Impact of shear-enhanced compaction bands on fluid flow via High-Speed Neutron Tomography

Shear-enhanced compaction bands were identified in porous sandstone outcrops and were also reproduced at the laboratory-scale. These bands shares common characteristics with shear bands in terms of the physical micro-processes occurring during their onset and evolution. At the laboratory-scale the micro-processes differ in proportions and order of occurrence. Shear-enhanced compaction bands are characterised by textural changes with respect to the host rock due to grain fragmentation and crushing. Such processes affect the pore connectivity within the bands and lead to changes in porosity and presumed permeability. Therefore, the presence of these bands in the reservoir scale may impact fluid flow. This could have implications for a number of geo-energy applications, such as hydrocarbon production in subsurface reservoirs and/or CO2 storage into aquifers of depleted reservoirs.
To capture the mechanical behaviour of shear-enhanced bands during their creation and evolution a number of non-destructive experimental techniques has been used, providing understanding of the 4D evolution of their properties. This work focuses on the impact of shear-enhanced compaction bands on the rock-fluid system. To do so, we performed water imbibition tests on sandstone samples with pre-existing lab-induced shear-enhanced compaction bands using High-Speed Neutron Tomography (HSNT). The objective is to observe the water-deformation interaction by tracking the water front. The advantage of using Neutrons (as opposed to x-rays) is the high contrast between material and water. Neutrons are sensitive to Hydrogen and thus, any fluid containing it.
Imbibition experiments with in-situ HSNT were performed at the CONRAD beamline at Helmholtz Zentrum Berlin (HZB). The neutron tomography included 300 radiographic projections acquired during rotation of the sample over 180o (back and forward since a full rotation was restricted by the experimental set-up). The acquisition time for each radiography was 0.2 sec, which results in a total time of 1 min /tomography. The tested specimens were wrapped in Teflon tape and placed in a heat-shrunk FEP membrane. Afterwards samples were settled in a cap sealed with silicon and fixed at the rotation stage. A small reservoir was connected to the cap through an electro-operated valve so that the water supply to the cap could be controlled during the experiment. Both image reconstruction and the flow front detection and tracking have been achieved by using in-house software (based on ASTRA libraries in the case of reconstruction).
Our results show that in-situ HSNT can capture successfully the 4D evolution of the water front in samples with shear-enhanced compaction bands. The water front has a piston-like shape in regions below the bands. The curvature of the flow front slightly changes as the front propagates through the network of bands. When the water front is approximately at the level of the bands, shear-enhanced compaction bands are visualised by HSNT, however, when the saturation of the sample increases, this is not feasible anymore. Flow speed maps demonstrate local increase in flow speed inside the bands. These bands have been previously characterised by intense grain fragmentation, compaction and porosity loss, presumably leading to local increase in capillary pressure, thus saturation locally accelerates.