holt et al, 2008, Sigernes, 2004Skurtveit et al, 2013, we can use very simple empirical relationships between finite strain (e.g. volumetric strain) and changes in the rocks physical properties, similar to those of .…”

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“…However, the influence of the physical characteristics of the granular geological materials is also very important. Although it has been reported that particle mineralogy and shape both affect the performance of an ultrasonic compaction 3 process [12,13], the influence of geological materials is unclear. There has been conflicting evidence between different studies with respect to the influence of vibrations on densification [14], and it has been proposed that significant differences between the physical properties of the particles, such as grain size and density, are contributory factors.…”