Estimation of Flow properties in Shear and Compaction Bands via Imaging Methods

Compaction and shear bands are modes of localisation phenomena that develop during bulk strain accumulation in geomaterials. In outcrops of porous sandstones, both types of deformation bands are often observed to be nearly-planar zones, 3-6 grain-diameters in thickness, that often develop multiple, over-printing strands to create cm-scale or thicker zones that are usually thought to inhibit fluid flow. Grain-scale mechanisms in such bands indicate local porosity reduction by grain crushing and movement of the resulting fragments into pores, along with grain movements allowing re-arrangement in tighter packing states. Triaxial compression experiments, enhanced by 3D Digital Image Correlation with the addition of Acoustic Emission location and micro-crack typing (before, during and after the lab-induced deformation), provide a means of understanding the micro-mechanics of the local deformation processes, which include self-organised regions of dilation and compaction, plus shear, for both types of bands.

Various research teams have undertaken laboratory tests, with the addition of in-experiment fluid flow measurements, which lead to a conclusion that the textural changes inside such bands cause orders-of-magnitude reductions of the permeability compared to the host-rock material. Here, we employ digital-rock methods in 3D to derive local estimates of the flow-property effects of the lab-induced localised deformation features and of the surrounding host-rock. This approach employs several scales of imaging methods, with most emphasis on the textures of the bands themselves. There, high-resolution maps of the deformation bands, plus local higher-resolution SEM images of sub-regions, enable the creation of mm-scale 3D models of deformed-rock materials. These models are used to calculate single- and multi-phase flow properties of the identified sub-regions of the bands that relate to local variations in strain states and Acoustic Emission locations. The permeability of such bands is, indeed, reduced by approx 10^-4 compared to the permeability of the host-rock material, confirming the received wisdom. The digital-rock method also allows an estimation of multi-phase flow properties, which are less easy to estimate via experiments. Here, we illustrate the value of the digital-rocks methods by predicting multi-phase flow in sample-sized regions that contain different and/or multiple bands. We plan to test these predictions by means of flow experiments where the fluid displacements are being directly imaged.