Investigatory Project “ Kaymito Leaves Decoction as Antiseptic Mouthwash ” Essay

Introduction1.1 Problem StatementFractures are prevalent in natural and synthetic structural media, scour in the best engineered materials. We find fractures in basic principle, in sandst iodine aquifers and embrocate reservoirs, in trunk layers and even in unconsolidated materials (Figures 1.1 to 1.4). Fractures are in any case common in concrete, used either as a structural material or as a liner for storage tanks (Figure 1.5). Clay liners used in landfills, soap and brine disposal pits or for underground storage tanks can fracture, releasing their liquid contents to the subsurface (Figure 1.6). Even flexible materials such as pave fracture with date (Figure 1.7).The fact that fractures are inevitable has led to spending billions of research dollars to construct safe long-term (10,000 years or more) storage for superior nuclear waste (Savage, 1995 IAEA, 1995), both to determine which construction techniques are least likely to result in failure and what are the implicatio ns of a failure, in wrong 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 to the highest degree cases, the material is flawed from its genesis. In crystalline materials, it may be the inclusion of bingle different atom or molecule in the structure of the growing crystal, or simply the juncture of two crystal tacks. In secureional materials, different grain types and sizes may be laid raft, 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 annihilation.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 fly the coop. 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 whatsoever 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 grime sites. Our picky focus in this study is the role that fractures play in the movement of contaminations in the subsurface. piddle supply from fractured bedrock aquifers is common in the linked States (Mutch and Scott, 1994). With increasing frequency contaminated fra ctured aquifers are detected (NRC, 1990). In m either 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 by means of 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 anatomy, 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 locomote for power from geothermal reservoirs, and the foreseeion of structural integrity or failure of large geotechnical structures, such as dams or foundations. Thus, the results of this study pretend a wide range of applications.The conceptual model of a t ypical 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 un look forable 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 dishonored 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 number one through the un arrant(a) zone, under three-phase flow conditions, displacing air and water. The variations in matrix perme business leader, due to the heterogeneity of the porous medium, result in additional deviations from vertical flow.If the NAPL encounters layers of middling less(prenominal) permeable materials (e.g. silt or clay lenses, or even tightly packed sand), or materials with smaller contracts and thus a higher capillary tubing 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 force s will allow it to 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 intensity level 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 disintegration 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 th e NAPL is denser than water (DNAPL, e.g. chlorinated essential 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 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 (Kuepe r 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 swerve 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 utility(a) 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 betation, 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 collectio n 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 study 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 othe r fractures, probably in other planes?How does one sample enough of the subsurface to generate a good humor of the complexity involved? Some techniques are beginning to emerge to determine some of the most important parameters. For example, pumping and tracer bullet tests (McKay et al., 1993 Hsieh et al., 1983) may provide enough study 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 conduction 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 contact it, how do we describe all these phenomena in a modeling framework? Clea rly, 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 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 dual porosity/dual permeability medium (Bai et al., 1993 Zimmerman et al., 1993 Johns and Roberts, 1991 rabbit 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 flo w occurs in the fractures. None of these models has yet been validated through controlled experiments.1.2 Research ObjectivesThe objectives of this research are To characterize the fracture aperture distribution of several fractured porous media at high colonisation 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 Approach7To 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 victimization CAT-scanning. With this information, I determined the geometry and permeability of the fractures, which I then use to construct a numeral 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 well-tried 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 micromodel, which is a simile of a real pore position in two dimensions, etched onto a silicon sub strate. 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).8In 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 OverviewThe work is presented in self-contained chapters. Chapter 2 deals with the high resolution mens uration 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 plan relevance of these studies.1.5 ReferencesAtkinson, B. K., 1989 Fra cture Mechanics of Rock, Academic Press, clean York, pp. 548 Abdul, A. 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