Compaction localization in high porosity sandstones with various degrees of heterogeneity: insight from X-ray Computed Tomography

Rock Engineering 2009 - Rock Engineering in Difficult Conditions
Patrick Baud, Alexandra Rolland, Teng-fong Wong,
Abstract Compaction localization may occur in geomaterials at relatively high confining pressure as pure volume reduction along a direction parallel to the maximum principal stress (compaction band) or associated with shearing (compactive shear band). Investigating the formation and geometric attributes (orientation, thickness, tortuosity, spatial distribution,…) of such potentially ubiquitous features is fundamental for the optimization of mechanical and fluid transport models. However, despite a growing body of experimental data, the parameters controlling the nucleation and propagation of localized compaction features as well as the associated spatial distribution of damage have not been clearly identified. Early studies suggested that porosity was exerting a major control on the occurrence of compaction localization. To date, discrete compaction bands were for example only observed in sandstones of porosity larger than 20%. More recent studies, including numerical simulations, demonstrated the importance of other microstructural parameters such as size distribution of grains, pores, grain contacts, or mineral composition.
X-ray computed tomography is an imaging technique that is sensitive to the average absorption coefficient of the materials contained in every voxel, which correlates well with density for matrix/void mixtures. This effect is particularly obvious in the case or rock samples that have failed by developing dilating shear bands. Can X-ray CT imaging as well be used for the study of localized compaction? More specifically, can it provide adequate parameters for predicting the development and geometric attributes of compaction features?
We present results obtained from X-ray CT scans of six sandstones acquired at 50 microns resolution in which various degrees of compaction localization (from highly localized to homogeneously distributed) have been observed. While raw X-ray attenuation data do not allow to discriminate between compacted and relatively undamaged zones, it has been shown by earlier studies that the coefficient of variation (COV) (standard deviation divided by the mean) calculated over volumes of 3*3*3 voxels could reveal significant changes in density homogeneity associated with compaction. When calculated on images of intact samples, we show that the COV provides information on grain scale heterogeneity, and this result, together with the average X-ray attenuation which provides a proxy for the porosity, can be compared with the type of compaction features that are later observed in the deformed samples (compaction bands, compactive shear bands, diffuse compaction bands or homogeneous compaction). In order to estimate the range of COV and mean attenuation values that are attained through compaction in a given sample, we also followed the progressive development of compaction localization in a sample of Bleurswiller sandstone of 25% porosity. This sample was deformed triaxially by steps, each step corresponding to an increasing level of plastic strain, and was imaged before the experiment and after each compression step.
Overall, our results suggest that X-ray CT data can readily provide two parameters (average attenuation value and coefficient of variation) involved in the development of localized compaction, and that different types of sandstones may be represented on plots made of such two variables, as well as progressive compaction for a given type.
Keywords: Strain localization, Sandstone, Heterogeneity, compaction, X-ray CT
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