Neutron imaging provides direct observation of evolving water–air and deuterated water–normal water exchanges in flow experiments performed on a laboratory-deformed, microporous, extremely fine-grained laminated limestone. First D2O, then H2O, are injected into the sample base, and the liquids' positions and concentrations are monitored by a combination of neutron radiography and tomography. The pre- neutron beam experimental deformation, at a confining pressure of 20MPa and with axial shortening, produced arrays of fractures and deformation bands superposed onto the undeformed, complex rock texture. The neutron images document significant, evolving, water speed and flow-direction variability in both D2O (figure 1a) and H2O (figure 1b) at the deci-micron scale, and spatially complex patterns of both increasing and decreasing water saturation in the matrix and in the induced features. We infer that what would normally be referred to as capillarity-driven and pressure-driven water movement occur concurrently, in close proximity and in competition, that fluid movement paths are very different between the base-to top matrix frontal advancement and in movement up semi-open fractures. It appears that as local and global water saturations evolve as these two drivers can change their dominance in both matrix and deformed components.
Thin sections have been used to obtain sub-micron resolution SEM images that provide multi-scale information on the textural features' spatial arrangements. The textural characteristics are consistent with the inferences made from the coarser-scale flow imaging. Alternating lamina types provide the primary lithological heterogeneity, while the experimentally created deformations lead to quasi-planar zones of highly comminuted matrix and fracture-like voids, each with characteristic scales ranging from sub-mm to cm. Together, the induced deformation features delineate a partially connected array. The interplay between fluid movement through deformation features, and flow into (and out of) the laminae, implies near-equivalence of local driving pressure and capillary related energies, with subtle shifts in this balance as water saturation increases. The insights gained invite a re-examination of common rules-of-thumb for multi-phase fluid flow often adopted in fractured, low-permeability microporous rocks.
