Publications

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Assessing the impact of parameter estimation accuracy in models of heterogeneous, three-dimensional (3-D) groundwater systems is critical for predictions of solute transport. A unique experimental data set provides concentration breakthrough curves (BTCs) measured at a 0.253 cm 3 scale over the 13 × 8 × 8 cm3 domain (∼53,000 measurement locations). Advective transport is used to match the first temporal moments of BTCs (or mean arrival times, m1) averaged at 0.253 and 1.0 cm3 scales through simultaneous inversion of highly parameterized heterogeneous hydraulic conductivity (K) and porosity (φ) fields. Pilot points parameterize the fields within eight layers of the 3-D medium, and estimations are completed with six different models of the K-φ relationship. Parameter estimation through advective transport shows accurate estimation of the observed m1 values. Results across the six different K-φ relationships have statistically similar fits to the observed m1 values and similar spatial estimates of m1 along the main flow direction. The resulting fields provide the basis for forward transport modeling of the advection-dispersion equation (ADE). Using the estimated K and φ fields demonstrates that advective transport coupled with inversion using dense spatial field parameterization provides an efficient surrogate for the ADE. These results indicate that there is not a single set of model parameters, or a single K-φ relationship, that leads to a best representation of the actual experimental sand packing pattern (i.e., nonuniqueness). Additionally, knowledge of the individual sand K and values along with their arrangement in the 3-D experiment does not reproduce the observed transport results at small scales. Small-scale variation in the packing and mixing of the sands causes large deviations from the expected transport results as highlighted in forward ADE simulations. Highly parameterized inverse estimation is able to identify those regions where variations in mixing and packing alter the expected property values and significantly improve results relative to the nave application of the experimentally derived property values. Impacts of the observation scale, the scale over which results are averaged and the number of observations and parameters on the final estimations are also examined. Results indicate existence of a representative element volume (REV) at 0.25 cm3, the existence of subgrid scale heterogeneity that impacts transport and the accuracy of highly parameterized models with even relatively small amounts of observations. Finally, this work suggests that local-heterogeneity features below the REV scale are difficult to incorporate into parameterized models, highlighting the importance of addressing prediction uncertainty for small-scale variability (i.e., uncaptured variability) in modeling practice. © 2012. American Geophysical Union. All Rights Reserved.

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We develop a 2-D pore scale model of coupled fluid flow, reactive transport, and calcium carbonate (CaCO 3) precipitation and dissolution. The model is used to simulate transient experimental results of CaCO 3 precipitation and dissolution under supersaturated conditions in a microfluidic pore network (i.e., micromodel) in order to improve understanding of coupled reactive transport systems perturbed by geological CO 2 injection. In the micromodel, precipitation is induced by transverse mixing along the centerline in pore bodies. The reactive transport model includes the impact of pH upon carbonate speciation and a CaCO 3 reaction rate constant, the effect of changing reactive surface area upon the reaction, and the impact of pore blockage from CaCO 3 precipitation on diffusion and flow. Overall, the pore scale model qualitatively captured the precipitate morphology, precipitation rate, and maximum precipitation area using parameter values from the literature. In particular, we found that proper estimation of the effective diffusion coefficient (D eff) and the reactive surface area is necessary to adequately simulate precipitation and dissolution rates. In order to match the initial phase of fast precipitation, it was necessary to consider the top and bottom of the micromodel as additional reactive surfaces. In order to match a later phase when dissolution occurred, it was necessary to increase the dissolution rate compared to the precipitation rate, but the simulated precipitate area was still higher than the experimental results after ∼30 min, highlighting the need for future study. The model presented here allows us to simulate and mechanistically evaluate precipitation and dissolution of CaCO 3 observed in a micromodel pore network. This study leads to improved understanding of the fundamental physicochemical processes of CaCO 3 precipitation and dissolution under far-from-equilibrium conditions. Copyright 2012 by the American Geophysical Union.

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This document summarizes research performed under the SNL LDRD entitled - Computational Mechanics for Geosystems Management to Support the Energy and Natural Resources Mission. The main accomplishment was development of a foundational SNL capability for computational thermal, chemical, fluid, and solid mechanics analysis of geosystems. The code was developed within the SNL Sierra software system. This report summarizes the capabilities of the simulation code and the supporting research and development conducted under this LDRD. The main goal of this project was the development of a foundational capability for coupled thermal, hydrological, mechanical, chemical (THMC) simulation of heterogeneous geosystems utilizing massively parallel processing. To solve these complex issues, this project integrated research in numerical mathematics and algorithms for chemically reactive multiphase systems with computer science research in adaptive coupled solution control and framework architecture. This report summarizes and demonstrates the capabilities that were developed together with the supporting research underlying the models. Key accomplishments are: (1) General capability for modeling nonisothermal, multiphase, multicomponent flow in heterogeneous porous geologic materials; (2) General capability to model multiphase reactive transport of species in heterogeneous porous media; (3) Constitutive models for describing real, general geomaterials under multiphase conditions utilizing laboratory data; (4) General capability to couple nonisothermal reactive flow with geomechanics (THMC); (5) Phase behavior thermodynamics for the CO2-H2O-NaCl system. General implementation enables modeling of other fluid mixtures. Adaptive look-up tables enable thermodynamic capability to other simulators; (6) Capability for statistical modeling of heterogeneity in geologic materials; and (7) Simulator utilizes unstructured grids on parallel processing computers.

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