Publications

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Nanogeochemistry-a newly emerging research field-attempts to understand geochemical reactions and mass transfers at nanometer scales, especially with regards to the formation of nanostructures in geochemical systems, emergent properties of these structures, and their controls on geochemical processes. The research also includes use of nanotechnology to design new materials and engineering approaches for effective natural resource extraction and environmental management. At the core of this new research field is the concept that, as the size of a material is reduced to nanometers, novel physical or chemical properties of the material may emerge that can be drastically different from those of the corresponding bulk phase and the material properties then become size-dependent. Nanostructures, which frequently occur in geologic materials, may directly control mineral phase stability, mineral-water interface chemistry, geochemical reaction kinetics, geo-fluid migration and transport, and even global biogeochemical cycles as a whole. This paper aims to provide a comprehensive review of recent progress in nanogeochemical research. The review is focused on two general types of nanostructures-nano solid phases and nanopores (nanofluids)-with an emphasis on the occurrence of each nanostructure in natural environments, the associated emergent properties, and the potential geochemical implications. Stemming from an increasing interest in shale gas research, a special discussion is provided on gas/oil disposition and migration in unconventional low-permeability reservoirs, wherein shale is treated as a nanocomposite material. Nanogeochemistry is a relatively young research field, and much remains to be explored. There is an urgent need for systematically characterizing specific nanostructures over the whole nanometer-size range and developing a general theoretical framework for data analysis and synthesis. There is also a need for developing experimental and modeling techniques to extrapolate the knowledge obtained from simple model systems to complex natural systems. © 2014 Elsevier B.V.

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The chemical potential of water may play an important role in adsorption and capillary condensation of water under multiphase conditions at geologic CO2 storage sites. Injection of large volumes of anhydrous CO 2 will result in changing values of the chemical potential of water in the supercritical CO2 phase. We hypothesize that the chemical potential will at first reflect the low concentration of dissolved water in the dry CO2. As formation water dissolves into and is transported by the CO2 phase, the chemical potential of water will increase. We present a pore-scale model of the CO2-water interface or menisci configuration based on the augmented Young-Laplace equation, which combines adsorption on flat surfaces and capillary condensation in wedge-shaped pores as a function of chemical potential of water. The results suggest that, at a given chemical potential for triangular and square pores, liquid water saturation will be less in the CO2-water system under potential CO2 sequestration conditions relative to the air-water vadose zone system. The difference derives from lower surface tension of the CO2-water system and thinner liquid water films, important at pore sizes <1 × 10 -6 m, relative to the air-water system. Water movement due to capillary effects will likely be minimal in reservoir rocks, but still may be important in finer grained, clayey caprocks, where very small pores may retain water and draw water back into the system via adsorption and capillary condensation, if dry-out and then rewetting were to occur. © 2014. American Geophysical Union. All Rights Reserved.

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The use of integrated probabilistic risk assessment tools to evaluate the barrier capability of a natural barrier system (NBS) in a nuclear waste repository is presented. The integrated risk assessment tools encompass highly detailed process models for flow and transport, probabilistic performance assessment (PA), and database management. In this paper development of an integrated tool for the modeling of far-field radionuclide transport in a generic salt repository is discussed. The tool was developed by wrapping the flow and transport reservoir simulator (FEHM) with the uncertainty quantification and optimization code (DAKOTA).

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In the supercritical CO2-water-mineral systems relevant to subsurface CO2 sequestration, interfacial processes at the supercritical fluid-mineral interface will strongly affect core- and reservoir-scale hydrologic properties. Experimental and theoretical studies have shown that water films will form on mineral surfaces in supercritical CO2, but will be thinner than those that form in vadose zone environments at any given matric potential. The theoretical model presented here allows assessment of water saturation as a function of matric potential, a critical step for evaluating relative permeabilities the CO2 sequestration environment. The experimental water adsorption studies, using Quartz Crystal Microbalance and Fourier Transform Infrared Spectroscopy methods, confirm the major conclusions of the adsorption/condensation model. Additional data provided by the FTIR study is that CO2 intercalation into clays, if it occurs, does not involve carbonate or bicarbonate formation, or significant restriction of CO2 mobility. We have shown that the water film that forms in supercritical CO2 is reactive with common rock-forming minerals, including albite, orthoclase, labradorite, and muscovite. The experimental data indicate that reactivity is a function of water film thickness; at an activity of water of 0.9, the greatest extent of reaction in scCO2 occurred in areas (step edges, surface pits) where capillary condensation thickened the water films. This suggests that dissolution/precipitation reactions may occur preferentially in small pores and pore throats, where it may have a disproportionately large effect on rock hydrologic properties. Finally, a theoretical model is presented here that describes the formation and movement of CO2 ganglia in porous media, allowing assessment of the effect of pore size and structural heterogeneity on capillary trapping efficiency. The model results also suggest possible engineering approaches for optimizing trapping capacity and for monitoring ganglion formation in the subsurface.

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