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We have improved ECO2N, the TOUGH2 fluid property module of the CO2-H2O-NaCl system. The major enhancements include: (i) the upper temperature limit is increased from 110 to about 300°C; (ii) the thermophysical properties of the CO2-rich phase are more accurately calculated as a non-ideal mixture of CO2 and H2O; (iii) the approach to calculate the specific enthalpy of dissolved CO2 has been improved to make the code more robust in modeling phase transitions under non-isothermal conditions; and (iv) more sophisticated models for effective heat conductivity of formations saturated with supercritical CO2 have been provided. The new module includes a comprehensive description of the thermodynamic and thermophysical properties of H2ONaClCO2 mixtures, that reproduces fluid properties largely within experimental error for the temperature, pressure and salinity conditions 10°C < T < 300°C, P < 600 bar, and salinity up to halite saturation. This includes density, viscosity, and specific enthalpy of fluid phases as functions of temperature, pressure, and composition, as well as partitioning of mass components H2O, NaCl and CO2 among the different phases. ECO2N with the TOUGH2 reservoir simulator can be applied to a wide range of problems in geologic sequestration of CO2 in saline aquifers, and in enhanced geothermal reservoirs. ECO2N can describe both sub- and supercritical states of CO2, but applications that involve subcritical conditions are limited to systems in which there is no change of phase between liquid and gaseous CO2, and in which no mixtures of liquid and gaseous CO2 occur. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In response to the urgent need for estimates of the oil and gas flow rate from the Macondo well MC252-1 blowout, we assembled a small team and carried out oil and gas flow simulations using the TOUGH2 codes over two weeks in mid-2010. The conceptual model included the oil reservoir and the well with a top boundary condition located at the bottom of the blowout preventer. We developed a fluid properties module (Eoil) applicable to a simple two-phase and two-component oil-gas system. The flow of oil and gas was simulated using T2Well, a coupled reservoir-wellbore flow model, along with iTOUGH2 for sensitivity analysis and uncertainty quantification. The most likely oil flow rate estimated from simulations based on the data available in early June 2010 was about 100,000 bbl/d (barrels per day) with a corresponding gas flow rate of 300 MMscf/d (million standard cubic feet per day) assuming the well was open to the reservoir over 30 m of thickness. A Monte Carlo analysis of reservoir and fluid properties provided an uncertainty distribution with a long tail extending down to 60,000 bbl/d of oil (170 MMscf/d of gas). The flow rate was most strongly sensitive to reservoir permeability. Conceptual model uncertainty was also significant, particularly with regard to the length of the well that was open to the reservoir. For fluid-entry interval length of 1.5 m, the oil flow rate was about 56,000 bbl/d. Sensitivity analyses showed that flow rate was not very sensitive to pressure-drop across the blowout preventer due to the interplay between gas exsolution and oil flow rate. [ABSTRACT FROM AUTHOR]

Simulation models for subsurface flow and transport require the estimation of many disparate parameters to capture properties and processes that cannot be explicitly represented in the model. The main challenges for model calibration reside in the formulation of the forward model, the identification of influential parameters, the selection of appropriate calibration data with sufficient information content about these parameters, and the performance of a robust and efficient minimization algorithm capable of handling complex topologies of the resulting objective function. The large variety of subsurface systems and related engineering problems that are addressed by numerical simulation prevents us from compiling definite lists of critical input parameters and desirable observation data to be used for model calibration. Instead, we describe the approach that we consider necessary to arrive at a defensible model for predictive simulations. This process includes sensitivity analyses and synthetic inversions prior to data collection, and detailed residual, error, and uncertainty analyses after automatic model calibration. We illustrate the approach by discussing applications of the tough simulation-optimization code, which supports the development of TOUGH2 models for nonisothermal, multiphase, multicomponent flow and transport processes in fractured porous geological media. We conclude that the calibration of models representing complex subsurface systems remains challenging and requires research that is targeted at the needs of scientists and engineers who address simulation problems of practical relevance. [ABSTRACT FROM AUTHOR]

Thermal single-well injection-withdrawal (SWIW) tests entail pumping cold water into a hot and usually fractured reservoir, and monitoring the temperature recovery during subsequent backflow. Such tests have been proposed as a potential means to characterize properties of enhanced geothermal systems (EGS), such as fracture spacing, connectivity, and porosity. In this paper we develop an analytical solution for thermal SWIW tests, using an idealized model of a single vertical fracture with linear flow geometry embedded in impermeable conductive wall rocks. The analytical solution shows that the time dependence of temperature recovery is dominated by the heat exchange between fracture and matrix rock, but strong thermal diffusivities of rocks as compared to typical solute diffusivities are not necessarily advantageous for characterizing fracture-matrix interactions. The effect of fracture aperture on temperature recovery during backflow is weak, particularly when the fracture aperture is smaller than 0.1 cm. The solution also shows that temperature recovery during backflow is independent of the applied injection and backflow rates. This surprising result implies that temperature recovery is independent of the height of the fracture, or the specific fracture-matrix interface areas per unit fracture length, suggesting that thermal SWIW tests will not be able to characterize fracture growth that may be achieved by stimulation treatments. [ABSTRACT FROM AUTHOR]

We have used the TOUGH2-MP/ECO2N code to perform numerical simulation studies of the long-term behavior of CO stored in an aquifer with a sloping caprock. This problem is of great practical interest, and is very challenging due to the importance of multi-scale processes. We find that the mechanism of plume advance is different from what is seen in a forced immiscible displacement, such as gas injection into a water-saturated medium. Instead of pushing the water forward, the plume advances because the vertical pressure gradients within the plume are smaller than hydrostatic, causing the groundwater column to collapse ahead of the plume tip. Increased resistance to vertical flow of aqueous phase in anisotropic media leads to reduced speed of up-dip plume advancement. Vertical equilibrium models that ignore effects of vertical flow will overpredict the speed of plume advancement. The CO plume becomes thinner as it advances, but the speed of advancement remains constant over the entire simulation period of up to 400 years, with migration distances of more than 80 km. Our simulations include dissolution of CO into the aqueous phase and associated density increase, and molecular diffusion. However, no convection develops in the aqueous phase because it is suppressed by the relatively coarse (sub-) horizontal gridding required in a regional-scale model. A first crude sub-grid-scale model was developed to represent convective enhancement of CO dissolution. This process is found to greatly reduce the thickness of the CO plume, but, for the parameters used in our simulations, does not affect the speed of plume advancement. [ABSTRACT FROM AUTHOR]

Correlations are presented to compute the mutual solubilities of CO2 and chloride brines at temperatures 12-300°C, pressures 1-600 bar (0.1-60 MPa), and salinities 0-6m NaCl. The formulation is computationally efficient and primarily intended for numerical simulations of CO2-water flow in carbon sequestration and geothermal studies. The phase-partitioning model relies on experimental data from literature for phase partitioning between CO2 and NaCl brines, and extends the previously published correlations to higher temperatures. The model relies on activity coefficients for the H2O-rich (aqueous) phase and fugacity coefficients for the CO2-rich phase. Activity coefficients are treated using a Margules expression for CO2 in pure water, and a Pitzer expression for salting-out effects. Fugacity coefficients are computed using a modified Redlich-Kwong equation of state and mixing rules that incorporate asymmetric binary interaction parameters. Parameters for the calculation of activity and fugacity coefficients were fitted to published solubility data over the P-T range of interest. In doing so, mutual solubilities and gas-phase volumetric data are typically reproduced within the scatter of the available data. An example of multiphase flow simulation implementing the mutual solubility model is presented for the case of a hypothetical, enhanced geothermal system where CO2 is used as the heat extraction fluid. In this simulation, dry supercritical CO2 at 20°C is injected into a 200°C hot-water reservoir. Results show that the injected CO2 displaces the formation water relatively quickly, but that the produced CO2 contains significant water for long periods of time. The amount of water in the CO2 could have implications for reactivity with reservoir rocks and engineered materials. [ABSTRACT FROM AUTHOR]

From a mass balance for water dissolved into the flowing CO2 stream and a consideration of saturation profiles from the Buckley-Leverett fractional flow theory, we derive an equation that directly relates gas saturation S g,d at the dry-out front to temperature, pressure, and salinity dependence of fluid properties. The equation is easily solved by iteration or interpolation. From gas saturation at the front we derive the average gas saturation in the dry-out region, from which we obtain the 'solid saturation' S S, i.e., the fraction of pore space filled with solid precipitate. Values of S S derived from this theory show excellent agreement with numerical simulations presented in the preceding companion paper. Thus, from relative permeabilities and fluid properties at in situ conditions prior to CO2 injection, it is possible to directly make an accurate estimate of solids precipitation, without having to perform a numerical simulation of the injection process. [ABSTRACT FROM AUTHOR]

Injection of CO2 into saline aquifers may cause formation dry-out and precipitation of salt near the injection well, which may reduce formation porosity, permeability, and injectivity. This paper uses numerical simulation to explore the role of different processes and parameters in the salt precipitation process and to examine injection strategies that could mitigate the effects. The main physical mechanisms affecting the dry-out and salt precipitation process include (1) displacement of brine away from the injection well by injected CO2, (2) dissolution (evaporation) of brine into the flowing CO2 stream, (3) upflow of CO2 due to gravity effects (buoyancy), (4) backflow of brine toward the injection point due to capillary pressure gradients that oppose the pressure gradient in the CO2-rich ('gas') phase, and (5) molecular diffusion of dissolved salt. The different mechanisms operate on a range of spatial scales. CO2 injection at constant rate into a homogeneous reservoir with uniform initial conditions is simulated in 1-D radial geometry, to resolve multiscale processes by taking advantage of the similarity property, i.e., the evolution of system conditions as a function of radial distance R and time t depends only on the similarity variable R2/t. Simulations in 2-D vertical cross sections are used to examine the role of gravity effects. We find that counterflow of CO2 and brine can greatly increase aqueous phase salinity and can promote substantial salt precipitation even in formations with low dissolved solids. Salt precipitation can accentuate effects of gravity override. We find that injecting a slug of fresh water prior to commencement of CO2 injection can reduce salt precipitation and permeability loss near the injection well. [ABSTRACT FROM AUTHOR]

Geologic storage of CO2 is expected to produce plumes of large areal extent, and some leakage may occur along fractures, fault zones, or improperly plugged pre-existing wellbores. A review of physical and chemical processes accompanying leakage suggests a potential for self-enhancement. The numerical simulations presented here confirm this expectation, but reveal self-limiting features as well. It seems unlikely that CO2 leakage could trigger a high-energy run-away discharge, a so-called “pneumatic eruption,” but present understanding is insufficient to rule out this possibility. The most promising avenue for increasing understanding of CO2 leakage behavior is the study of natural analogues. [ABSTRACT FROM AUTHOR]

The transport of chemicals or heat in fractured reservoirs is strongly affected by the fracture-matrix interfacial area. Under unsaturated conditions, such as in vapor-dominated geothermal reservoirs, this area can be estimated from inert gas tracer tests that produce a characteristic tail in tracer breakthrough curves (BTCs). For water-saturated conditions, molecular diffusion is orders of magnitude smaller, and tails in BTCs from diffusive exchange between fractures and matrix are too weak to be practically useful as a characterization tool for fracture spacings of practical interest. However, recent studies suggest that reversibly sorbing solute tracers can generate strong tails in BTCs that may allow a determination of the fracture-matrix interfacial area. To theoretically explore such a useful phenomenon, this paper develops an analytical solution for BTCs in slug tracer tests in a water-saturated fractured reservoir, idealized here as a set of uniformly spaced plane, parallel, identical fractures. The solution shows that increased sorption should have the same effect on BTCs as an increase of the diffusion coefficient. The solution is useful for understanding transport mechanisms, verifying numerical codes, and identifying appropriate chemicals as tracers for the characterization of fractured reservoirs. [ABSTRACT FROM AUTHOR]

Injection of CO2 into saline aquifers is described by mass conservation equations for the three components water, salt (NaCl), and CO2. The equations are discretized using an integral finite difference method, and are solved using methods developed in geothermal and petroleum reservoir engineering. Phase change processes are treated through switching of primary thermodynamic variables. A realistic treatment of PVT (fluid) properties is given which includes salinity and fugacity effects for partitioning of CO2 between gaseous and aqueous phases. Chemical reactions and mechanical stress effects are neglected. Numerical simulations are presented for injection of CO2 into a brine aquifer, and for loss of CO2 from storage through discharge along a fault zone. It is found that simulated pressures are much more sensitive to space discretization effects than are phase saturations. CO2 discharge along a fault is a self-enhancing process whose flow rates can increase over time by more than an order of magnitude, suggesting that reliable containment of CO2 will require multiple barriers. [ABSTRACT FROM AUTHOR]

Laboratory experiments were conducted to study solute transport along preferential flow paths in unsaturated, inclined fractures. Qualitative aspects of solute transport were identified in a miscible dye tracer experiment conducted in a transparent replica of a natural granite fracture. Additional experiments were conducted to measure the breakthrough curves of a conservative tracer introduced into an established preferential flow path in two different fracture replicas and a rock-replica combination. The influence of gravity was investigated by varying fracture inclination. The relationship between the travel times of the solute and the relative influence of gravity was substantially affected by two modes of intermittent flow that occurred: the snapping rivulet and the pulsating blob modes. The measured travel times of the solute were evaluated with three transfer function models: the axial dispersion, the reactors-in-series, and the lognormal models. The three models described the solute travel times nearly equally well. A mechanistic model was also formulated to describe transport when the pulsating blob mode occurred which assumed blobs of water containing solute mixed with residual pools of water along the flow path. [ABSTRACT FROM AUTHOR]

Pyrite oxidation (FeS2) causes acidification and mobilization of metals. Mathematical modeling of pyrite oxidation in variably saturated reactive flow systems is challenging because (1) it occurs through a complex interplay of multi-phase flow and transport processes, and (2) aqueous concentrations of key species vary over tens orders of magnitude in different redox conditions. Here, we present a general multi-phase reactive transport model for redox processes. Two alternative implementations were made in the TOUGHREACT and TOUGH2-CHEM simulation codes which use sequential iteration and simultaneous solution, respectively. Both codes are used to simulate a fully and a variably saturated pyrite oxidation problem with simple 1-D flow and reaction conditions. Results from both codes indicate that the effects of oxygen partial pressure reduction due to reactions on the fluid flow is not significant under ambient conditions. However, it must be noted that when fluid flow and chemical reactions are strongly coupled, such as when boiling takes place in geothermal reservoirs, this could be essential. The fully simultaneous approach has a complete process description. The sequential iteration approach is found to be more efficient computationally. The oxygen gas diffusion process plays a dominant role in the chemical evolution for pyrite oxidation in unsaturated conditions. An example in 2-D fractured rock is presented to demonstrate pyrite oxidation under complex flow and geochemical conditions. This example shows that pyrite oxidation exerts strong influence on hydrogeochemical evolution in variably saturated flow systems. The alteration of primary rock minerals and the development of secondary mineral assemblages predicted are consistent with field observations. This example serves as a prototype for oxidative weathering processes with broad significance for geoscientific, engineering, and environmental applications. [ABSTRACT FROM AUTHOR]

A capillary barrier arises in unsaturated conditions at the contact of a fine layer of soil overlying a coarse layer of soil. If such a contact is tilted, infiltrating moisture in the fine layer will be diverted and flow down the contact as capillary diversion. Capillary barriers can occur naturally in layered heterogeneous systems, or they can be engineered for the purpose of diverting infiltration, for example, away from hazardous wastes. Recent theoretical analyses of capillary diversion have revealed some simple relations for the behavior of an idealized capillary barrier. Meanwhile, laboratory experiments and field investigations have shown much more complicated behavior. We have attempted to bridge the gap between the simple theoretical results and the more complicated laboratory and field observations by means of numerical experiments. Numerical modeling of capillary barriers presents significant challenges because of the potential for gravitational instability which can give rise to strong space discretization and grid orientation effects. We have examined the ability of different finite difference techniques, such as harmonic weighting, upstream weighting, and higher-order differencing, to represent flow exclusion and leakage effects at capillary barriers. The numerical experiments show that analytical results previously obtained are reasonable, but that leakage occurs upstream of the theoretical breakthrough point. Furthermore, in the breakthrough region and downstream from it, the behavior is considerably more complicated than assumed in the theoretical analysis. Numerical experiments, with careful consideration of space discretization and grid orientation effects, reveal that the initial breakthrough allows sufficient leakage at the contact to partially dry the upper layer, making the barrier active again downstream. Finite difference schemes specifically designed to handle transient multiphase flows can adequately reproduce the broad features of flow diversion and leakage at capillary barriers. [ABSTRACT FROM AUTHOR]

A multidimensional integral finite difference numerical simulator is developed for modeling the steam displacement of nonaqueous phase liquid (NAPL) contaminants in shallow subsurface systems. This code, named STMVOC, considers three flowing phases, gas, aqueous, and NAPL; and three mass components, air, water, and an organic chemical. Interphase mass transfer of the components between any of the phases is calculated by assuming local chemical equilibrium between the phases, and adsorption of the chemical to the soil is included. Heat transfer occurs due to conduction and multiphase convection and includes latent heat effects. A general equation of state is implemented in the code for calculating the thermophysical properties of the NAPL/chemical. This equation of state is primarily based on corresponding states methods of property estimation using a chemical's critical constants. The necessary constants are readily available for several hundred hazardous organic liquid chemicals. In part 2 (Falta et al., this issue), the code is used to simulate two one-dimensional laboratory steam injection experiments and to examine the effect of NAPL properties on the steam displacement process. [ABSTRACT FROM AUTHOR]

The multiphase steam injection simulator developed in part 1 (Falta et al., this issue) is used to simulate two laboratory column steam displacement experiments. In the first simulation, steam is injected into a clean, water-saturated column, while in the second simulation, steam is injected into a column containing both water and separate phase trichloroethylene. In both cases, the numerical results are in good quantitative agreement with the experimental data. Based on the assumption of local chemical equilibrium between the phases, a simple criterion is derived for determining the major mechanism of nonaqueous phase liquid (NAPL)/chemical transport during the steam displacement process. Several one-dimensional simulations of the steam displacement of high-boiling-point NAPLs are discussed. These results are consistent with theoretical predictions and indicate that steam may efficiently displace organic liquids having boiling points substantially greater than that of water. [ABSTRACT FROM AUTHOR]

A theoretical investigation of factors affecting the gas phase transport of evaporating organic liquids in the unsaturated zone is presented. Estimates of density-driven advective gas flow using a simple analytic expression indicate that significant advective gas flow will result from the evaporation of volatile liquids in soils having a high permeability. Numerical simulations using a two-dimensional cylindrical geometry and including the effects of phase partitioning between the solid, gas, water, and organic liquid phases show that mass transfer due to density-driven flow may dominate the gas phase transport of some organic chemical vapors in the unsaturated zone. [ABSTRACT FROM AUTHOR]

Flow visualization experiments were conducted on a transparent replica of a natural, rough-walled rock fracture from the Stripa Mine, Sweden, for inlet conditions of constant pressure and flow rate over a range of angles of inclination. The experiments demonstrated that infiltrating water proceeds through unsaturated rock fractures along nonuniform, localized preferential flow paths. Even though constant inlet conditions were maintained, pervasive unsteady or intermittent flow was also observed in these experiments, where portions of the flow channel underwent cycles of snapping and reforming. Experiments conducted on parallel plates with a sequence of apertures progressing from small to large to small reproduced intermittent flow. Measurements of the frequency of intermittent flow events and the volume of water metered between events were obtained from the fracture replica and parallel plate experiments and related to the Bond and capillary numbers to generalize the results. The frequency data from the fracture replica experiments did not follow the same trend as the data from the parallel plate experiments for similar Bond and capillary numbers, but the volume of water metered data was consistent in these experiments. [ABSTRACT FROM AUTHOR]

Water flow in partially saturated fractures under thermal drive may lead to fast flow along preferential localized pathways and heat pipe conditions. At the potential high-level nuclear waste repository at Yucca Mountain, water flowing in fast pathways may ultimately contact waste packages and transport radionuclides to the accessible environment. Sixteen experiments were conducted to visualize heat-driven liquid flow in fracture models that included (1) assemblies of roughened glass plates, (2) epoxy replicas of rock fractures, and (3) a fractured specimen of Topopah Spring tuff. Continuous rivulet flow was observed for high liquid flow rates, intermittent rivulet flow and drop flow for intermediate flow rates, and film flow for lower flow rates and wide apertures. Heat pipe conditions (vapor-liquid counterflow with phase change) were identified in five of the seven experiments in which spatially resolved thermal monitoring was performed but not when vapor-liquid counterflow was hindered by very narrow apertures and when an inadequate working fluid volume was used. [ABSTRACT FROM AUTHOR]

In this paper a procedure is presented to solve the estimation-identification problem in two-phase flow modeling. Given discrete observations made on the system response, an optimum parameter set is derived for an appropriate conceptual model by solving the inverse problem using standard optimization techniques. Subsequently, a detailed error analysis is performed, and nonlinearity effects are considered. We discuss the iterative process of model identification and parameter estimation for a ventilation test performed at the Grimsel Rock Laboratory, Switzerland. A numerical model of the ventilation drift and the surrounding crystalline rock matrix is developed. Evaporation of moisture at the drift surface and the propagation of the unsaturated zone into the formation are simulated. A sensitivity analysis is performed to identify the parameters to be estimated. Absolute permeability and two parameters of van Genuchten's characteristic curves are subsequently determined based on measurements of negative water potentials, evaporation rates, and gas pressure data. The performance of the minimization algorithm and the system behavior for the optimum parameter set are discussed. The study shows that a field experiment conducted under two-phase flow conditions can be successfully reproduced by taking into account a variety of physical processes and that it is possible to reliably determine the two-phase hydraulic properties that are related to the given conceptual model. [ABSTRACT FROM AUTHOR]

Many problems in subsurface hydrology involve the flow and transport of solutes that affect liquid density. When density variations are large (>5%), the flow and transport are strongly coupled. Density variations in excess of 20% occur in salt dome and bedded-salt formations which are currently being considered for radioactive waste repositories. The widely varying results of prior numerical simulation efforts of salt dome groundwater-brine flow problems have underscored the difficulty of solving strongly coupled flow and transport equations. We have implemented a standard model for hydrodynamic dispersion in our general purpose integral finite difference simulator, TOUGH2. The residual formulation used in TOUGH2 is efficient for the strongly coupled flow problem and allows the simulation to reach a verifiable steady state. We use the model to solve two classic coupled flow problems as verification. We then apply the model to a salt dome flow problem patterned after the conditions present at the Gorleben salt dome, Germany, a potential site for high-level nuclear waste disposal. Our transient simulations reveal the presence of two flow regimes: (1) recirculating and (2) swept forward. The flow dynamics are highly sensitive to the strength of molecular diffusion, with recirculating flows arising for large values of molecular diffusivity. For pure hydrodynamic dispersion with parameters approximating those at Gorleben, we find a swept-forward flow field at steady state rather than the recirculating flows found in previous investigations. The time to steady state is very sensitive to the initial conditions, with long time periods required to sweep out an initial brine pool in the lower region of the domain. Dimensional analysis is used to demonstrate the tendency toward brine recirculation. An analysis based on a dispersion timescale explains the observed long time to steady state when the initial condition has a brine pool in the lower part of the system. The nonlinearity of the equations and the competing effects of dispersion and gravity make this variable-density problem a challenge for any numerical simulation method. [ABSTRACT FROM AUTHOR]

Thermohaline convection occurs in hypersaline geothermal systems due to thermal and salinity effects on liquid density. Because of its importance in oceanography, thermohaline convection in viscous liquids has received more attention than thermohaline convection in porous media. The fingered and layered convection patterns observed in viscous liquid thermohaline convection have been hypothesized to occur also in porous media. However, the extension of convective dynamics from viscous liquid systems to porous media systems is complicated by the presence of the solid matrix in porous media. The solid grains cause thermal retardation, hydrodynamic dispersion, and permeability effects. We present simulations of thermohaline convection in model systems based on the Salton Sea Geothermal System, California, that serve to point out the general dynamics of porous media thermohaline convection in the diffusive regime, and the effects of porosity and permeability, in particular. We use the TOUGH2 simulator with residual formulation and fully coupled solution technique for solving the strongly coupled equations governing thermohaline convection in porous media. We incorporate a model for brine density that takes into account the effects of NaCl and CaCl2. Simulations show that in forced convection, the increased pore velocity and thermal retardation in low-porosity regions enhances brine transport relative to heat transport. In thermohaline convection, the heat and brine transport are strongly coupled and enhanced transport of brine over heat cannot occur because buoyancy caused by heat and brine together drive the flow. Random permeability heterogeneity has a limited effect if the scale of flow is much larger than the scale of permeability heterogeneity. For the system studied here, layered thermohaline convection persists for more than one million years for a variety of initial conditions. Our simulations suggest that layered thermohaline convection is possible in hypersaline geothermal systems provided the vertical permeability is smaller than the horizontal permeability, as is likely in sedimentary basins such as the Salton Trough. Layered thermohaline convection can explain many of the observations made at the Salton Sea Geothermal System over the years. [ABSTRACT FROM AUTHOR]

Gas flow in porous media differs from liquid flow because of the large gas compressibility and pressure-dependent effective permeability. The latter effect, named after Klinkenberg, may have significant impact on gas flow behavior, especially in low permeability media, but it has been ignored in most of the previous studies because of the mathematical difficulty in handling the additional nonlinear term in the gas flow governing equation. This paper presents a set of new analytical solutions developed for analyzing steady-state and transient gas flow through porous media including Klinkenberg effects. The analytical solutions are obtained using a new form of gas flow governing equation that incorporates the Klinkenberg effect. Additional analytical solutions for one-, two- and three-dimensional gas flow in porous media could be readily derived by the following solution procedures in this paper. Furthermore, the validity of the conventional assumption used for linearizing the gas flow equation has been examined. A generally applicable procedure has been developed for accurate evaluation of the analytical solutions which use a linearized diffusivity for transient gas flow. As application examples, the new analytical solutions have been used to verify numerical solutions, and to design new laboratory and field testing techniques to determine the Klinkenberg parameters. The proposed laboratory analysis method is also used to analyze data from steady-state flow tests of three core plugs from The Geysers geothermal field. We show that this new approach and the traditional method of Klinkenberg yield similar results of Klinkenberg constants for the laboratory tests; however, the new method allows one to analyze data from both transient and steady-state tests in various flow geometries. [ABSTRACT FROM AUTHOR]

Water injection into unsaturated fractured rock at above-boiling temperatures gives rise to complex fluid flow and heat transfer processes. Examples include water injection into depleted vapor-dominated geothermal reservoirs, and emplacement of heat-generating nuclear wastes in unsaturated fractured rock. We conceptualize fractures as two-dimensional heterogeneous porous media, and use geostatistical techniques to generate synthetic permeability distributions in the fracture plane. Water flow in hot high-angle fractures is simulated numerically, taking into account the combined action of gravity, capillary, and pressure forces, and conductive heat transfer from the wall rocks which gives rise to strong vaporization. In heterogeneous fractures boiling plumes are found to have dendritic shapes, and to be subject to strong lateral flow effects. Fractures with spatially-averaged homogeneous permeabilities tend to give poor approximations for vaporization behavior and liquid migration patterns. Depending on water flow rates, rock temperature, and fracture permeability, liquid water can migrate considerable distances through fractured rock that is at above-boiling temperatures and be only partially vaporized. [ABSTRACT FROM AUTHOR]

Infiltration of water and non-aqueous phase liquids (NAPLs) in the vadose zone gives rise to complex two- and three-phase immiscible displacement processes. Physical and numerical experiments have shown that ever-present small-scale heterogeneities will cause a lateral broadening of the descending liquid plumes. This behavior of liquid plumes infiltrating in the vadose zone may be similar to the familiar transversal dispersion of solute plumes in single-phase flow. Noting this analogy we introduce a mathematical model for 'phase dispersion' in multiphase flow as a Fickian diffusion process. It is shown that the driving force for phase dispersion is the gradient of relative permeability, and that addition of a phase-dispersive term to the governing equations for multiphase flow is equivalent to an effective capillary pressure which is proportional to the logarithm of the relative permeability of the infiltrating liquid phase. The relationship between heterogeneity-induced phase dispersion and capillary and numerical dispersion effects is established. High-resolution numerical simulation experiments in heterogeneous media show that plume spreading tends to be diffusive, supporting the proposed convection-dispersion model. Finite difference discretization of the phase-dispersive flux is discussed, and an illustrative application to NAPL infiltration from a localized source is presented. It is found that a small amount of phase dispersion can completely alter the behavior of an infiltrating NAPL plume, and that neglect of phase-dispersive processes may lead to unrealistic predictions of NAPL behavior in the vadose zone. [ABSTRACT FROM AUTHOR]

The transport and mixing of solutes undergoing first-order decay is central to many problems in groundwater hydrology. Mixing in porous media flow occurs due to advective dilution, hydrodynamic dispersion, and molecular diffusion. Mixing is stronger in regions of higher velocity, and weaker in slower-moving regions. Two-dimensional numerical experiments show that concentration profiles normal to the flow direction are displaced toward regions of slower flow in a flow field with a velocity gradient. Variable-velocity flow fields occur in subsurface flow around permeability heterogeneities, between recirculation cells, and in flow driven by natural convection. We examine two-dimensional solute concentration fields rather than breakthrough curves since for many complicated flow patterns, the breakthrough curve cannot discern important details of the concentration field. For the case of a species undergoing first-order decay, the effect of parent accumulation in regions of low velocity is enhanced for the daughter species because (i) the rate of daughter production is proportional to the local concentration of parent, and (ii) mixing is proportional to the local velocity. The resulting displacement of concentration profiles toward low-velocity regions may have important consequences for subsurface radionuclide transport and also for flows in chemically reactive systems and strongly coupled systems. [ABSTRACT FROM AUTHOR]

In this study, new elements were developed to generalize the dual-porosity model for moisture infiltration and solute transport in unsaturated rocks, taking into account fractal aspects of the percolation process. Random advection was considered as a basic mechanism of solute transport in self-similar fracture systems. In addition to spatial variations in the infiltration velocity field, temporal fluctuations were also taken into account. The rock matrix, which is a low-permeability component of the heterogeneous geologic medium, acts as a trap for solute particles and moisture. Scaling relations were derived for the moisture infiltration flux, the velocity correlation length, the average velocity of infiltration, and the velocity correlation function. The effect of temporal variations in precipitation intensity on the infiltration processes was analyzed. It showed that the mode of solute transport is determined by the power exponent in the advection velocity correlation function and the dimensionality of the trapping system, both of which may change with time. Therefore, depending on time, various transport regimes may be realized: superdiffusion, subdiffusion, or classical diffusion. The complex structure of breakthrough curves arising from changes in the transport regimes was also examined. A renormalization of the solute source strength due to characteristic fluctuations of highly disordered media was established. [ABSTRACT FROM AUTHOR]

We present an overview of the problem of solute transport in unsaturated heterogeneous media. We first review field and laboratory observations that demonstrate nonclassical flow and transport behavior. The main physical principles causing anomalous transport regimes in fractured rock media are identified. The basic factors and physical concepts needed to describe anomalous transport in saturated and unsaturated fractured rock are discussed in detail. [ABSTRACT FROM AUTHOR]

Numerical simulation has become a widely practiced and accepted technique for studying flow and transport processes in the vadose zone and other subsurface flow systems. This article discusses a suite of codes, developed primarily at Lawrence Berkeley National Laboratory (LBNL), with the capability to model multiphase flows with phase change. We summarize history and goals in the development of the TOUGH codes, and present the governing equations for multiphase, multicomponent flow. Special emphasis is given to space discretization by means of integral finite differences (IFD). Issues of code implementation and architecture are addressed, as well as code applications, maintenance, and future developments. [ABSTRACT FROM AUTHOR]

We investigate the physical processes that occur during the sequestration of CO2 in liquid-saturated, brine-bearing geologic formations using the numerical simulator TOUGH2. Carbon dioxide is injected in a supercritical state that has a much lower density and viscosity than the liquid brine it displaces. In situ, the supercritical CO2 forms a gas-like phase, and also partially dissolves in the aqueous phase, creating a multiphase, multicomponent environment that shares many important features with the vadose zone. The flow and transport simulations employ an equation of state package that treats a two-phase (liquid, gas), three-component (water, salt, CO2) system. Chemical reactions between CO2 and rock minerals that could potentially contribute to mineral trapping of CO2 are not included. The geological setting considered is a fluvial/deltaic formation that is strongly heterogeneous, making preferential flow a significant effect, especially when coupled with the strong buoyancy forces acting on the gas-like CO2 plume. Key model development issues include vertical and lateral grid resolution, grid orientation effects, and the choice of characteristic curves. [ABSTRACT FROM AUTHOR]

Liquid flow through unsaturated fractures often proceeds as fingers or preferential flow paths. During the invasion of liquid fingers into an initially dry, nonhorizontal fracture, fingers may drain, forming a narrow thread of liquid called a rivulet that connects to a wider portion of liquid at the advancing front, defined as a blob. Experimental studies using idealized fractures were performed to investigate the effects of wettability, surface roughness, and aperture size on several important features of gravity-driven flow in fractures: liquid drainage, blob migration, and rivulet flow. The experiments demonstrate that the critical length of the blob before drainage occurred was significantly longer on surfaces with intermediate wettability and on surfaces with roughness on the order of 100 µm than on a smooth, flat water-wetting surface. However, drainage did not occur on surfaces with smaller-scale roughness on the order of 10 µm. Blob velocities were also measured and were always less than the saturated gravity-driven flow velocity, even when a liquid with a static contact angle of zero was used. This reduction in velocity was attributed to contact angle hysteresis. Rivulet widths measured as a function of flow rate between glass and acrylic parallel plates were generally larger on the acrylic plates than the glass plates for a particular flow rate, demonstrating the sensitivity of rivulet flow to wettability. In addition, the cubic law overpredicted the measured rivulet widths, except for the widths measured between the acrylic plates at 20°. The effect of aperture variability on rivulet flow was also examined. At a critical aperture ranging between 0.25 and 0.37 mm, the liquid in the rivulet did not completely span the aperture, forming two streamlets of liquid on either side of the fracture. [ABSTRACT FROM AUTHOR]