X-ray visualized unstable displacements of water by gas in sandstone slabs for subsurface gas storage

A series of experiments have been performed studying the displacement of water by gas in consolidated porous media at an adverse viscosity ratio of μw/μg = 100, from the perspective of subsurface gas storage. The gas/water relative permeability functions (RPs) produced in the laboratory to model these processes generally use short cores from which the RPs are derived using conventional analysis and assumptions. In this work, we present results that challenge some of these assumptions and bring into question some of the currently used RP functions to design storage scenarios gas/water systems. Using a novel visualization technique, large two-dimensional sandstone slabs are imaged via x-rays during the gas → water unstable drainage processes. Three experiments were carried out evaluating the impact of rate and vertical flow direction. In the bottom-to-top experiments, we observed the rate dependence on the evolution of viscous fingers from an initially stable bank resulting from the effect of capillary dispersion stabilizing the early finger growth. In the case of top-to-bottom displacement, we observe that the combined capillary and gravity forces are not sufficient to fully stabilize the system, although a visible “stable” bank is formed prior to the emergence of the gas fingering instability. Finally, these results are compared to a water → oil drainage carried out under the same conditions and viscosity ratio. The results are then discussed in the context of subsurface gas storage, and recommendations are made for future experiments designed to derive appropriate gas/water RP functions and for upscaling the results from the laboratory to the field.