CONTRACTOR: SANDIA NATIONAL LABORATORIES
Lockheed Martin
Albuquerque, New Mexico 87185
CONTRACT: DE-AC04-94AL85000
CATEGORY: Geophysics and Earth Dynamics
PERSON IN CHARGE: H. R. Westrich
A. Shear Strain Localization and Fracture Evolution in Rocks (W. A. Olsson [505-844-7344; Fax 505-844-7345; E-mail waolsso@sandia.gov], D. J. Holcomb, and J. T. Fredrich)
Objectives: This research seeks an improved understanding of the mechanism of the formation of faults and fractures in rock and the prediction of their causative stresses, location, orientation, thickness, and spacing. The research examines the applicability of a theory that describes faulting as a constitutive instability leading to a localization of shear deformation from a homogeneous pattern of deformation.
Project Description: An experimental program is undertaken that entails four parts: (1) A systematic evaluation of the Rudnicki-Rice constitutive parameters under axisymmetric and truly multiaxial stress states. Several rock types are used to cover a range of porosities, including low-porosity Tennessee marble, intermediate porosity Gosford sandstone, and higher porosity Castlegate sandstone. (2) Experiments to measure and compare the actual and predicted development of strain localization in axisymmetric and plane-strain compression (3) Multiaxial stress tests on pressurized thin-walled cylinders with superimposed torsion to investigate special phenomena, vertex hardening, leading to strain localization at relatively small deformations. (4) Post test petrographic observations concerning the details of shear banding and the potentially accelerating effects of imperfections in experimental boundary conditions. The foregoing research is integrated with a parallel theoretical study by Rudnicki.
Results: Previous work has shown that a vertex forms at the load point on the yield surface for Tennessee marble. Further work on Gosford and Berea sandstones showed that vertex hardening also occurred in these porous, granular rocks. This year improved experimental techniques were used to search for vertex hardening in Castlegate sandstone having 25 to 30% porosity; as yet the results are not conclusive. Whether this result is related to the high porosity needs to be determined. Acoustic emission location techniques were used to demonstrate that localization occurred post-peak (softening) for Gosford sandstone under conventional triaxial conditions, but pre-peak (hardening) for plane strain conditions. Recent results by Rudnicki show that a detailed analysis of the constitutive behavior is required to determine if the yield surface was actually expanding during the plane strain tests (true hardening) or whether the increasing mean stress required to maintain plane strain conditions created a false hardening. Constitutive models were developed for Tennessee marble using triaxial test data and are being used to predict the results of plane strain and torsion tests. Multiaxial constitutive models are needed for predicting localization conditions for comparison with experimental results.
B. Inversion of Full Waveform Seismic Data for Shallow Subsurface Properties (D.F. Aldridge [505-284-2823; Fax 505-844-0240; E-mail dfaldri@sandia.gov], G. J. Elbring, and D. E. Womble)
Objective: This research plans to demonstrate the applicability of full waveform seismic inversion methodologies to shallow subsurface environmental characterization or remediation targets.
Project Description: Full waveform inversion of seismic data offers the potential for providing improved subsurface images, especially for shallow seismic surveys, where reflected energy may be poor quality or even nonexistent. We propose to implement a seismic waveform inversion algorithm that is specifically adapted to shallow seismic exploration. The algorithm will accommodate an isotropic elastic earth with three-dimensional heterogeneities, and will be able to handle the variety of sources, receivers, and recording geometries commonly used in shallow seismic surveying. State of the art numerical techniques will be used to model elastic wave propagation and to perform data inversion. In particular, a finite-difference synthetic seismogram algorithm based on the velocity-stress equations of elastodynamics will enable rapid and realistic simulations of the seismic wavefield expected in the shallow subsurface. Compositing (or stacking) of common-receiver gathers of seismic traces will allow simultaneous, rather than sequential, inversion of data acquired from numerous sources and receivers. This novel approach implies a significant reduction in computer run time. Investigation of this computationally intensive nonlinear inverse problem is well suited to Sandia's established infrastructure and expertise in supercomputing. In order to demonstrate the applicability of the inversion approach to shallow seismic targets, a limited-sized seismic data set will be recorded from a site with particular interest in the environmental characterization and/or remediation fields.
Results: This project has only recently been initiated. To date, efforts have been devoted to implementing and testing a three-dimensional, elastic, finite-difference seismic wave propagation algorithm in a computer workstation environment.
C. Micromechanics of Failure in Brittle Geomaterials (J. T. Fredrich [505-844-2096; Fax 505-844-7354; E-mail fredrich@sandia.gov] and T. Wong [State University of New York, Stony Brook])
Objectives: This project provides a fundamental understanding of the effects of grain boundary structure and cementation, damage state, and load path on the deformation and failure mode of brittle porous and nonporous geologic materials by measurement of mechanical behavior under high pressure and deviatoric stress, quantitative microstructural sample characterization, and theoretical analysis.
Project Description: The experimental investigation will provide a detailed understanding of the micromechanical processes associated with the brittle failure of geomaterials and includes triaxial tests following various load paths which are defined by the ratio K of the change in the radial confining (horizontal) stress to the change in the axial (overburden) stress. Tests are conducted to various stages of failure and include measurement of strain and acoustic emission. The micromechanical failure process are further elucidated and characterized quantitatively using light microscopy, laser scanning confocal microscopy, and scanning electron microscopy. Work focuses on porous carbonate and siliciclastic rocks, although related experiments are also being performed on low-porosity crystalline rocks in order to study completely the effect of certain parameters. The results of the laboratory tests and microstructural studies are used to guide analyses using fracture mechanics and continuum plasticity theories.
Results: (1) The high resolution imaging technique developed previously under this project can be more fully exploited by extracting quantitative statistical descriptions of the microgeometry from the volumetric image data (e. g., systematic studies of damage evolution with stress, or, correlation of pore geometry with permeability). To this end, a 3dma code that extracts quantitative statistical geometric descriptions from synchrotron microtomographic data was modified to support volumetric data acquired using laser scanning confocal microscopy; volumetric data sets were processed to investigate detailed aspects of the image processing and binarization, and to verify the robustness of the extracted microgeometric statistical descriptions. (2) Analysis of the experimental test series conducted on Westerly granite with differing initial damage states is being pursued. (3) Quantitative microstructural analyses of samples deformed in a triaxial test series designed to investigate the effect of ductile grain-boundary lining second phases on brittle failure in compression is also in progress. The experimental investigation included tests to axial strains of ~10 % at confining pressures up to 600 MPa and encompassed conditions at which the macroscopic failure mode changes from unstable work softening behavior to stable work hardening behavior. Three-dimensional imaging using laser scanning confocal microscopy is being performed to elucidate the micromechanics and to statistically characterize the damage structure.
D. Resolution and Accuracy of 3D Electromagnetic Imaging (G. A. Newman [505-844-8158; Fax 505-844-7354; E-mail ganewma@sandia.gov] and D. L. Alumbaugh)
Objectives: Non-linear electromagnetic inversion is an important tool for subsurface imaging. The objective of this research is to develop and analyze techniques for quantifying the resolution of, and appraising the accuracy of images produced by 2D and 3D electromagnetic inversion schemes, and to apply these techniques to field data.
Project Description: Non-linear electromagnetic inversion for 2D and 3D subsurface imaging of electromagnetic properties has rapidly evolved over the last decade due to its potential benefit in the areas of contaminant waste site characterization, oil and mineral exploration and delineation, and ground water resource evaluation. However, before we can proceed on inverting field data and interpreting the resulting images with any level of confidence, methods of appraisal and error analysis must be developed and tested. The purpose of this project is to examine established methods for this purpose such as calculating the model covariance and model resolution matrices through direct matrix inversion using Bakus-Gilbert theory , and examine procedures that estimate these resolution parameters without direct matrix inversion; the latter is required when iterative techniques such as Conjugate Gradients are employed within the inversion scheme. Finally, the resulting inversion schemes and methods of appraisal will be applied to data sets deemed to be of sufficient quality as they become available.
Results: An iterative 2D non-linear inversion scheme has been implemented using the same general structure as employed in the 3D scheme except that there are options to use direct matrix solution techniques as well as the iterative Conjugate Gradient (CG) technique to arrive at a solution; the 3D scheme employs only the CG method. The inclusion of both the direct and iterative techniques allows researchers to ascertain the accuracy of the Monte Carlo/Spectral Lanczos techniques that will bee employed to estimate the model resolution and covariance matrices.
Direct calculations of these matrices show that the two most useful attributes are the diagonals of the covariance matrix to ascertain parameter uncertainty and the individual rows of the resolution matrix to resolve parameter averaging (image smoothing). Initial attempts at generating 2D and 3D images of data collected over the INEL Cold Test Pit with the Apex Parametrics MaxMin I-8S system have proven to be disappointing relative to image quality and computational time. The data may not be of high enough quality to resolve pertinent features that investigators require. Because of the lack of high quality frequency domain EM data required by existing schemes, methods have been implemented to image 3D magnetotelluric data and are currently being tested. Both linearized and non-linear CG methods are being examined, with the latter appearing to be more computationally efficient.
CATEGORY: Geochemistry
PERSON IN CHARGE: H. R. Westrich
A. Cation Diffusion Rates in Selected Minerals (R.T. Cygan [505-844-7216; Fax 505-844-7216; E-mail rtcygan@sandia.gov], D. K. Fisler, and H.R. Westrich)
Objectives: Determine experimental cation diffusion coefficients for enstatite and carbonate minerals at temperatures less than 1000°C for evaluating disequilibrium behavior in geological, nuclear waste, energy, and materials concerns.
Project Description: Evaluation and modeling of geochemical processes related to nuclear waste, energy, and materials problems will require the accurate determination of cation diffusion data in silicate minerals and carbonates. A new technique for the preparation of diffusion couples using thin film technology was developed in an effort to evaluate the relatively slow diffusion of Mg2+ and Ca2+ in phases such as pyroxene and calcite where diffusion rates are on the order of 10-22 to 10-16 m2/s in the temperature range of 700 to 1000°C. Resistive evaporation of enriched stable isotopes onto polished mineral surfaces is used to create a thin film-mineral diffusion couple, which were annealed in a controlled oxygen fugacity furnace for periods up to three months in order to provide a diffusive penetration depth of approximately 0.2 microns. Depth profiles of the tracer isotope are obtained using an ion microprobe and are then fit to appropriate diffusion models to obtain precise and reproducible diffusion coefficients.
Results: Diffusion rates of magnesium and calcium in calcite have been determined at 550 to 700oC. These results have been applied to obtain the cooling history of the Allan Hills meteorite ALH84001 (possible fossil evidence for life on Mars). The preservation of zoning in carbonates within the meteorite is possible only with either slow cooling rates or a low temperature formation of the carbonates. The diffusion data have been used to calculate grain size-temperature-cooling rate relations. The results indicate that proposals for high temperature formation of the carbonates (greater than 800oC) require cooling rates faster than 1o/my. Any models incorporating a low temperature formation of these carbonates are consistent with preservation of the zoning based on the diffusion data. The experimental work has been complemented by atomistic simulations of calcium self-diffusion in calcite. Lattice energy, defect formation energies, and activation energy for a cation vacancy migration have been calculated from these models, and demonstrate the mechanism and direction of "hopping" of cations in the calcite structure (see Figure). The rigid ion model does not incorporate electronic polarization, however, it shows that ionic polarization (i.e., relaxation of atomic sites in the vicinity of a cation vacancy) is an important contribution to the energy for the migration of cations.
B. Organic Anion-Mineral Surface Interactions During Diagenesis (P. V. Brady [505-844-7146; Fax 505-844-7354; E-mail pvbrady@sandia.gov], R. T. Cygan, H. R. Westrich, and S. D. Balsley)
Objectives: Identification of mechanism(s) of organic acid control of dissolution and growth of silicates, and linkage of surface chemistry to global processes.
Project Description: Rates are measured in the laboratory as a function of mineral, temperature, organic acid levels, pH and time, and molecular modeling is done to constrain reaction stoichiometries.
Results: Experimental work on kaolinite, albite, enstatite, and Al2O3, and modeling work on kaolinite and enstatite are done. Important conclusions from our work are:
1. Weathering of seafloor basalts is an important control over global climate,
2. Sulfate, at high enough levels, accelerates dolomite growth,
3. Al sites on kaolinite are inordinately acidic,
4. The activation energy of volcano weathering in Hawaii is ~26 kcal/mol,
5. Organic acids probably don't have a big effect on weathering.
C. Heterogeneous Nucleation and Growth Kinetics of Clays (K. L. Nagy [505-844-5337; Fax 505-844-7354; E-mail klnagy@sandia.gov], and R. T. Cygan)
Objectives: Clay nucleation occurs heterogeneously and growth may occur epitaxially on detrital minerals. However, the role of nucleating substrates on the kinetics is unknown. Our goal is to quantify clay growth on various substrates, to assess "reactive" surface area, and to provide kinetic data for accurate modeling of weathering and diagenesis.
Project Description: Experiments are performed in which the growth surface (single or powdered crystals) is characterized before and after reaction by various techniques used by the surface physics community to investigate thin films. These include Tapping Mode Atomic Force Microscopy (TMAFM), ion beam analyses, rotating anode and synchrotron X-ray reflectivity and X-ray diffraction (XRD). Comparison is made with results from standard powder experiments in which rates are quantified from solution chemistry changes. Clay growth occurs under controlled solution composition and temperature conditions that mimic nature. Nucleation site densities and nucleated crystal morphologies are monitored to acquire information on reactive surface areas, a parameter considered typically by bulk measurements such as gas adsorption. Molecular modeling of the bonding interactions between substrate and overgrowth as well as overgrowth morphology provides a fundamental basis for interpreting the experimental results.
Results: The di-octahedral sheet of many clays is gibbsite-like in structure and composition. Therefore, on these clays gibbsite should nucleate and grow epitaxially. Gibbsite growth rates were measured in stirred-flow reactors at 80°C, pH 3 on powdered kaolinite and single crystal muscovite using two new techniques. In the first, gibbsite mass formed on kaolinite was quantified using rotating anode X-ray diffraction, and agreed well with that calculated from Al mass balance between inlet and outlet solutions. In the second, gibbsite mass was determined from TMAFM images by integrating the crystallite volume above the muscovite surface. Seven rates from two samples run at the same supersaturation but for different times agreed within the error typical for stirred-flow experiments. Compared to published rates for gibbsite growth on gibbsite powders, the new rates showed the same linear dependence on Gibbs free energy of reaction. Evidence for epitaxy was determined from TMAFM images as elongated gibbsite crystals oriented according to the distorted Si/Al tetrahedral sheet of muscovite. An important implication for reactive-transport modelers is that nucleation and growth of sheet-structured phases must take into account the role of epitaxy: reactive surface area for growth is not limited to the selfsame phases.
CATEGORY: Hydrology
PERSON IN CHARGE: H. R. Westrich
A. Two-Phase Immiscible Fluid Flow in Fractured Rock: The Physics of Two-Phase Flow Processes in Single Fractures (R. J. Glass [505-848-0556; Fax 505-848-0558; E-mail rjglass@nwer.sandia.gov], H. Rajaram [University of Colorado, Boulder], and M. J. Nicholl [Oklahoma State University])
Objectives: Develop a quantitative understanding of the critical processes controlling two-phase flow and transport in fractures based on detailed physical experiments and high resolution numerical simulations. This understanding may subsequently be abstracted for use in conceptual models applied at large-scale to applied problems in petroleum extraction and the isolation of hazardous or radioactive waste.
Project Description: Under two-phase, immiscible fluid-flow conditions, phase-geometry within the fracture (i.e., geometry saturated with each phase) ultimately controls: the permeability to each phase, fluid pressure/saturation relations, and solute dispersion within each phase. Phase geometry is a function of both the aperture field and the two-phase flow processes themselves. Phase "fingering", separate from the single phase concept of flow channelization, occurs where capillary fingering, gravity-driven fingering, or viscous fingering is dominant, emphasizing the importance of understanding the two-phase displacement processes. Systematic physical experimentation is coupled with concurrent numerical simulation to explore the interplay between capillary, gravitational, and viscous forces in the control of phase structure, and thus flow and transport in rough walled fractures. Understanding of capillary, viscous, and gravity fingering is explored in order to identify mechanisms that may be used to damp or enhance fingered flow. In addition, a conceptual understanding is developed for the phase invasion processes controlling replacement of a fully entrapped, immobile phase as it dissolves into the flowing phase.
Results: High resolution (time, space, concentration, aperture) full-field light transmission techniques to make such measurements in transparent fractures constructed out of analog materials have been developed and evaluated. Aperture and solute concentration fields are being used to test and develop capabilities for large scale modeling of single phase flow, solute transport (diffusive to advective dominated) and phase dissolution in fractures containing entrapped phase structures. We incorporated a conceptual model for calculating interface curvature into a Modified Invasion Percolation (MIP) solver. A dimensionless parameter that weighs the relative importance of aperture induced curvature (between the two fracture surfaces perpendicular to the plane of the fracture) and in-plane curvature (interfacial curvature in the plane of the fracture) arises and adds rich structural behavior to the phase invasion process. We have compared simulation results to all data available from past experiments in our lab and find MIP to simulate our experiments very well. Experiments were designed that fully test the model under conditions of small capillary number (ratio of viscous forces to capillary forces) systematically varying the bond numbers (ratio of gravity to capillary forces). We explored the idea of upscaling MIP to fracture networks for evaluating deep and rapid movement of water and contaminants in fractured rock formations, and find MIP to predict gravity-driven flow features within fracture networks.
B. Laboratory Investigation of Constitutive Property Upscaling (Vincent C. Tidwell [505-848-0574; Fax 505-848-0558; E-mail vctidwe@sandia.gov] and John L. Wilson [New Mexico Institute of Mining and Technology])
Objectives: The objectives of this research program are: (1) physically investigate permeability upscaling for a variety of geologic materials, (2) correlate differences in measured permeability upscaling with specific characteristics of the porous medium, instrument, and sampling strategy, and (3) use the acquired information to test theoretical upscaling models and explore alternative measures and models of permeability upscaling.
Project Description: Physical investigation of permeability upscaling is accomplished by means of a specially adapted minipermeameter, which we have termed the multi-support permeameter (MSP). The MSP allows rapid, precise, non-destructive measurement of permeability over a range of discrete sample supports (i.e., sample volumes). By varying the size of the minipermeameter tip seal measurements spanning five orders of magnitude on a per volume basis are made subject to consistent boundary conditions and flow geometry. Thousands of measurements on multiple faces of meter-scale blocks of rock are collected with each of five different tip seals (0.31 - 5.08 cm ID) plus a single large-scale (15.24 cm ID) measurement designed to integrate over the entire sampling domain. This process is repeated on multiple rock samples, each of which exhibit different textural and structural characteristics. To quantify permeability upscaling, key summary statistics are calculated from the acquired data sets and analyzed with reference to their corresponding sample support.
Results: To date, spatially-exhaustive, multi-support permeability data sets have been acquired from three different blocks of rock, a strongly cross-bedded Massillon Sandstone, a weakly laminated Berea Sandstone, and a volcanic Topopah Spring Tuff. Summary statistics, semivariograms, indicator semivariograms, spectral densities, fractal dimensions, etc. have been calculated for each sample support, rock face, and rock sample. Distinct trends relating these key statistical measures to sample support have been identified and quantified. Comparisons are drawn between the measured trends and that predicted by a series of theoretical models which differ according to their fundamental assumptions concerning the spatial structure of the underlying permeability field. Linear filter analysis of the multi-support permeability data is employed to characterize what is "measured" by the minipermeameter. Calculated filter functions, which quantify how heterogeneities populating the sample support are spatially weighted, exhibit an approximate Gaussian structure. This spatial weighting patter is consistent with the divergent flow field imparted by the minipermeameter. The shape of the filter function is also seen to be closely related to the size of the inner tip seal radius. These initial results also suggest that the heterogeneous structure of the porous medium influences the shape (i.e., spread and symmetry) of the filter function.
C. Multicomponent Convection in Porous Media and Fractures (R. J. Glass [505-848-0556; Fax 505-848-0558; E-mail rjglass@nwer.sandia.gov], H. W. Stockman, and S. W. Tyler [Desert Research Institute])
Objectives: This research seeks to understand the physical and chemical processes controlling multicomponent convection in natural porous and fractured media and to develop quantitative relationships between system parameters (solute concentrations, permeability and system geometry) and convective transport. From experimental and numerical relationships, estimates of the magnitude of convective mass transport will be developed.
Project Description: We combine high resolution, full field light transmission techniques and concurrent numerical simulation to study the onset and development of convection in simulated porous media (Hele-Shaw cells) and fractures. The light transmission technique allows quantitative measurement of the solute concentrations which can be extrapolated to develop the mass flux of components as they develop in time. The numerical modeling efforts focus on the use of lattice gas automata techniques to capture the fine scale structure of the onset and development of convection. The stability of a stratified solution in a porous medium has been theoretically shown to be a function of the solute concentrations, solute diffusivities and the medium's permeability. However the theoretical stability field had not been experimentally verified. Confirmation of the theoretical stability was crucial to determine under what hydrologic conditions would double diffusive convection occur. Experiments have been conducted to verify the theoretical onset of double diffusive convection in porous media as a function of permeability, solute concentration and solution properties.
Results: The results demonstrated that convective stability could be estimated using a simple inequality based on the buoyancy ratio, Rp(ratio of the contribution to overall fluid density of each solute) and the ratio of the solute diffusivities, t(0<t<1). For many solutes found in natural porous media, double diffusive convection will occur when Rp<t-1. This implies that the hydrodynamic stability is controlled by the initial solute concentrations and that only order-of-magnitude estimates of aquifer permeability may be needed. We have also investigated the rates of mass flux which occur after the initial instability develops. High resolution light transmission digital images were obtained throughout a series of experiments using sucrose and sodium chloride as solutes, using a dye to track the mass flux as each Hele-Shaw experiment. Initial conditions were varied from nearly stable to highly unstable. Analysis of the Hele-Shaw cell experiments has been initiated using a LB model. For the experiments considered, we were able to match the structural results of the flow field reasonably well however the time scale was off up to about a factor of 4. (See http://www.sandia.gov/eesector /gs/gc/movies.htm). We believe that this is most likely due to the Taylor-Aris dispersion effect which so far is not incorporated in the LB modeling.
Figure 1. Dimensionless NaCl flux (filled circles) across double diffusive experiment as a function of time from onset of convection. For comparison, the corresponding diffusive flux is shown by the solid line. The convective flux reaches a steady rate after approximately 100 minutes and is approximately seven times the diffusive flux at that time.
D. Continuum and Particle Level Modeling of Concentrated Suspension Flows (L. Mondy [505-844-1755; Fax 505-844-8251; E-mail lamondy@sandia.gov], A. Graham [Los Alamos National Laboratory], M. Ingber [University of New Mexico])
Objectives: The purpose of this program is to combine experiments, computations, and theory to make fundamental advances in our ability to predict transport phenomena in concentrated, multiphase, disperse systems, particularly when flowing through geologic media.
Project Description: The proposed research will elucidate the underlying physical principles that govern concentrated multiphase systems in areas essential to continued progress in geosciences. Continuum-level modeling of the flow of suspensions has recently been shown to be capable of accurately analyzing a variety of laboratory-controlled experiments. However, in order to be of use in real world applications, significant enhancements to currently available models will be required. We will use both experimentation and high performance computing to obtain microstructural information that is necessary to the development and refinement of the continuum models. For example, we expect to use this microstructural information to gain insight into the physics of particle bridge formation and collapse and particle sedimentation, which are particularly important in sand control issues found in petroleum production. Further, we expect that continuum-level modeling could eventually be directly implemented in codes currently used to predict hydraulic fracturing operations in the petroleum industry. The understanding gained about the physics of multiphase flows will, however, have much broader application in geosciences.
Results: This project has only recently been initiated.