Stephen W. Webb

Gas transport in porous media

Table of Contents 1. Introduction C.K. Ho and S.W. Webb Part 1: Processes and Models 2. Gas Transport Mechanisms S.W. Webb 3. Vapor Transport Processes C.K. Ho 4. Solid/Gas Partitioning S.K. Ong 5. Two-Phase Gas Transport S.W. Webb 6. Conservation Equations S. Whitaker 7. Gas-Phase Dispersion in Porous Media M.S. Costanza-Robinson and M.L. Brusseau 8. Gas Injection and Fingering in Porous Media M. Sahimi, M. Reza Rasaei, and M. Haghighi 9. Unstable and Fingering Gas Flow in Fractures P. Persoff 10. Natural Convection Gas Transport in Porous Media K. Khanafer and K. Vafai 11. Scaling Issues in Porous and Fractured Media V.C. Tidwell 12. Numerical Codes for Continuum Modeling of Gas Transport in Porous Media K. Pruess 13. Lattice Boltzmann Method for Calculating Fluid Flow and Dispersion in Porous and Fractured Media H.W. Stockman Part 2: Measurement and Monitoring 14. Experimental Determination of Transport Parameters O. Solcova and P. Schneider 15. Air Permeability Measurements in Porous Media V.C. Tidwell 16. Analyzing Barometric Pumping to Characterize Subsurface Permeability J. Rossabi 17. Subsurface Flow Measurements J. Rossabi 18. Measurement of Vapor Concentrations C.K. Ho, M. Kelly and M.T. Itamura 19. In-Situ Measurement of InducedContaminant Flux A. Tartre Part 3: Applications 20. Radon Transport B.W. Arnold 21. Gas Transport Issues in Landmine Detection J. Phelan 22. Environmental Remediation of Volatile Organic Compounds R. Falta 23. Yucca Mountain Heater Tests Y.Y.W. Chang 24. Impact of Gas Generation on the Performance of the Waste Isolation Pilot Plant P. Vaughn 25. Oil and Gas Industry Applications of Gas Flow in Porous Media D.J. Borns 26. Geological Carbon Sequestration: CO2 Transport in Depleted Gas Reservoirs C.M. Oldenburg 27. Industrial Gas Transport Processes Involving Heat Transfer O.A. Plumb Index

A simple extension of two-phase characteristic curves to include the dry region

Two-phase characteristic curves are necessary for the simulation of water and vapor flow in porous media. Existing functions such as van Genuchten, Brooks and Corey, and Luckner et al. have significant limitations in the dry region as the liquid saturation goes to zero. This region, which is important in a number of applications including liquid and vapor flow and vapor-solid sorption, has been the subject of a number of previous investigations. Most previous studies extended standard capillary pressure curves into the adsorption region to zero water content and required a refitting of the revised curves to the data. In contrast, the present method provides for a simple extension of existing capillary pressure curves without the need to refit the experimental data. Therefore, previous curve fits can be used, and the transition between the existing fit and the relationship in the adsorption region is easily calculated. The data-model comparison shows good agreement. This extension is a simple and convenient way to extend existing curves to the dry region.

The Use of Fick's Law for Modeling Trace Gas Diffusion in Porous Media

Two models for combined gas-phase diffusion and advection in porous media, the advective-diffusive model (ADM) and the dusty-gas model (DGM), are commonly used. The ADM is based on a simple linear addition of advection calculated by Darcys law and ordinary diffusion using Ficks law with a porosity–tortuosity–gas saturation multiplier to account for the porous medium. The DGM applies the kinetic theory of gases to the gaseous components and the porous media (or ‘dust’) to develop an approach for combined transport due to diffusion and advection that includes porous medium effect. The ADM and Ficks law are considered to be generally inferior for gas diffusion in porous media, and the more mechanistic DGM is preferred. Under trace gas diffusion conditions, Ficks law overpredicts the gas diffusion flux compared to the DGM. The difference between the two models increases as the permeability decreases. In addition, the difference decreases as the pressure increases. At atmospheric pressure, the differences are minor (<10%) for permeabilities down to about 10−13 m2. However, for lower permeabilities, the differences are significant and can approach two orders of magnitude at a permeability of 10−18 m2. In contrast, at a pressure of 100 atm, the maximum difference for a permeability of 10−18 m2 is only about a factor of 2. A molecule–wall tortuosity coefficient based on the DGM is proposed for trace gas diffusion using Ficks law. Comparison of the Knudsen diffusion fluxes has also been conducted. For trace gases heavier than the bulk gas, the ADM mass flux is higher than the DGM. Conversely, for trace gases lighter than the bulk gas, the ADM mass flux is lower than the DGM. Similar to the ordinary diffusion variation, the differences increase as the permeability decreases, and get smaller as the pressure increases. At atmospheric pressure, the differences are small for higher permeabilities (>10−13 m2) but may increase to about 2.7 for He at lower permeabilities of about 10−18 m2. A modified Klinkenberg factor is suggested to account for differences in the models.

Capillary barrier performance in heterogeneous porous media

The effects of heterogeneities on the performance of capillary barriers is investigated by numerically simulating three systems comprised of a fine soil layer overlying a coarse gravel layer with (1) homogeneous, (2) layered heterogeneous, and (3) random heterogeneous property fields. The amount of lateral diversion above the coarse layer under steady state infiltration conditions is compared among the simulations. Results indicate that the performance of capillary barriers can be significantly influenced by the spatial variability of hydraulic properties. In the layered heterogeneous systems, realizations with highly stratified regions within the fine layer performed best and resulted in localized capillary barriers that delayed breakthrough into the coarse layer. In contrast, realizations of the random heterogeneous system performed worst because of channeled flow that produced numerous localized regions of breakthrough into the coarse layer. Results of the homogeneous model were comparable to the mean results of the layered heterogeneous realizations, but homogeneous results underpredicted the frequency and amount of breakthrough for all realizations of the random heterogeneous system. These results indicate that homogeneous models can be used to estimate the average behavior of layered heterogeneous systems with reasonable accuracy. In addition, engineered capillary barriers may be improved through emplacement and packing methods that induce highly stratified features within the fine layer of a capillary barrier system.

Mixing of Stably Stratified Gases in Subsurface Reservoirs: A Comparison of Diffusion Models

Numerical simulations of the mixing of carbon dioxide (CO2) and methane (CH4) in a gravitationally stable configuration have been carried out using the multicomponent flow and transport simulator TOUGH2/EOS7C. The purpose of the simulations is to compare and test the appropriateness of the advective–diffusive model (ADM) relative to the more accurate dusty-gas model (DGM). The configuration is relevant to carbon sequestration in depleted natural gas reservoirs, where injected CO2 will migrate to low levels of the reservoir by buoyancy flow. Once a gravitationally stable configuration is attained, mixing will continue on a longer time scale by molecular diffusion. However, diffusive mixing of real gas components CO2 and CH4 can give rise to pressure gradients that can induce pressurization and flow that may affect the mixing process. Understanding this coupled response of diffusion and flow to concentration gradients is important for predicting mixing times in stratified gas reservoirs used for carbon sequestration. Motivated by prior studies that have shown that the ADM and DGM deviate from one another in low permeability systems, we have compared the ADM and DGM for the case of permeability equal to 10−15 m2 and 10−18 m2. At representative reservoir conditions of 40 bar and 40°C, gas transport by advection and diffusion using the ADM is slightly overpredicted for permeability equal to 10−15 m2, and substantially overpredicted for permeability equal to 10−18 m2 compared to DGM predictions. This result suggests that gas reservoirs with permeabilities larger than approximately 10−15 m2 can be adequately simulated using the ADM. For simulations of gas transport in the cap rock, or other very low permeability layers, the DGM is recommended.

Development of a mechanistic model for the movement of chemical signatures from buried land mines/UXO

The detection and removal of buried landmines and unexploded ordnance (UXO) is one of the most important problems facing the world today. Numerous detection strategies are being developed, including IR, electrical conductivity, ground- penetrating radar, and chemical sensor. Chemical sensor rely on the detection of explosive chemical molecules, which are transported from buried UXO/landmines by advection and diffusion in the soil. As part of this effort, numerical models are being developed to predict explosive chemical signature transport in soils. Modifications have been made to TOUGH2, a general-purpose porous media flow simulator, for application to the chemical sensing problem resulting in the T2TNT code. Understanding the fate and transport of explosive signature compounds in the solid will affect the design, performance, timing and operation of chemical sensing campaigns by indicating preferred sensing strategies.

Effect of diurnal and seasonal weather variations on the chemical signatures from buried land mines/UXO

The chemical signature form buried landmines/UXO is affected by a number of environmental fate and transport processes in the soil such as vapor-solid and liquid-solid sorption, diffusion, biodegradation, and water movement. For shallow burial depths, land surface processes, such as wind, solar and long-wave radiation, and precipitation play an important role. The impact of these land surface processes has been evaluated for a landmine/UXO buried 5 cm below the surface using actual weather data for an entire year using the T2TNT computer code. The gas-phase concentration of the chemical signatures, which is used by most chemical sensors currently being developed, shows appreciable diurnal variation and minimum seasonal changes due to the change in the weather. The most dramatic variation in the gas-phase concentration occurs immediately after a rainfall following a long dry period. This information will impact the use of chemical sensors by indicating the best times of the day and best times of the year to sense these signatures.

Laboratory data and model comparisons of the transport of chemical signatures from buried land mines/UXO

Sensing the chemical signature emitted from the main charge explosives from buried landmines and unexploded ordnance (UXO) is being considered for field applications with advanced sensors of increased sensitivity and specificity. The chemical signature, however, may undergo many interactions with the soil system, altering the signal strength at the ground surface by many orders of magnitude. The chemidynamic processes are fairly well understood from many years of agricultural and industrial pollution soil physics research. Due to the unique aspects of the surface soil environment, computational simulation is being used to examen the breadth of conditions that impact chemical signature transport, from the buried location to a ground surface release. To provide confidence in the information provided by simulation modeling, laboratory experiments have been conducted to provide validation of the model under well-constrained laboratory testing conditions. A soil column was constructed with soil moisture monitoring ports, a bottom porous plate to regulate the soil moisture content, and a top plenum to collect the surface flux of explosive chemicals. The humidity of the air flowing through the plenum was set at about 50 percent RH to generate an upward flux of soil moisture. A regulated flux of aqueous phase 2,4-DNT was injected into the soil at about ten percent of the upward water flux. Chemical flux was measured by sampling with solid phase microextraction devices and analysis by gas chromatography/electron capture detection. Data was compared to model results from the T2TNT code, specifically developed to evaluate the buried landmine chemical transport issues. Data and model results compare exceptionally well providing additional confidence in the simulation tool.

Effect of soil layering on NAPL removal behavior in soil-heated vapor extraction

Abstract Soil heating has been proposed as a method to enhance the vapor extraction of NAPLs from contaminated soils. Three-dimensional fluid flow and heat transfer simulations have been performed for soil-heated vapor extraction to determine the transient system performance for a hypothetical configuration. Soil layering has been considered in evaluation of the initial non-aqueous phase liquid (NAPL) distribution and in evaporation and transport to the vapor extraction location. Results from this layered model are compared with results for a homogeneous system with an initially uniform NAPL, indicating the influence of layering, the initial NAPL distribution, the type of NAPL, and the possibility of enhanced vapor diffusion. Not only is the NAPL removal time reduced significantly with the addition of heat, but the uncertainty in the removal time owing to a number of difficult to characterize in situ factors, such as layering and the initial NAPL distribution, is much less than for standard soil vapor extraction without heating, owing to the rise in temperature and increase in NAPL vapor pressure with time.

Thermally induced natural convection effects in Yucca Mountain drifts.

Thermally induced natural convection from the heat produced by emplaced waste packages is an important heat and mass transfer mechanism within the Yucca Mountain Project (YMP) drifts. Various models for analyzing natural convection have been employed. The equivalent porous medium approach using Darcys law has been used in many YMP applications. However, this approach has questionable fidelity, especially for turbulent flow conditions. Computational fluid dynamics (CFD), which is based on the fundamental Navier-Stokes equations, is currently being evaluated as a technique to calculate thermally induced natural convection in YMP. Data-model comparisons for turbulent flow conditions show good agreement of CFD predictions with existing experiments including YMP-specific data.