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A Review of Laboratory and Numerical Simulations of Hydrocarbons Migration in Subsurface Environments
Abstract
The leaking from underground storage and surface spills of various hydrocarbon sources has caused the hazardous subsurface contamination. The toxic compounds of chemicals have made field study infeasible and it has been replaced by laboratory and numerical simulations. This study introduces the methodology for two-dimensional non-aqueous phase liquid experiments with the application of light reflection and light transmission methods associated with image analysis methods. In addition, this study emphasizes the experiments with numerical simulations in which data acquisition is essential for verification and validation of numerical models. The numerical efforts are supported by basic formulation, with existing codes and its application for light hydrocarbon migration simulation. Overall, this study discussed the laboratory works and numerical simulations using current visualization techniques and makes suggestions for future research.
... By using the principle of thermodynamics, a multiphase model was developed by Corapcioglu and Panday (1991) to quantify the partitioning of the mass of each constituent between phases for analyzing the problem of hydrocarbon migration in the unsaturated zone. The formulation of many relationships, such as capillary pressure-saturation, relative permeability-saturation and its hysteresis effect (Helmig, 1997;Kamaruddin et al., 2011), and fingering effect (Bouwer, 1991;Butts and Jensen, 1996), has been studied and developed for the future advancement of multiphase modeling. ...
... The benefit of the PCFF method is that it is easy to use; each constituent in different phases can be modeled separately in a preselected number of time periods and can be applied to any single constituent multiphase numerical simulator. Kamaruddin et al. (2011) reviews the literature up to 2008 and highlights the procedures involved in both the laboratory and mathematical modeling with the numerical formulation of existing models. Parikh et al. (2012a) formulated a mathematical equation for immiscible flow by considering mean capillary pressure of two phases, which has been presented, including its solution techniques using an iterative method. ...
... Hence, it becomes inevitable to conduct an experiment so that the quantitative information of flow behavior is generated through laboratory studies. Later, the migration of NAPL through a porous medium was quantitatively determined either by using conventional methods, such as dual gamma attenuation techniques and X-ray attenuation methods Host-Madsen and Jensen, 1992;Tidwell and Glass, 1994;Illangasekare, 1995;Chevalier and Petersen, 1999;Fagerlund et al., 2006;Yoon et al., 2009), or with the aid of image analysis techniques (IATs) (Schincariol et al., 1993;Van Geel and Sykes, 1994a;Papafotiou et al., 2008;Jaeger et al., 2009;Kamaruddin et al., 2011). Tidwell and Glass (1994) compared the IAT to standard methods (gravimetric and gamma densitometry methods), which show very close agreement between these techniques with minimal effort in the case of photographic methods. ...
Groundwater resources have been polluted by many sources. Among them, spillages of oil and similar petroleum products now become a common experience worldwide. This nonaqueous phase liquid (NAPL) can easily migrate downward through the unsaturated (vadose) zone and become widely distributed in the water table. Therefore, it is important to develop a methodology for monitoring and analyzing the movement of these contaminants through the vadose zone for the effective design of remediation schemes. This review discusses the equations involved in the numerical simulation of multiphase flow and the recent development of various multiphase models. In addition, this
study emphasizes the advancement of laboratory works using image analysis techniques. Overall, this study reviews the important features and limitations of existing remediation methods and highlights the applicability of natural fibers for the development of a sustainable cleanup technology against oil spill problems
... According to the annual report 2017À18 of the Ministry of Petroleum and Natural Gas, Government of India, 247.6 million metric tons per year addition capacity of all refineries were developed in 2017. Contamination of soilÀwater systems due to the release of hydrocarbons, commonly referred to as dense and light nonaqueous phase liquids (LNAPL), is an emerging problem Gupta, Ranjan, et al., 2019;Kamaruddin et al., 2011). When released to land, these contaminants migrate downward through the unsaturated zone and consequently light hydrocarbons float and move on top of the water table, while dense hydrocarbons move downward through the water table and penetrate into the saturated zone (Yadav & Hassanizadeh, 2011). ...
Overturning of hydrocarbon carriers (trains, tractors, tricks) and leakage of pipelines are very common which may cause spotting of these pollutants on surface. If large amount of hydrocarbons is spilled on the surface, they start infiltrating soil and finally reach to groundwater resources. Any ex situ technique to remediate such hydrocarbon spills may disturb the natural setting of the subsurface and also results in high cost, whereas natural attenuation has been reported to be very slow and sometimes results in incomplete removal. Enhanced bioremediation has proven a positive and potential role to accelerate the biodegradation rate of hydrocarbon compounds. Thus this study aimed to evaluate the performance of bioremediation products. The study may help to frame low-cost remediation and management plan for hydrocarbon-polluted sites.
... A wide range of the models developed for representing the immiscible flow in the subsurface zone were reviewed and presented (Kamaruddin et al., 2011;Praseeja and Sajikumar, 2019). The models designed for simulating the flow of NAPL through porous media based on FEMs (finite element methods) are (i) areal multiphase organic simulator (ARMOS) (Kaluarachchi et al., 1990), (ii) multiphase analysis of groundwater, NAPL, and soluble components (MAGNAS) (Huyakorn et al., 1992), (iii) multiphase flow and transport (MOFAT) (Katyal et al., 1991), and (iv) NAPL simulator (Guarnaccia et al., 1997), and based on FDMs (finite difference methods) are (i) subsurface transport over multiple phases (STOMP) (White et al., 1995), (ii) TOUGH2 (Pruess et al., 1999), (iii) VENT2D (Benson, 1994), and (iv) No. Greek symbols Δ differential operator ρ density (kg/m 3 ) ε porosity χ adsorption function Ψ pressure head (m) θ volumetric water content ϑ decay rate SURFACT (Panday and Huyakorn, 2008). ...
The study on the migration behaviour of light non-aqueous phase liquid (LNAPL) through the subsurface system is essential for implementing a proper remedial measure against groundwater contamination. A FEM-based flow and transport model FEFLOW (Finite Element subsurface FLOW and transport system) was utilised in this study to model the migration of LNAPL, through the unsaturated zone, where the LNAPL is modelled as a single-phase contaminant with the least water solubility. Further, it evaluates the utility of a natural fibre, coir geotextile (CG) in controlling the migration of the LNAPL through the subsurface system based on an experimental study. The predictions from the numerical model are compared and found matching with the experimental results. The CG layer is also modelled similarly to the soil layer with appropriate values of the parameters, which defines the novelty of this study. Hence, the developed numerical model is used to simulate the actual field conditions to assess how long the coir geotextile can sustain as a remedial measure for controlling LNAPL migration through the soil. The provision of two layers of CG with a vertical layer on both sides as a box-like containment could hold LNAPL up to 7.5 years in the wet condition and 5 years in the dry condition of soil.
... When the upward movement of the water level occurs, part of the NALP follows the rise of the water table, and another part is trapped below it, due to a capillary hysteresis that reduces its mobility. In the opposite direction, when the water level drops, the water drains from the porous medium and the NALP agglutinate, increasing their saturation and mobility [19][20][21][22][23]. However, NALP presence as an independent phase was not fixed in the model, so groundwater level was considered steady. ...
The aim of this work is to assess the risk of groundwater contamination associated with BTEX dissolution from fuels as a residual phase. Numerical simulations of sixty scenarios were carried out with the software HYDRUS 2D/3D. Groundwater contamination risk was analyzed given the combination of different porous media textures (silt loam, sandy loam and clay), water fluxes (0.5%, 1% or 3% Rainfall), water table depths (1.5, 2.5, 5 or 8 m below ground surface) and biodegradation rate (active or null). Risk was calculated comparing leachate concentrations to the aquifer and limits established by an international guideline for human drinking water. In all cases, benzene and toluene had the highest mobility in the dissolved phase. Contrary, xylene and ethylbenzene tended to concentrate close to the source zone. These two compounds predominantly concentrated in the solid phase. Calculated risk was proportional to the water flux rate and inversely proportional to the unsaturated thickness. Without biodegradation, in fine-grained sediments risk was very high for shallow aquifers (> 1.5 m depth) and moderate or low for deeper aquifers. However, in sandy loam sediments risk was classified as very high for aquifers up to 8 m deep. When biodegradation was considered, leached concentrations were greatly reduced in the three textures. BTEX concentration in Bahía Blanca City´s aquifer showed acceptable agreement with simulated scenarios. The most sensitive parameters to model results were biodegradation > foc > water table depth > Ks. This study is important for assessing the risks and developing management strategies for fuel contaminated sites.
... Recently, image analysis techniques or digital image techniques (DITs) are used to investigate the migration of fluids within soils [23]. The application of DIT is widely used for double-porosity soil [22][23][24][25][26][27][28][29][30]. In this research, a noninvasive technique is applied using a DIT that captures the successive migration of leachate through compacted laterite soil in the laboratory to simulate the leachate transportation in actual landfill. ...
As leachate has been a source of groundwater contamination worldwide, this paper examines the phenomenon of leachate migration on different gradations of compacted laterite soil used as sanitary landfill liners. Three different soil gradations (30%, 40% and 50% with respect to fines content) used in this study were compacted in circular acrylic columns to provide a clear visualization of leachate migration into the soils. Digital image technique was used in capturing photos at successive time intervals to monitor the leachate migration. The captured digital images were fed into Matlab and converted into hue-saturation-intensity (HSI) format. Surfer software then read the HSI and generated 2D contour plots. The results of the experiments showed that the leachate moves downward faster in the soil gradation with the least fines content. Hydraulic conductivity values decrease with increase in time duration and equally with increase in fines content. The hydraulic conductivities of the leachate for 30%, 40% and 50% fines were 3.64×10-9m/s, 2.40×10-9m/s, and 1.24×10-9m/s respectively. This reveals that for tropical laterite soils, gradation containing 50% fines content provides better hydraulic conductivity. The use of noninvasive digital image technique can enable designers/engineers to monitor and visualize the leachate migration in compacted soils in waste containment application systems.
... Diversos pesquisadores (Abriola e Pinder, 1985;Kaluarachi e Parker, 1989, Kueper et al., 1989Oostrom et al., 2007;Kamaruddin et al., 2011; dentre outros) têm realizado estudos relevantes nesta linha com trabalhos teóricos, na forma de modelos matemáticos e computacionais elaborados para descrever o fluxo de contaminantes em sub superfície. Também constam trabalhos experimentais, que envolvem estudos laboratoriais e de campo, com ensaios de infiltração de diferentes líquidos orgânicos em solos de diferente natureza, simulando o fluxo do contaminante. ...
Results of column tests performed on large undisturbed samples are presented, focusing on the behavior of the hydrodynamic dispersion coefficient (\(D_h\)). Tests were performed employing petroleum produced water from onshore facilities percolating sandy soils with different fine contents. To measure the organic content of the produced water,
this work used the parameter TPH (total petroleum hydrocarbons). The obtained results show that the longitudinal dispersion coefficient (\(\alpha _L\)) varies with flow velocity (\(v_s\)) and that both the ratio between the hydrodynamic dispersion and diffusion coefficients (\(D_h/D\)) and \(\alpha _L\) are approximately two orders of magnitude higher than the values normally found in the literature for the same type of soil. This is probably related to the fact that the organic compounds measured by TPH in the produced water are partially in dissolved form, but dispersed particles are also transported by water flow, increasing the experimental values of \(D_h\).
... Diversos pesquisadores (Abriola e Pinder, 1985;Kaluarachi e Parker, 1989, Kueper et al., 1989Oostrom et al., 2007;Kamaruddin et al., 2011; dentre outros) têm realizado estudos relevantes nesta linha com trabalhos teóricos, na forma de modelos matemáticos e computacionais elaborados para descrever o fluxo de contaminantes em sub superfície. Também constam trabalhos experimentais, que envolvem estudos laboratoriais e de campo, com ensaios de infiltração de diferentes líquidos orgânicos em solos de diferente natureza, simulando o fluxo do contaminante. ...
This work developed two-dimensional flow experiments with water and diesel in non-saturated sandy soil, using an instrumented flow channel monitored by tensiometer and visual analysis. The soil was compacted with 5% moisture and dry mass density of 1.62 g.cm-3. The fluids migration occurred uniformly, both, in vertical and horizontal directions, with symmetrical spread of the infiltrating fronts. The ratio between the arrival time of the infiltrating fronts to the capillary fringe of diesel and water (td/tw) was 2.3; compatible with the ratio (2.16) between the liquid properties (ρ/μ/σ) for diesel and water. The behavior of the water and diesel isochrones were adequately explained by the joint effect of gravity and friction on the liquids vertical flow, represented by the mobility (ρ/μ), and the lateral scattering by the capillarity effect, controlled by surface tension (s). The use of one-dimensional models to simulate the vertical advance of the two-dimensional infiltrating front, independent of the fluid, resulted in considerable delay using the Brutsaert equation (1977), while the equation from Philip (1969) provided a considerable advance. The simulations using the Green Ampt model (1911) were close to the experimental data. However, the Philip model (1969) with the correction for the porosity, due to lateral scattering, resulted in better simulations, with error of about 10%. Philip's (1969) model, corrected for lateral scattering, is more adequate for soils with fine.
... Recently, image analysis techniques were used to investigate the migration of fluids within soils [5]. The application of digital image technique is usually used for double-porosity soil [5][6][7][8][9][10][11][12][13]. ...
To protect groundwater from leachate contamination in sanitary landfill involve the use of hydraulic barriers i.e. liners and covers. Nonetheless, can these barriers continue to impede the migration of leachate over a long period? A full-scale experiment would be prohibitively costly and time consuming. The only feasible recourse therefore is to construct a model, which reasonably portray the behaviour of the full-scale system and simulate the relevant physical parameters and describes the overall significant characteristics of the transport phenomena. This research investigates the long-term performance of compacted tropical laterite soil liners at various gradations against leachate migration in sanitary landfills using numerical modeling. Series of laboratory experimentation were carried out using three different laterite soil gradations (30%, 40% and 50% with respect to fines content) compacted at optimum moisture content using British Standard light energy. Leachate was poured on the compacted soil in an acrylic column and its migration was monitored using Digital Image Technique (DIT). Subsequently, PetraSim computer software a graphical interface used to solve problems related to contaminant transport was applied to predict the velocity of leachate migration. The predicted velocity values for 30%, 40% and 50% fines are 4.5 x 10 ⁻⁷ m/s, 7 x 10 ⁻⁹ m/s, and 8 x 10 ⁻¹⁰ m/s, respectively. This shows that the laterite soil with 50% fines content is more compatible with the leachate and can be used as soil liner. The outcome of this research would enable designers to use non-destructive method to monitor and predict leachate migration in compacted soil liners to simulate leachate migration in waste containment applications.
Disaster such as earthquake has an impact to water resources particularly groundwater. The migration of pollutant in groundwater is crucial particularly due to earthquake impact. In addition, the soil moisture content also might influence the pollutant migration. Therefore, this paper presents the investigation of light non-aqueous phase liquid (LNAPL) migration in laterite soil with two dissimilar moisture contents with and without vibration. The study of LNAPL migration was investigated using digital image analysis. The lab-scale experiments were conducted by using soil column, mirror, LNAPL and Nikon D90 digital camera. Unsaturated laterite soil was poured in the acrylic column then compressed until 100 mm height. Then, LNAPL was poured onto the soil column surface instantaneously. The pattern and behavior of LNAPL migration in laterite soil was monitored and recorded using digital image processing technique (DIPT) at certain time intervals. The images were processed with Surfer software and Matlab routine inorder to plot the LNAPL migration pattern using hue-saturation-intensity (HSI) value. The study found that higher rate migration of LNAPL with high moisture content, and the rate of LNAPL migration decreases with presence of vibration and with lower moisture content. The migration time required to reach bottom of soil was longer for low moisture content with/without vibration as compared to the high moisture content with/without vibration.KeywordsMigrationLight non-aqueous phase liquidUnsaturated laterite soilMoisture contentWith/without vibrationDigital image processing technique
Variations in groundwater flow regimes due to direct draining/pumping and surrounding climatic variabilities may significantly affect the spatial and temporal distribution of hydrocarbon compounds in subsurface. Fate and transport of these contaminants have been studied quite adequately, however the behavior of these pollutants under varying groundwater flow regimes has not been investigated in the past. Therefore, an extensive experimental investigation is made to study the effect of changes in groundwater flow velocity on fate and transport of light non-aqueous phase liquids (LNAPL) using a 3-D sand tank setup. The tank setup of size 60cm-L×30cm-W×60cm-D embedded with sampling ports was packed with homogeneous sand having grain size of ranges 0.5-1.0 mm. Pure phase toluene, a representative of LNAPL, was released from top port of the tank setup to create a pure phase pool of the LNAPL around the groundwater table. A constant water flux was allowed to flow first for maintaining a base groundwater flow velocity in the horizontal direction. The flow velocity was then increased/decreased by changing the water flux passing through the saturated zone by keeping the water table location at the same height. Groundwater samples were collected routinely and were analyzed using gas chromatography-mass spectrometry (GC-MS). A series of batch experiments were also performed using the same groundwater and porous media used in the tank setup to estimate the biodegradation rate at different dissolved LNAPL concentrations. It was observed that biodegradation rate increases upto 50 ppm concentration and remain almost the same till 100 ppm and then decreases with increasing concentration of the LNAPL. The biodegradation rates corresponding to the observed concentration of LANAPL in tank setup was then used for conducting the simulation experiments. Results show that dissolution rate of the LNAPL increases linearly with groundwater velocity and was estimated for the three different groundwater flow regimes varying from 0.083 to 0.129 cm/h. The observed high rate of degradation of the LNAPL for faster flow velocities shows the dependency of the degradation kinetics on dissolved LNAPL concentration. The observed breakthrough curves at different ports showed that horizontal and transverse transport of the LNAPL was more prominent as compared to its vertical movement. The observed concentration of dissolved toluene compared well with the simulated curves for the considered cases of groundwater flow velocities. The results of this study are of direct use in applying bioremediation techniques for field problems subjected to dynamic groundwater flow conditions.
A simplified nonwetting liquid entrapment model for three fluid phase porous media is presented. The model requires one additional parameter in the constitutive relationship, effective residual oil saturation, and can be implemented in a numerical multiphase flow model. The validity of the model was verified with good agreement between the predicted and observed saturations. The laboratory column experiment which showed percentage of trapped oil volume under a given set of boundary conditions is very sensitive and varies in a highly nonlinear manner with the effective residual oil saturation. -Authors
One- and two-dimensional experiments were conducted to examine differences in the behavior of gasoline and gasohol (10% ethanol by volume) as they infiltrate through the unsaturated zone and spread at the capillary fringe. Ethanol in the spilled gasohol quickly partitions into the residual water in the vadose zone and is retained there as the gasoline continues to infiltrate. Under the conditions tested, over 99% of the ethanol was initially retained in the vadose zone. Depending on the volume of gasoline spilled and the depth to the water table, this causes an increase in the aqueous-phase saturation and relative permeability, thus allowing the ethanol-laden water to drain into the gasoline pool. Under the conditions tested, the presence of ethanol does not have a significant impact on the overall size or shape of the resulting gasoline pool at the capillary fringe. Residual gasoline saturations in the vadose zone were significantly reduced however because of reduced surface and interfacial tensions associated with high ethanol concentrations. The flux of ethanol in the effluent of the column ranged from 1.4 x 10(-4) to 4.5 x 10(-7) g/(cm(2) min) with the LNAPL and from 6 x 10(-3) to 3.0 x 10-4 g/(cm(2) min) after water was introduced to simulate rain infiltration. The experimental results presented here illustrate that the dynamic effects of ethanol partitioning into the aqueous phase in the vadose zone create an initial condition that is significantly different than previously understood.
A hysteretic model for two-phase permeability (k)-saturation (S)-pressure (P) relations is outlined that accounts for effects of nonwetting fluid entrapment. The model can be employed in unsaturated fluid flow computer codes to predict temporal and spatial fluid distributions. Consideration is given to hysteresis in S-P relations caused by contact angle, irregular pore geometry, and nonwetting fluid entrapment effects and to hysteresis in k-S relations caused by nonwetting fluid entrapment effects. An air-water flow experiment is conducted with a 72-cm vertical soil column where the water table is fluctuated to generate scanning S-P paths. Water contents are measured via a gamma radiation system, and water pressures are measured via pressure transducers connected to ceramic tensiometers inserted in the soil column. Computer simulations of the experiment employing the hysteretic k-S-P model and a nonhysteretic k-S-P are compared with measured water contents and pressures. Close agreement is found between experimental water contents and those predicted by a numerical code employing the hysteretic k-S-P relations. When nonhysteretic k-S-P constitutive relations are utilized, there is poor agreement between measured and predicted water saturations of the scanning paths. Only one more parameter is needed to model two-phase hysteretic fluid behavior than to model nonhysteretic behavior. Results of this study suggest that consideration should be given to effects of hysteresis in k-S-P relations to accurately predict fluid distributions.
A finite element model has been developed to simulate simultaneous flow of water and light hydrocarbon in an areal flow region of an unconfined aquifer for analyses of hydrocarbon spreading from subsurface leaks or spills and for use in design of free product recovery systems. Vertically integrated governing equations for water and oil flow are employed which assume local vertical equilibrium and negligible gas pressure gradients. Multiple water and free product recovery wells are handled as internal type-I boundary conditions by stipulating air-oil table elevation and free product height with corrections to convert grid averaged nodal heads to actual well bore fluid levels. An automatic updating scheme for well bore correction factors is introduced which ensures consistency of well flux calculations with the global mass balance. Areal model predictions are compared with two dimensional vertical cartesian and radial simulations with multiphase seepage faces for hypothetical trench and well free product recovery systems, respectively. The results indicate that the assumption of vertical equilibrium and lack of explicit treatment of seepage faces in the areal model produce minor loss in accuracy while conferring major reductions in computational effort. Simulations of various spill spreading and free product recovery scenarios with multiple pumping wells are investigated to demonstrate the model capabilities.
Physical experiments were conducted to investigate the transport of a dissolved volatile organic compound (trichloroethylene, TCE) from shallow groundwater to the unsaturated zone under a variety of conditions including changes in the soil moisture profile and water table position. Experimental data indicated that at moderate groundwater velocities (0.1 m/d), vertical mechanical dispersion was negligible and molecular diffusion was the dominant vertical transport mechanisms. Under these conditions, TCE concentrations decreased nearly 3 orders of magnitude across the capillary fringe and soil gas concentrations remained low relative to those of underlying groundwater. Data collected during a water table drop showed a short-term increase in concentrations throughout most of the unsaturated zone, but these concentrations quickly declined and approached initial values after the water table was returned to its original level. In the deep part of the unsaturated zone, the water table drop resulted in a long-term decrease in concentrations, illustrating the effects of hysteresis in the soil moisture profile. A two-dimensional random walk advection-diffusion model was developed to simulate the experimental conditions, and numerical simulations agreed well with experimental data. A simpler, one-dimensional finite-difference diffusion-dispersion model was also developed. One-dimensional simulations based on molecular diffusion also agreed well with experimental data. Simulations which incorporated mechanical dispersion tended to overestimate flux across the capillary fringe. Good agreement between the one- and two-dimensional models suggested that a simple, one-dimensional approximation of vertical transport across the capillary fringe can be useful when conditions are appropriate.
Two independent techniques, X ray absorption and light transmission, are developed and demonstrated for imaging transient saturation fields in thin-slab porous systems. The techniques yield full two-dimensional saturation fields with high spatial and temporal resolution. In the time required to make a single measurement by one of the traditional methods (e.g., gravimetric or gamma densitometry) and entire image consisting of hundreds of thousands of points is acquired by either the X ray or light technique. These methods are also very sensitive, capable of resolving a hundred or more levels of saturation at each of these points. Evaluation of these techniques is accomplished by direct comparison of X ray and light data as well as comparison with gravimetric and gamma densitometry data. Results of the comparison show very close agreement between the four techniques (on average within 5% saturation). These techniques represent useful tools for investigating processes governing unsaturated flow and transport through porous media.
A two-dimensional finite element model based on Galerkin's weighted residual approach and an upstream weighting technique was developed to predict simultaneous flow of water and oil in a three-fluid phase system with gas assumed at constant pressure. Element matrices were computed using the influence coefficient method for both Picard and Newton-Raphson nonlinear iteration schemes. A number of hypothetical simulations were performed in both one and two dimensions to evaluate the accuracy and efficiency of the various schemes with respect to handling of nonlinear soil properties, time marching, mass balance errors, soil nonhomogeneity, and effects of upstream weighting. Results indicate that the Picard scheme appears to be as effective as the Newton-Raphson scheme while requiring substantially less computational effort if upstream weighting is employed. The Picard method without upstream weighting did not provide satisfactory convergence behavior. Results from problems involving extreme soil nonhomogeneity indicate that accurate solutions can be obtained with the Picard method with the proper use of upstream weighting. Saturation-pressure derivative terms in the formulation can be evaluated using both analytical and chord-slope methods. Results indicate that time averaging of these terms is critical for good mass balance results especially during redistribution. Overall mass balance is also sensitive to the fluid-dependent scaling parameters of the constitutive relationships.
A fully coupled numerical model has been developed which describes multiphase fluid flow through soil: namely gas, water and a nonaqueous phase liquid (NAPL) in a deforming porous media for subsurface systems. A multiphase flow model, based on the two-phase flow model of Brooks and Corey, is presented to express the dependence of saturation and relative permeability on the capillary pressure. Nonlinear saturation and relative permeability on the capillary pressure. Nonlinear saturation and relative permeability functions are incorporated into a Galerkin finite element model which is subsequently used to simulate multiphase immiscible fluid flow under saturated and unsaturated conditions in porous media. The governing partial differential equations, in terms of soil displacements and fluid pressures, which are coupled and non-linear, are solved by the finite element method. Numerical implementation of the formulation are discussed and example problems are performed to demonstrate the model and solution procedure.
Little quantitative experimental data are available describing the behavior of immiscible contaminants in unsaturated heterogeneous porous media. Such data are, however, essential to the fundamental understanding of the processes governing nonaqueous phase liquid behavior and for the validation of modeling tools. The effect of macro-heterogeneity on light nonaqueous phase liquid (LNAPL) flow and distribution in the unsaturated zone was investigated experimentally by simulating LNAPL spills in layered soil systems consisting of sands with various textures. Two multiphase flow experiments were conducted in a two-dimensional flume ( 180 x 120 x 8 cm). The vertical distribution of water and LNAPL pressure were measured using hydrophilic and hydrophobic tensiometers. An image analysis technique was used to estimate the saturation distribution of the fluids in a two-dimensional vertical plane. The experiments show that LNAPL entrapment, which contributes to long-term soil and water contamination, depends strongly on the initial water saturation and water pressure at the layer interfaces and on the texture contrasts between the soil layers, which lead to permeability and capillary barrier effects. Thus, the knowledge of the initial water pressure and saturation distribution in unsaturated layered soil formations is critical to the correct prediction of LNAPL infiltration and drainage.
Numerical simulations with the multifluid flow simulator STOMP are compared with quantitative results of a detailed lighter-than-water nonaqueous-phase liquid (LNAPL) infiltration and redistribution experiment in a 1.67-m-long 1-m-high, and 0.05-m-wide flow cell. The experiment was performed to test the ability of commonly used nonhysteretic and hysteretic constitutive relations between relative permeability (k), fluid saturation (S), and capillary pressure head (h) to describe multifluid flow in two dimensions. The fluid and sand parameters necessary to apply the constitutive relations were obtained independently. The Bow cell was filled with a homogeneous sand mixture under saturated conditions. After partial drainage of the sand, the LNAPL was slowly injected for 12 h from a small source area located at the surface. A dual-energy gamma radiation system was used to determine LNAPL and water saturations at 255 locations during infiltrations and redistribution. The results show a reasonable match between the experimental and numerical data, indicating that the constitutive relations used are adequate to describe relatively slow LNAPL infiltration and redistribution. The differences between the nonhysteretic and hysteretic simulations are small. This implies that hysteresis, a result of nonwetting fluid entrapment anti pore geometry, was not important in this experiment.