Effect of fissure drainage on dispersive mixing in porous media gravity currents: an experimental investigation

Abstract

The prospect of storing hydrogen in depleted natural gas reservoirs requires that the dispersive mixing between injected hydrogen and the cushion gas resident within the porous medium be well understood. Motivated by this observation, we report upon a series of similitude laboratory experiments employing salt and fresh water; experiments use optical techniques to characterize the dispersive mixing that results from porous media gravity current flow. Importantly, the gravity current is supposed to propagate over a `"`leaky" boundary, i.e. one containing an isolated fissure that allows drainage into an underlying (and dynamically-isolated) layer. Thus do we find that drainage and dispersion are connected: as more (relatively undiluted) fluid drains through the fissure, the remaining gravity current fluid is disproportionately comprised of fluid that has been significantly diluted through dispersive mixing. A novel metric for comparing the relative amount of dispersed fluid present is proposed. We then characterize the variation of this so-called dispersed buoyancy fraction with the gravity current density and source height, the dip angle and the fissure geometry. Thus do we find, for example, that dispersion increases sharply for down-dip flow whereas dispersion may plateau as the fissure area is increased. Select experimental data are contrasted against the predictions of a previously-published theoretical model. The implications of said comparison are then discussed e.g. in the context of model refinement.