MISCIBLE FLUID DISPLACEMENT STABILITY IN UNCONFINED POROUS-MEDIA - 2-DIMENSIONAL FLOW EXPERIMENTS AND SIMULATIONS

In situ flushing groundwater remediation technologies, such as cosolve nt flushing, rely on the stability of the interface between the reside nt and displacing fluids for efficient removal of contaminants. Contra sts in density and viscosity between the resident and displacing fluid s can adversely affect the stability of the displacement front. Petrol eum engineers have developed techniques to describe these types of pro cesses; however, their findings do not necessarily translate directly to aquifer remediation. The purpose of this laboratory study was to in vestigate how density and viscosity contrasts affected cosolvent displ acements in unconfined porous media characterized by the presence of a capillary fringe. Two-dimensional flow laboratory experiments, which were partially scaled to a cosolvent flushing field experiment, were c onducted to determine potential implications of flow instabilities in homogeneous sand packs. Numerical simulations were also conducted to i nvestigate the differential impact of fluid property contrasts in unco nfined and confined systems. The results from these experiments and si mulations indicated that the presence of a capillary fringe was an imp ortant factor in the displacement efficiency. Buoyant forces can act t o carry a Lighter-than-water cosolvent preferentially into the capilla ry fringe during displacement of the resident groundwater. During subs equent water flooding, buoyancy forces can act to effectively trap the cosolvent in the capillary fringe, contributing to the inefficient re moval of cosolvent from the aquifer. (C) 1998 Elsevier Science B.V.