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X-Ray Imaging for Compaction Band Characterisation in Porous Sandstones


Compaction bands in porous sandstones are planar zones of finite thickness that form almost normal to the direction of maximum compression. Grain-scale mechanisms indicate porosity reduction by grain crushing and movement of fragments into pores, along with grain movements. Such bands can have important impacts on fluid flow in sandstone reservoirs. We investigate by experiments the grain-scale processes occuring during compaction band formation in two porous (~22%) sandstones with similar grain-sizes (~300μm). Vosges sandstone is 93% quartz, 5% microcline, 1% kaolinite and 1% micas; it is moderately sorted with sub-angular to rounded shapes, and cement occurs mostly as quartz overgrowths. The texture of Vosges is more heterogeneous than the Bentheim, which has rounded grains, and is composed of 95% quartz, 3% feldspar and 2% kaolinite. Cylindrical samples of 40mm/80mm (diameter/length) for Vosges, and 50mm/100 mm for Bentheim, were loaded under triaxial compression with confining pressure of 130–190 MPa. All specimens had a circumferential notch at their mid-length to force localization. X-ray CT was obtained before and after the experiments, defining the changes caused by deformation. The standard deviation of the grey-scale values in Vosges is smaller than that in Bentheim (undeformed), which implies a more homogeneous density for the former due to cementation. Compaction bands created in both sandstones are identified as zones of low standard deviation values, due to compacted material and associated local porosity reduction. In the Bentheim raw X-ray images, compaction bands are also visible as higher-density regions. In both sandstones, compaction bands have curved or irregular shapes rather than the planar ones expected, and have orientations as low as 66o to the shortening. Acoustic emission data shows that the deformation mechanisms involve shear motions as well as volume loss. Local strains calculated with Digital Image Correlation confirm the AE interpretations, revealing both volumetric and shear strains that vary within and near the bands. Pore networks extracted from the 5μm resolution X-ray images of the localized zones allow us to calculate the flow properties of compaction bands and matrix in both sandstones. Permeability reduction is significant (~3 orders), and calculated relative permeability provides insights into the multi-phase flow effects of such bands.