Grantee Research Project Results

1999 Progress Report: Theoretical Evaluation of the Interfacial Area between Two Fluids in Soil

EPA Grant Number: R827116
Title: Theoretical Evaluation of the Interfacial Area between Two Fluids in Soil
Investigators: Bryant, Steven
Institution: The University of Texas at Austin
EPA Project Officer: Aja, Hayley
Project Period: October 1, 1998 through September 30, 2001 (Extended to November 30, 2002)
Project Period Covered by this Report: October 1, 1998 through September 30, 1999
Project Amount: $246,378
RFA: Exploratory Research - Physics (1998) RFA Text |  Recipients Lists
Research Category: Air , Safer Chemicals , Land and Waste Management

Objective:

The overall rate of mass transfer of a chemical species between a nonaqueous phase liquid (NAPL) and an immiscible fluid phase (water, air) is a critical parameter in several environmental applications. The rate of mass transfer depends upon the thermodynamic driving force and the area of the interface between the phases. The interfacial area depends very strongly upon the geometry of the pore space of the host soil or rock and is consequently very difficult to measure directly. This project will develop a novel mathematical modeling technique to predict the interfacial area from first principles.

Progress Summary:

The area of the interface between two immiscible phases in a porous medium depends on the geometric configuration of the phases. This configuration is governed by the pressure difference between the phases, the geometry of the pore space, and the history of fluid displacement within the medium. In naturally occurring granular porous media, the locations of the grains are random, and consequently, the pore space is highly irregular. A physically representative analogue of such media is the random, dense packing of equal spheres described by Finney. Techniques have been developed previously for uniquely locating pore throats and pore bodies in this packing, for extracting network representations of the pore space, and for simulating drainage (displacement of the wetting phase by the nonwetting phase) in these networks. These provide a sound theoretical basis for quantifying the area of the fluid-fluid interface in simple porous media.

During the first year of this project, we have extended these drainage simulations to locate and quantify trapped volumes of wetting phase. These volumes are trapped when they become disconnected from the bulk wetting phase. The morphology of the trapped wetting phase depends strongly on the assumptions made regarding wetting phase connectivity. We have evaluated two possibilities: trapping only at a grain contact, and trapping at a grain contact and within pore throats associated with the grain contact. Trapping at a grain contact occurs when all the pore throats surrounding a grain contact have been drained (invaded by nonwetting phase), leaving the wetting phase as a pendular ring at the contact. The shape of the pendular ring depends on the grain size, the separation between the grains, and the capillary pressure (pressure in the nonwetting phase less the pressure in the wetting phase) at which the last pore throat associated with the contact was drained. Not every grain contact will support a pendular ring; for any given grain separation, there is a maximum value of capillary pressure above which a pendular ring is not stable.

Trapping within a pore throat arises when we relax the criterion for trapping at a grain contact. The criterion described above assumes that the wetting phase in pore throats associated with the grain contact is connected to the bulk wetting phase, even when all the pore bodies associated with the contact have been drained. Under that criterion, all the pore throats can eventually be drained as the capillary pressure increases. An alternative assumption is that the wetting phase in undrained pore throats associated with a grain contact becomes trapped when all the pore bodies associated with the contact are drained. The wetting phase morphology in this case is a pendular ring at the contact connected to lenses in the undrained throats.

The local configuration of the phases, and hence the area of the interface between them, is computed on a pore-by-pore basis at each stage of a drainage simulation. The area of the pendular rings is readily determined from classical methods. The interface in undrained pore throats is assumed to be a spherical cap. The total interfacial area includes contributions from isolated (trapped) volumes of wetting phase and from the bulk wetting phase (connected to an external sink). The relative magnitudes of these contributions depend strongly on the assumptions made regarding the connectivity of the wetting phase during the drainage simulation. If the wetting phase is assumed to be trapped only at grain contacts (pendular rings only), the contribution of the isolated wetting phase to the total interfacial area is very small for wetting phase fractions greater than 0.1. On the other hand, if the wetting phase is assumed to be trapped at grain contacts and in associated pore throats (pendular rings and lenses), the isolated wetting phase makes a significant contribution to the total interfacial area during the entire drainage process. Regardless of the mode of trapping, the total interfacial area exhibits a maximum at low wetting phase saturation. This maximum is about one order of magnitude smaller than the grain surface area.

Measurements of interfacial area with tracers will not distinguish the contributions of isolated and connected volumes of wetting phase. This distinction may be critical, however, in evaluating a remediation or contamination process, depending on the relative importance of transfer to/from the bulk phase and transfer to/from volumes of trapped phase. These predictions will, thus, be useful in interpreting measurements both of interfacial area and of mass transfer rates.

Future Activities:

Given the apparent importance of the trapping criteria in determining interfacial area, we will implement a high-resolution drainage algorithm that will allow more accurate determinations of the order in which pores and pore throats are drained. This will, in turn, yield a more accurate assessment of the contributions of lenses and pendular rings. We also will consider a more refined evaluation of wetting phase connectivity so that a more accurate identification of trapped wetting phase (disconnected from the bulk phase) can be made. We also will begin evaluating interfacial area during imbibition (displacement of nonwetting phase by wetting phase).

Journal Articles:

No journal articles submitted with this report: View all 15 publications for this project

Supplemental Keywords:

remediation, nonaqueous phase liquid, NAPL, soil, chemical transport, physics, modeling., RFA, Scientific Discipline, Air, Waste, Toxics, Mathematics, chemical mixtures, Chemistry, HAPS, Engineering, Chemistry, & Physics, Groundwater remediation, Physics, chemical transport modeling, porus media, soil , groundwater contamination, thermodynamics, NAPL, fate and transport, interfacial phenomena, interwall partitioning tracer tests, NAPLs, mass transfer

Relevant Websites:

http://www.ticam.utexas.edu/CSM/EPA/area/index.html
http://www.ticam.utexas.edu/CSM/EPA/connectivity/index.html
http://www.ticam.utexas.edu/CSM/EPA/critcurv/index.html

Progress and Final Reports:

Original Abstract

2000 Progress Report

2001 Progress Report

2002

Final Report