Investigatory Project “ Kaymito Leaves Decoction as Antiseptic Mouthwash ” Essay

Introduction 1.1 Problem Statement Fractures are prevalent in natural and synthetic structural media, even in the best engineered materials. We find fractures in bedrock, in sandstone aquifers and oil reservoirs, in clay layers and even in unconsolidated materials (Figures 1.1 to 1.4). Fractures are also common in concrete, used either as a structural material or as a liner for storage tanks (Figure 1.5). Clay liners used in landfills, sludge and brine disposal pits or for underground storage tanks can fracture, releasing their liquid contents to the subsurface (Figure 1.6). Even â€Å"flexible† materials such as asphalt fracture with time (Figure 1.7). The fact that fractures are inevitable has led to spending billions of research dollars to construct â€Å"safe† long-term (10,000 years or more) storage for high-level nuclear waste (Savage, 1995; IAEA, 1995), both to determine which construction techniques are least likely to result in failure and what are the implications of a failure, in terms of release to the environment and potential contamination of ground water sources or exposure of humans to high levels of radioactivity. Why do materials fail? In most cases, the material is flawed from its genesis. In crystalline materials, it may be the inclusion of one different atom or molecule in the structure of the growing crystal, or simply the juncture of two crystal planes. In depositional materials, different grain types and sizes may be laid down, resulting in layering which then becomes the initiation plane for the fracture. Most materials fail because of mechanical stresses, for example the weight of the overburden, or heaving (Atkinson, 1989; Heard et al., 1972). Some mechanical stresses are applied constantly2 until the material fails, others are delivered in a sudden event. Other causes of failure are thermal stresses, drying and wetting cycles and chemical dissolution. After a material fractures, the two faces of the fracture may be subject to additional stresses which either close or open the fracture, or may subject it to shear. Other materials may temporarily or permanently deposit in the fracture, partially or totally blocking it for subsequent fluid flow. The fracture may be almost shut for millions of years, but if the material becomes exposed to the surface or near surface environment, the resulting loss of overburden or weathering may allow the fractures to open. In some cases, we are actually interested in introducing fractures in the subsurface, via hydraulic (Warpinski, 1991) or pneumatic fracturing (Schuring et al., 1995), or more powerful means, to increase fluid flow in oil reservoirs or at contaminated sites. Our particular focus in this study is the role that fractures play in the movement of contaminants in the subsurface. Water supply from fractured bedrock aquifers is common in the United States (Mutch and Scott, 1994). With increasing frequency contaminated fractured aquifers are detected (NRC, 1990). In many cases, the source of the contamination is a Non-Aqueous Phase Liquid (NAPL) which is either in pools or as residual ganglia in the fractures of the porous matrix. Dissolution of the NAPL may occur over several decades, resulting in a growing plume of dissolved contaminants which is transported through the fractured aquifer due to natural or imposed hydraulic gradients. Fractures in aquitards may allow the seepage of contaminants, either dissolved or in their own phase, into water sources. Fluid flow in the fractured porous media is of significance not only in the context of contaminant transport, but also in the production of oil from reservoirs, the generation of steam for power from geothermal reservoirs, and the prediction of structural integrity or failure of large geotechnical structures, such as dams or foundations. Thus, the results of this study have a wide range of applications. The conceptual model of a typical contaminant spill into porous media has been put forward by Abriola (1989), Mercer and Cohen (1990), Kueper and McWhorter (1991) and Parker et al. (1994). In some cases, the contaminant is dissolved in water and thus3 travels in a fractured aquifer or aquitard as a solute. Fractures provide a fast channel for widely distributing the contaminant throughout the aquifer and also result in contaminant transport in somewhat unpredictable directions, depending on the fracture planes that are intersected (Hsieh et al., 1985). More typically a contaminant enters the subsurface as a liquid phase separate from the gaseous or aqueous phases present (Figure 1.8). The NAPL may be leaking from a damaged or decaying storage vessel (e.g. in a gasoline station or a refinery) or a disposal pond, or may be spilt during transport and use in a manufacturing process (e.g. during degreasing of metal parts, in the electronics industry to clean semiconductors, or in an airfield for cleaning jet engines). The NAPL travels first through the unsaturated zone, under three-phase flow conditions, displacing air and water. The variations in matrix permeability, due to the heterogeneity of the porous medium, result in additional deviations from vertical flow. If the NAPL encounters layers of slightly less permeable materials (e.g. silt or clay lenses, or even tightly packed sand), or materials with smaller pores and thus a higher capillary entry pressure (e.g. NAPL entering a tight, water-filled porous medium), it will tend to flow mostly in the horizontal direction until it encounters a path of less resistance, either more permeable or with larger pores. Microfractures in the matrix are also important in allowing the NAPL to flow through these lowpermeability lenses. When the NAPL reaches the capillary fringe, two scenarios may arise. First, if the NAPL is less dense than water (LNAPL, e.g. gasoline, most hydrocarbons), then buoyancy forces will allow it to â€Å"float† on top of the water table. The NAPL first forms a small mound, which quickly spreads horizontally over the water table (Figure 1.9). When the water table rises due to recharge of the aquifer, it displaces the NAPL pool upward, but by that time the saturation of NAPL may be so low that it becomes disconnected. Disconnected NAPL will usually not flow under two-phase (water and NAPL) conditions. Connected NAPL will move up and down with the movements of the water table, being smeared until becomes disconnected. If the water table goes above the disconnected NAPL, it will begin to slowly dissolve. NAPL in the unsaturated zone will4 slowly volatilize. The rates of dissolution and volatilization are controlled by the flow of water or air, respectively (Powers et al., 1991; Miller et al., 1990; Wilkins et al., 1995; Gierke et al, 1990). A plume of dissolved NAPL will form in the ground water, as well as a plume of volatilized NAPL in the unsaturated zone. If the NAPL is denser than water (DNAPL, e.g. chlorinated organic solvents, polychlorinated biphenyls, tars and creosotes), then once it reaches the water table it begins to form a mound and spread horizontally until either there is enough mass to overcome the capillary entry pressure (DNAPL into a water saturated matrix) or it finds a path of less resistance into the water-saturated matrix, either a fracture or a more porous/permeable region. Once in the saturated zone, the DNAPL travels downward until either it reaches a low enough saturation to become disconnected (forming drops or â€Å"ganglia†) and immobile, or it finds a low-permeability layer. If the layer does not extend very far, the DNAPL will flow horizontally around it. In many cases, the DNAPL reaches bedrock (Figure 1.10). The rock usually contains fractures into which the DNAPL flows readily, displacing water. The capillary entry pressure into most fractures is quite low, on the order of a few centimeters of DNAPL head (Kueper and McWhorter, 1991). Flow into the fractures continues until either the fracture becomes highly DNAPL saturated, or the fracture is filled or closed below, or the DNAPL spreads thin enough to become disconnected. The DNAPL may flow into horizontal fractures within the fracture network. In terms of remediation strategies, DNAPLs in fractured bedrock are probably one of the most intractable problems (National Research Council, 1994). They are a continuous source of dissolved contaminants for years or decades, making any pumping or active bioremediation alternative a very long term and costly proposition. Excavation down to the fractured bedrock is very expensive in most cases, and removal of the contaminated bedrock even more so. Potential remediation alternatives for consideration, include dewatering the contaminated zone via high-rate pumping and then applying Soil Vapor Extraction to remove volatile DNAPLs, or applying steam to mobilize and volatilize the DNAPL towards a collection well. An additional option is to use5 surfactants, either to increase the dissolution of DNAPL or to reduce its interfacial tension and thus remobilize it (Abdul et al., 1992). An issue with remobilizing via surfactants is the potential to drive the DNAPLs further down in the aquifer or bedrock, complicating the removal. If an effective remediation scheme is to be engineered, such as Soil Vapor Extraction, steam injection or surfactant-enhanced dissolution or mobilization, we need to understand how DNAPLs flow through fractures. Flow may be either as a solute in the aqueous phase, as two separate phases (DNAPL-water) or as three phases (DNAPL, water and gas, either air or steam). Another complication in any remediation scheme, not addressed in this study, is how to characterize the fracture network. Which are the fractures that carry most of the flow? What is their aperture and direction? What is the density of fracturing in a particular medium? Are the fractures connected to other fractures, probably in other planes? How does one sample enough of the subsurface to generate a good idea of the complexity involved? Some techniques are beginning to emerge to determine some of the most important parameters. For example, pumping and tracer tests (McKay et al., 1993; Hsieh et al., 1983) may provide enough information to determine the mean mechanical and hydraulic aperture of a fracture, as well as its main orientation. Geophysical techniques like seismic imaging, ground-penetrating radar and electrical conductivity tests are being improved to assist in the determination of fracture zones (National Research Council, 1996). However, there is room for significant improvement in our current ability to characterize fractures in the subsurface. Even if we come to understand how single and multiphase flow occurs in a fracture, and the interactions between the fracture and the porous matrix surrounding it, how do we describe all these phenomena in a modeling framework? Clearly, we cannot describe every fracture in a model that may consider scales of tens, hundreds or thousands of meters in one or more directions. One approach is to consider the medium as an â€Å"equivalent continuum† (Long, 1985), where the small-scale properties are somehow averaged in the macroscopic scale. Probably the best solution for averaging properties is to use a stochastic description of properties such as permeability (or6 hydraulic conductivity) including the effect of fractures on overall permeability, diffusivity, sorption capacity, grain size, wettability, etc. Another approach, first developed in the petroleum industry, is to consider a â€Å"dual porosity/dual permeability† medium (Bai et al., 1993; Zimmerman et al., 1993; Johns and Roberts, 1991; Warren and Root, 1963), referring to the porosity and permeability of the matrix and the fracture. Diffusive or capillary forces drive the contaminants, or the oil and its components, into or out of the matrix, and most advective flow occurs in the fractures. None of these models has yet been validated through controlled experiments. 1.2 Research Objectives The objectives of this research are:  · To characterize the fracture aperture distribution of several fractured porous media at high resolution;  · To study the transport of a contaminant dissolved in water through fractured media, via experimental observation;  · To study the physical processes involved in two- and three-phase displacements at the pore scale;  · To observe two- and three-phase displacements in real fractured porous media;  · To bring the experimental observations into a modeling framework for predictive purposes. 1.3 Approach7 To understand single and multiphase flow and transport processes in fractures, I first decided to characterize at a high level of resolution the fracture aperture distribution of a number of fractured rock cores using CAT-scanning. With this information, I determined the geometry and permeability of the fractures, which I then use to construct a numerical flow model. I also use this information to test the validity of predictive models that are based on the assumed statistics of the aperture distribution. For example, stochastic models (Gelhar, 1986) use the geometric mean of the aperture distribution to predict the transmissivity of a fracture, and show that the aperture variance and correlation length can be used to predict the dispersivity of a solute through a fracture. These models have not been, to my knowledge, been tested experimentally prior to this study. I compare these theoretical predictions of fracture transmissivity and dispersivity of a contaminant, with experimental results, both from the interpretation of the breakthrough curve of a non-sorbing tracer and from CAT-scans of the tracer movement through the fractured cores. To study multiphase displacements at the pore scale, we use a physical â€Å"micromodel†, which is a simile of a real pore space in two dimensions, etched onto a silicon substrate. The advantage of having a realistic pore space, which for the first time has the correct pore body and pore throat dimensions in a micromodel, is that we can observe multiphase displacements under realistic conditions in terms of the balance between capillary and viscous forces. I conduct two- and three-phase displacements to observe the role that water and NAPL layers play in the mobilization of the various phases. The micromodels are also used to study the possible combinations of double displacements, where one phase displaces another which displaces a third phase. The pore scale observations have been captured by Fenwick and Blunt (1996) in a threedimensional, three-phase network model which considers flow in layers and allows for double displacements. This network model then can produce three-phase relative permeabilities as a function of phase saturation(s) and the displacement path (drainage, imbibition or a series of drainage and imbibition steps).8 In addition, I use the fracture aperture information to construct capillary pressuresaturation curves for two phase (Pruess and Tsang, 1990) and three phases (Parker and Lenhard, 1987), as well as three-phase relative permeabilities (Parker and Lenhard, 1990). The fracture aperture distribution is also an input to a fracture network model which I use to study two-phase displacements (drainage and imbibition) under the assumption of capillary-dominated flow. To observe two- and three-phase displacements at a larger scale, in real fractured cores, I use the CAT-scanner. I can observe the displacements at various time steps, in permeable (e.g. sandstones) and impermeable (e.g. granites) fractured media, determining the paths that the different phases follow. These observations are then compared with the results of the network model as well as with more conventional numerical simulation. 1.4 Dissertation Overview The work is presented in self-contained chapters. Chapter 2 deals with the high resolution measurement and subsequent statistical characterization of fracture aperture. Chapter 3 uses the fracture aperture geostatistics to predict transmissivity and diffusivity of a solute in single-phase flow through a fracture, which is then tested experimentally. We also observe the flow of a tracer inside the fracture using the CAT-scanner, and relate the observations to numerical modeling results. Chapter 4 presents the theory behind the flow characteristics at the pore scale as well as the micromodel observations of two- and three-phase flow. In Chapter 5, twophase flow in fractured and unfractured porous media is presented, comparing CATscanned observations of various two-phase flow combinations (imbibition, drainage and water flooding) against numerical modeling results. Chapter 6 presents three-phase flow9 in fractures, comparing numerical results against CAT-scanner observations. Finally, Chapter 7 considers the engineering relevance of these studies. 1.5 References Atkinson, B. K., 1989: Fracture Mechanics of Rock, Academic Press, New York, pp. 548 Abdul, A. S., T. L. Gibson, C. C. Ang, J. C. Smith and R. E. Sobczynski, 1992: Pilot test of in situ surfactant washing of polychlorinated biphenyls and oils from a contaminated site, Ground Water, 30:2, 219-231 Abriola, L.,: 1989: Modeling multiphase migration of organic chemicals in groundwater systems – A review and assessment, Environmental Health Perspectives, 83, 117-143 Bai, M., D. Elsworth, J-C. Roegiers, 1993: Multiporosity/multipermeability approach to the simulation of naturally fractured reservoirs, Water Resources Research, 29:6, 1621-1633 Fenwick, D. H. and M. J. Blunt: 1996, Three Dimensional Modeling of Three Phase Imbibition and Drainage, Advances in Water Resources, (in press) Gelhar, L. W., 1986: Stochastic subsurface hydrology: From theory to applications., Water Resources Res., 22(9), 1355-1455. Gierke, J. S., N. J. Hutzler and J. C. Crittenden, Modeling the movement of volatile organic chemicals in columns of unsaturated soil, Water Resources Research, 26:7, 1529-1547 Heard, H. C., I. Y. Borg, N. L. Carter and C. B. Raleigh, 1972: Flow and fracture of rocks, Geophysical Monograph 16, American Geophysical Union, Washington, D. C. Hsieh, P. A., S. P. Neuman, G. K. Stiles and E. S. Simpson, 1985: Field determination of the threedimensional hydraulic conductivity of anisotropic media: 2. Methodology and application to fracture rocks, Water Resources Research, 21:11, 1667-1676 Hsieh, P. A., S. P. Neuman and E. S. Simpson, 1983: Pressure testing of fractured rocks- A methodology employing three-dimensional cross-hole tests, Report NUREG/CR-3213 RW, Dept. of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721 IAEA, 1995: The principles of radioactive waste management, International Atomic Energy Agency, Vienna Johns, R. A. and P. V. Roberts, 1991: A solute transport model for channelized flow in a fracture. Water Resources Res. 27(8): 1797-1808. Kueper, B. H. and D. B. McWhorter, 1991: The behavior of dense, nonaqueous phase liquids in fractured clay and rock, Ground Water, 29:5, 716-728 Long, J. C. S., 1985: Verification and characterization of continuum behavior of fractured rock at AECL Underground Research Laboratory, Report BMI/OCRD-17, LBL-14975, Batelle Memorial Institute, Ohio McKay, L. D., J. A. Cherry and R. W. Gillham, 1993: Field experiments in a fractured clay till, 1. Hydraulic conductivity and fracture aperture, Water Resources Research, 29:4, 1149-1162 Mercer, J. W. and R. M. Cohen, 1990: A review of immiscible fluids in the subsurface: properties, models, characterization and remediation, J. of Contaminant Hydrology, 6, 107-163 Miller, C. T., M. M. Poirier-McNeill and A. S. Mayer, 1990: Dissolution of trapped nonaqueous phase liquids: mass transfer characteristics, Water Resources Research, 26:11, 2783-2796 Mutch, R. D. and J. I. Scott, 1994: Problems with the Remediation of Diffusion-Limited Fractured Rock Systems. Hazardous Waste Site Soil Remediation: Theory and Application of Innovative Technologies. New York, Marcel Dekker, Inc. National Research Council, 1994: Alternatives for ground water cleanup, National Academy Press, Washington, D. C. National Research Council, 1996: Rock Fracture and Fracture Flow: Contemporary Understanding and Applications, Committee on Fracture Characterization and Fluid Flow, National Academy Press, Washington, D. C. (in press). Parker, J. C. and R. J. Lenhard, 1987: A model for hysteretic constitutive relations governing multiphase flow: 1. Saturation-pressure relations, Water Resources Research, 23:12, 2187-219610 Parker, J. C. and R. J. Lenhard, 1990: Determining three-phase permeability-saturation-pressure relations from two-phase system measurements, J. Pet. Sci. and Eng., 4, 57-65 Parker, B. L., R. W. Gillham and J. A. Cherry, 1994: Diffusive disappearance of immiscible-phase organic liquids in fractured geologic media, Ground Water, 32:5, 805-820 Powers, S. E., C. O. Loureiro, L. M. Abriola and W. J. Weber, Jr., 1991: Theoretical study of the significance of nonequilibrium dissolution of nonaqueous phase liquids in subsurface systems, Water Resources Research, 27:4, 463-477 Pruess, K. and Y. W. Tsang, 1990: On two-phase relative permeability and capillary pressure of roughwalled rock fractures, Water Resources Research, 26:9, 1915-1926 Reitsma, S. and B. H. Kueper, 1994: Laboratory measurement of capillary pressure-saturation relationships in a rock fracture, Water Resources Research, 30:4, 865-878 Savage, D., 1995: The scientific and regulatory basis for the geological disposal of radioactive waste, John Wiley, New York Schuring, J. R., P. C. Chan and T. M. Boland, 1995: Using pneumatic fracturing for in-situ remediation of contaminated sites, Remediation, 5:2, 77-90 Norman R. Warpinski, 1991: Hydraulic fracturing in tight, fissured media, SPE 20154, J. Petroleum Technology, 43:2, 146-209 Warren , J. E. and P. J. Root, 1963: The behavior of naturally fractured reservoirs, Soc. Pet. Eng. J., 3, 245-255 Wilkins, M. D., L. M. Abriola and K. D. Pennell, 1995: An experimental investigation of rate-limited nonaqueous phase liquid volatilization in unsaturated porous media: Steady state mass transfer, Water Resources Research, 31:9, 2159-2172 Zimmerman, R. W., G. Chen, T. Hadgu and G S. Bodvarsson, 1993: A numerical dual-porosity model with semianalytical treatment of fracture/matrix flow, Water Resources Research, 29:7, 2127-2137