Influence of wettability on dense nonaqueous phase liquid (DNAPL) constitutive relationships in saturated porous media.

Abstract

Considerable attention has been focused on issues relating to groundwater protection and remediation. As a result, significant research has explored on the processes governing dense nonaqueous phase liquid (DNAPL) contaminant migration and entrapment following its release to the subsurface environment. Much of this work has assumed that subsurface materials are completely water-wet even though variations in wettability are likely in the contaminated subsurface. In this work a series of one and two-dimensional experiments was conducted to quantify the effects of solid wettability on DNAPL migration and entrapment in saturated porous media. In the two-dimensional infiltration study the organic-wet sands acted as a very effective capillary barrier, retaining the DNAPL and inhibiting its downward migration. Numerical simulations of this sand box infiltration experiment employing the wettability modified van Genuchten and Brooks-Corey retention functions, in conjunction with the Burdine model, were able to bracket observed DNAPL migration and entrapment behavior. This infiltration study also highlighted the need for a simple predictive retention function model for the broad range of wettabilities that may be encountered in the contaminated subsurface. Such a model, based upon the Leverett and Cassie equations, was developed to predict drainage and imbibition retention functions in fractional water, intermediate and organic-wet systems. This Leverett-Cassie equation was validated with data from number of water/intermediate and water/organic fractional wettability DNAPL/water systems measured in this study, as well as data from two published studies. A series of multistep outflow experiments was also conducted to determine the effects of wettability on fluid flow. A sensitivity analysis found that outflow predictions were very sensitive to the retention function, but fairly insensitive to the selected relative permeability model. In addition simulation results, based upon traditional multiphase flow equations, failed to adequately fit observed outflow rates. To improve the outflow fit, the multiphase flow governing equations were modified to incorporate dynamic capillary pressure effects. Inclusion of a saturation-dependent dynamic capillary pressure effect that significantly improved water outflow and outflow rate fits. Endpoint permeability measurements from these outflow experiments indicate that endpoint relative permeabilities do not approach a value of one at residual saturations, as is typically assumed.