Cosolvent-Water Displacement in One-Dimensional Soil Column

Abstract

A one-dimensional flow and transport model with dynamic fluid density and viscosity terms is proposed for modeling cosolvent flushing in a water-saturated porous medium. Given knowledge of the density and viscosity functions for cosolvent-water mixtures, the model is controlled by two flow parameters (specific storage and permeability) and one transport parameter (dispersivity). Sensitivity analysis demonstrates that the model solution is relatively insensitive to the flow parameters for the imposed constant head, constant flow conditions. The dynamic density and viscosity model is tested against the conventional transport model in simulating breakthrough data from soil column experiments in which water is displaced by pure methanol pulses (or slugs). Methanol slug breakthrough behavior is first predicted using independent parameter estimates (dispersivity obtained from tracer tests), then the dispersivity was adjusted to obtain optimal fits. The dynamic model provided slightly better predictions than the conventional transport model but failed to accurately reproduce methanol breakthrough behavior. Irregularities in observed slug breakthrough curves suggest that frontal instabilities may have been the cause of the discrepancy between the model and observations. Cosolvent overriding may have also contributed to the discrepancy in the horizontal displacement case.