H11E-0332 0800h
Coupling Flow and Thermal and Reactive Geochemical Transport
The couplings among fluid flows, thermal transport, geochemical reactions, advective and diffusive transport of solutes in fractured media or soils, and changes in hydraulic properties due to precipitation and dissolution along fractures and rock matrix are important for understanding interplays between fluid flows and dynamic transport processes. This paper describes the development and demonstrative applications of a mechanistic-based numerical model of coupled fluid flow and thermal and reactive geochemical transport, including both fast and slow reactions, in variably saturated media. Theoretical bases, numerical implementations, and two numerical experiments using the model will be presented. The first example deals with the effect of precipitation-dissolution on fluid flow and matrix diffusion in a two-dimensional fractured media. Because of the precipitation and decreased diffusion of solute from the fracture into the matrix, retardation in the fractured medium is not as large as the case wherein interactions between geochemical reactions and transport are not considered. The second example focuses on a complicated but realistic advective-dispersive-reactive transport problem. This example exemplifies the need for innovative numerical algorithms to solve problems involving stiff geochemical reactions.
H11E-0333 0800h
Dedolomitization and Flow in Fractures
The evolution of porosity and hydraulic conductivity resulting from the incongruent dissolution of fractured dolomite was studied experimentally. Two types of flow experiments were carried out: (1) reactive flow in quasi-2D rock fractures (geometries consisted of a through-flow fracture that connected the inlet and outlet, and an oblique fracture that did not connect them), and (2) 3D fractured/intact linear corefloods. Experimental results demonstrated a strong influence of flow conditions on aragonite/calcite formation processes in fractured dolomite. Mixing at fracture intersections induced aragonite precipitation that led to an irreversible reduction of sample hydraulic conductivity. Clogging of the samples was more rapid for oblique fractures than for through-flow fractures. Oblique fractures were always filled with precipitated material in both 2D and 3D fracture flow experiments. To further study the dynamics of porosity evolution during mixing induced precipitation of calcite at a fracture intersection, a 2D finite element numerical model was developed. The influence of two different expressions that relate specific surface area to porosity was also explored. During the simulations, porosity was reduced primarily in regions in which significant degrees of mixing occurred, effectively creating a barrier to further mass transport. The results demonstrate that mixing induced precipitation can account for systems in which only portions of high porosity regions are filled. Mixing induced precipitation thus represents a viable mechanism that can account for porosity infilling in both fractured and porous geological systems. The simulations also indicate that the choice of functional form for the specific surface area plays an important role in controlling porosity patterns. As specific surface area is currently one of the least constrained parameters in models of porosity evolution, this result highlights the need for future experimental studies.
H11E-0334 0800h
Infiltrative Instability Near a Topographic Jump. Implication for the Underground Drainage of Soluble Rocks
The infiltrative instability takes place when an reactive fluid is injected into a soluble porous matrix. It has been widely studied using either experimental or numerical approaches, when the fluid is injected along the axis of a core sample or radially from it's axis of symmetry. However it is known that in the case of a limestone formation, the initiation of a drainage network is associated to topographic gradients due either to a tectonic event or to a sea level change. Recently, the evolution of a fissure network submitted to dissolution by meteoric water has been simulated and compared with observations of speleologists. But in this case channel orientations result largely from the initial fissure distribution. We propose that the infiltration instability should be also studied in a initially homogeneous formation to assess the contribution of initial heterogeneities. Therefore, we have simulated the infiltrative instability near a topography jump, with simplified chemical system and boundary conditions. The dissolution is assumed to occur either instantaneously or with a first order kinetics, and the porous medium is assumed to be permanently saturated by the fluid and to lie above a perfectly impervious medium. Then the characteristic numbers of our study reduce to the P\'{e}clet and Dahmkohler numbers and to the permeability ratio between the initial porous medium and the dissolution channels. The numerical method involves a multigrid solver for the flow equation and a Volume of Fluid method in the case of instantaneous dissolution. Our models produce first a short channel near the edge of the jump, emerging at mid-height of the cliff (non-dimensional time : 0.06) while the formation of the drainage of the whole formation involves a much longer time scale (non-dimensional time : 13 for the drainage of a length of twice the height of the topographic jump). Moreover, the style of dissolution instabilities varies in the upstream direction and tends to the formation of large zones of pervasive dissolution far from the jump. This is viewed as the result of the decreasing filtration velocity inside the initial homogenous porous medium that implies a decrease of the P\'{e}clet number and an increase of the Dahmkohler number. Possible consequences on the drainage structure of carbonated or silicated soluble rocks are discussed.
H11E-0335 0800h
Acoustic Effects on Colloid/Surface Interactions and Porous-Media Permeability
Acoustic and seismic waves have been observed to influence porous fluid-flow behavior in the Earth and geomaterials over a wide range of scale lengths (microns to kilometers). Examples include oil reservoir production increases induced by seismic (1 to 500 Hz) waves, and mobilizing colloidal clays in sandstone cores by ultrasonic (10 to 50 kHz) energy. The effects of stress-wave propagation on both colloid electrokinetics and fluid-flow dynamics in porous media are not understood. In particular, the coupling of acoustic and seismic waves with colloid behavior is an important mechanism to understand because the distribution of colloids in a porous medium will directly affect its permeability. Recent experimental observations indicate that very-high-frequency (0.5 to 5 MHz) acoustic energy can induce attachment and detachment of micron-size colloids at solid surfaces. Using a microscopic, video image-processing system focused on a glass flow-visualization cell, the behavior of 0.5- to 3-micron diameter polystyrene spheres suspended in 0 to 0.1 M aqueous solution was observed. Initial image-processing-based analysis of acoustically-induced colloid/surface detachment events indicates that very-high-frequency acoustics not only increases particle detachment, but may also permanently "deactivate" colloid attachment (or "active") sites on the glass cell surface. The ability of acoustics to attach or detach colloids also appears to depend on the colloid size and ionic strength of the suspending solution. Other experiments show that seismic-band (1 to 1000 Hz) mechanical stress oscillations can change the permeability of centimeter-size sandstone cores due to mobilization of micron-size colloids contained in the pore space. A unique core-holder apparatus that mechanically strains 2.54-cm-diameter porous rock samples during constant-rate fluid flow was used for these experiments. During single-phase brine flow through sandstone, axial stress oscillations at 50 Hz mobilized in-situ clay colloids and increased the absolute permeability of the rock by 10 to 20 percent. New experiments are being performed by injecting into clean core samples the same polystyrene colloid suspensions used in the microscopic visualization experiments. Observations of both micro- and core-scale behavior will be presented and discussed.
H11E-0336 0800h
Permeability Changes Due to Secondary Mineral Precipitation During Weathering of Low-Level Radioactive Waste Glass in the Vadose Zone
A film depositional model has been incorporated into the Subsurface Transport Over Reactive Multi-phases (STORM) code, which simulates coupled unsaturated flow, solute transport, energy transport, geochemical reactions, and porosity/permeability changes due to mineral precipitation and dissolution. STORM has been applied to simulations of the weathering of low-level radioactive glass disposed in the shallow subsurface, conducted over geologic time scales. As the glass dissolves, secondary minerals precipitate, which consist mostly of clays. The film depositional model is based on the assumption that the clays are deposited on the pore walls as a continuous film, which may cause a reduction in permeability. The film depositional model is developed for a discrete pore-size distribution, which is determined using the unsaturated hydraulic properties of the porous medium. This facilitates the process of dynamically updating the unsaturated hydraulic parameters used to describe fluid flow through the media. Results are presented from two-dimensional, field scale simulations of the simultaneous dissolution of low-level radioactive waste glass and secondary mineral precipitation using the film depositional model. These results are used to assess the impact of permeability changes on glass weathering rates over a 10,000-year period. This research was performed in part using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy's Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for the Department of Energy by Battelle.
H11E-0337 0800h
Hydrodynamics and Long-term Permeability Evolution in Clogging Porous Media
Permeability reduction caused by colloid deposition in porous media, or clogging, is important in water treatment, aquifer hydraulics, and subsurface remediation. Analysis of six published data sets, representing a variety of particles, porous media and fluids, indicates greater clogging at lower fluid velocity. There is a unique relationship between a clogging parameter in a modified O'Melia and Ali model and the depositing particle's Peclet number. The adopted Peclet number is the ratio of advective to Brownian particle transport within a porous medium and includes hydrodynamics, particle size, and the grain size of the porous medium. Although these data quantify the dependence of clogging on Peclet number, they do not describe steady-state clogging, achieved under conditions of constant flow, with a constant permeability and a constant mass of deposited particles. Data and models for steady-state clogging are lacking because classical filtration research focused on water treatment filters, which are backwashed periodically, and so are not allowed to reach steady state. Steady-state clogging is relevant to flow in natural subsurface environments as an important limiting case in the feedback process between particle deposition and permeability evolution, and as an initial condition for models that describe permeability alteration from mechanical or seismic forces. A model for steady-state clogging assumes deposit permeability to be negligible, then calculates Poiseuille flow in open tubes through the clogged media. The radius of these tubes is a characteristic pore size; the number of these tubes is determined by the applied flow rate and the deposit shear strength, using published estimates. With these assumptions, the model predicts that the pressure drop across the clogged sample is independent of the imposed flow rate, so steady-state permeability will be proportional to flow rate. Deposition experiments (destabilized montmorillonite on quartz sand) to test the power-law relationship between hydrodynamics and permeability and the steady-state clogging model are in progress.
H11E-0338 0800h
Simulation Study of Micro Particles Behavior in Fluid Flow Using Lattice Boltzmann Method
Evaluation of underground hydraulic characteristics has been a key issue not only for hydrogeology but for various fields of geo-engineering. We have been investigating hydraulic properties, such as permeability, of fractures and porous rocks using a 3D lattice Boltzmann method (LBM) for recent several years. In this paper, we propose a coupling method of LBM and DEM (distinct element method) to incorporate dynamic interaction of fluid flow and particles. This coupling technique brings new insights into the effect of micro particles in the hydraulic properties, such that migration and sedimentation of solid particles remarkably decreases permeability. We present two simulation examples; I) sedimentation of micro particles by the gravity in dead water, II) behaviour of micro particles in fluid flow through a porous media. In the simulation-I, surface geometry of the particle assembly shows a gentle 'sag' with a subtle subsidence at its center, suggesting that the upward fluid expulsion causes slightly uplifted geometry. Such geometry of particles can be commonly seen in natural sedimentary rocks that deformed due to fluid expulsion at its unconsolidated stages. The simulation-II clearly showed some conditions of pore throat plugging by the micro particles. The fluid flow pattern should be significantly affected by the moving particles, as well as the pressure difference (an input parameter). The percolation distance of solid particles was well controlled with the pressure difference and throat geometries. We concluded that the coupling simulation of LBM and DEM has extremely high potential to investigate the behavior of solid and fluid interactions. The technique can simulate permeability changes precisely, that are affected by dynamic or physical factors such as compaction. Fluid flow simulations with the technique can be directly applied for plugging of solid particles within a reservoir, which is significant for petroleum production and drill-hole completion. The particle behavior can also be used to design appropriate pressure to enhance or prevent such plugging of fluid paths. This approach can thus also be a powerful tool to simulate plugging of solid particles within fractures known as grouting.
H11E-0339 0800h
Lattice Boltzmann simulation of coupled flow and evolution of porous media at the pore scale
Reactive flows coupled to the evolution of porous formations are a topic of great importance for a wide range of scientific problems, such as acid stimulation of petroleum reservoirs, environmental contaminant transport, mineral mining, geologic sequestration of carbon dioxide, chemical weathering, and dissolution/formation of clathrate hydrates. Because of the complexity of natural porous media and the vast span in scales needed to be addressed (the ratio between the largest and the smallest scales can be as large as 105), pore-scale simulations of the problem in a domain of macroscopic dimensions are essentially impossible even with modern supercomputers. Current modeling approaches describing reactive flows and the evolution of porous materials are based on a macro-scale continuum representation of fluid flow, transport and reaction. In these approaches, phenomenological coefficients governing macroscopic process are needed. Constitutive relations for these coefficients can be derived from the solution at the microscopic (pore) scale over a representative control volume. Thus, a detailed understanding of the phenomena at the pore scale is important for solving the problem at the macro scale of practical interest. Mineral precipitation/dissolution reactions result in physical changes of the porous medium over time with consequent changes in porosity and permeability. Clogging can result in rapid reduction in permeability without completely filling the available pore space. In this study, we present a Lattice Boltzmann model to numerically simulate coupled flow and evolution of the porous media at the pore scale and investigate changes in porosity/permeability of the medium due to mineral precipitation/dissolution. We performed a set of sensitivity tests varying the Peclet and Damkohler numbers and saturation and examined the effects of these parameters on the patterns of mineral dissolution/precipitation based on a first-order kinetic rate law or a first order heterogeneous reaction between the aqueous solution and mineral surface. Several examples are compared and contrasted ranging from reaction instability in limestone leading to the formation of wormholes, to uniform reaction over a macro scale control volume. The limitations of the applicability of the continuum formulation are discussed and constitutive relations for changes in porosity and permeability are derived.
H11E-0340 0800h
Engineered Calcite Precipitation in Porous Media: Effects on Flow and Vice Versa
Engineered precipitation of minerals in the subsurface offers a potential means to control the mobility of some trace metal and radionuclide contaminants in groundwater. Examples include biological reduction of U(VI) to insoluble UO$_{2}$, and co-precipitation of $^{90}$Sr and other divalent metals in calcite. In order to take advantage of in situ precipitation in field-scale remediation technologies we must be able to control the onset and distribution of mineral precipitation within a porous medium. This is challenging because precipitation can alter permeability and flow paths. Models predicting the coupling between mineral precipitation, delivery mechanisms, and changes in local flow behavior are not yet well developed, and improved information on mechanistic relationships linking parameters and processes is needed. We are currently conducting laboratory investigations to provide data that will support the development of both improved models for coupling between precipitation and flow, and techniques for monitoring precipitation events in the field. The model experimental system that we are using is calcite precipitation induced by enzymatic hydrolysis of urea to bicarbonate and ammonium, catalyzed by the enzyme urease. Urea hydrolysis thus results in the in situ generation of the reactant (bicarbonate) necessary for calcite precipitation. For these experiments we are using urease immobilized on macroporous Eupergit C beads mixed with quartz sand and packed within a defined zone in a sand column. Varying the flux of urea and calcium through the urease zone provides a means to affect the distribution and extent of calcite precipitation. Column experiments are underway to test some basic hypotheses related to precipitation as a function of hydrolysis kinetics, precipitation kinetics and flow rates. X-ray tomography and complex resistivity are being tested as methods to monitor and map precipitation within porous media at different scales.
H11E-0341 0800h
Enigmatic Permeability Switching in Fractures Under Net Dissolution
Results are reported for water flow-through experiments conducted on a natural fracture in Arkansas Novaculite at temperatures of 20C, 40C, 80C, and 120C and under ambient confining stress of 1.72 MPa and flow-rates of Q = 1.0 to 0.0625 mL/min. Measurements of fluid and dissolved mass fluxes, and concurrent x-ray CT imaging are used to constrain the progress of dissolution and its effect on transport properties throughout the 3150 hr duration of the experiment. Changes in differential pressure between sample inlet and outlet are recorded and used as a proxy for evolution in fracture aperture (i.e., permeability). Measurements of effluent dissolved mineral mass flux provide an independent measure of aperture changes with time. These measurements of evolving aperture are further constrained through non-destructive imaging by x-ray CT. The hydraulic aperture monotonically decreases from 18 to 7 um during the first 1500 hrs (20C), despite net dissolution of Si. As temperature is incremented to 120C, the fracture switches from net closure to gaping, resulting in a final aperture of 13 um at the end of the 3150 hr duration experiment. Effluent Si concentrations increase with temperature, and imply that precipitation remained inactive within the fracture, and highlighting the potential for the activation of dissolution at fracture void surfaces. The phenomenon of switching from fracture closure to gaping occurred even in the absence of changes in experimental conditions of flow-rate or applied effective stress and is due to a switch in dominant processes; from mineral dissolution dominant at the contacting asperities to free-face dissolution-dominant at the fracture void surfaces.
H11E-0342 0800h
Biofilm Growth Induced Transformation of Porous Media Dynamics
Magnetic resonance microscopy (MRM) has been applied to study hydrodynamic dispersion in porous media impacted by biofilms growth. MRM measures the averaged propagator of motion which provides the probability of displacements to occur over experimentally controlled times. The transition from pre-asymptotic to asymptotic hydrodynamic dispersion in a homogeneous porous medium constructed from monodisperse spheres is clearly visualized by the time evolution of the propagator to a Gaussian distribution. The growth of biofilms in the porous media induces a transition in the hydrodynamic dispersion from normal to anomalous transport which is visualized by the propagator transition from Gaussian to that modeled by a subdiffusive fractal kinetics model based on continuous time random walks (CTRW's). This transition is consistent with the porous media structure changing from homogeneous to nonhomogeneous and connections to fractal dimensions are discussed. The MRM data can be analyzed in the q-space domain, i.e. the wavelength space reciprocal to displacement, and provides information on the dynamics on scales above and below a single pore. Fractional kinetics models for subdiffusive processes predict stretched exponential Gaussian behavior and the q-space data fits to strectched exponentials exhibit a transition from Gaussian to subdiffusion due to biofilm growth.