Basic Research Needs for Geosciences - Energetics Meetings and ...

CROSSCUTTING ISSUE:HIGHLY REACTIVE SUBSURFACE MATERIALS AND ENVIRONMENTSquantify rate-limiting mechanisms of transient reactions and the cumulative effects of compositereactions.The impact of perturbations on chemical migration may be much greater in fluids located in tightgrain boundaries or within single mineral grains compared to fluids flowing through open cracks.Nanoscale confinement will alter the physiochemical properties of complex fluids, and highlylocalizedfluctuations in chemical conditions may occur due to leaching of mineral surfaces,nucleation and growth of new solids, and introduction of exotic elements from the repository.Recent advances in chemical imaging capabilities offer excellent opportunities for a fresh look atmineral-solution reactions in confined spaces. These tools include neutron, synchrotron X-ray,NMR, and other probes of surface reactions at the atomic scale, along with tomographic methodsfor in situ characterization of porosity/permeability and chemical/mineralogical heterogeneitywithin dense geologic media from either laboratory experiments and or natural systems (Fenteret al. 2002; Wenk 2006).It is probable that the equilibrium state between a fluid in a nanosized pore and the confiningsolid phases is significantly different from that between the bulk fluid and minerals at the samesolution composition, temperature and pressure, due to the effects of confinement onthermodynamic properties. It is also likely that reaction mechanisms will change when thestructure and solvating properties of fluids change under confinement. The influence ofconfinement on the structure and dynamics of geofluids is now starting to receive more attention(e.g., Sposito et al. 1999; Wang et al. 2004a, b 2006; Mamontov and Cole 2006). Many factorsdictate how fluids, and with them reactants and products of intrapore transformations, migratethrough nanoenvironments, wet, and ultimately adsorb and react with the solid surfaces. Theseinclude the size, shape, distribution and interconnectedness of the confined spaces, andcomposition and physical properties of the solids and fluids (Cole et al. 2004, 2006). Thedynamical behavior of fluids and gases in confined spaces is controlled by processes occurring atthe interface with solids. A fluid can exhibit unexpected confinement-promoted phasetransitions, including freezing, boiling or condensation, immiscibility, and other phenomenawhich are intrinsic to the fluid-confining surface interactions (such as layering and wetting) (e.g.,Gelb et al. 1999; Melnichenko et al. 2006).RESEARCH APPROACHESA combination of unique and novel experimental and analytical techniques coupled withtheoretical and modeling approaches is required to gain a fundamental understanding of thestructures, dynamics, and reactivity of geologically representative fluids. Complexintermolecular interactions of C-O-H-N-S fluids (H 2 O, CO 2 , H 2, H 2 S, N 2 , CH 4 ) result in theirunique thermophysical properties, including large deviations in the volumetric properties fromideality, vapor-liquid equilibria, and critical phenomena. The scaling from molecular tomacroscopic properties of these fluids can be obtained by first making accurate determinations ofthese properties over wide ranges of temperature, density and composition, using novel methodssuch as densimetry (Blencoe et al. 1996) or synthetic fluid inclusions (Bodnar et al. 1985). Theseresults can be used construct more robust Equations of State and to validate results obtained byab initio modeling and molecular dynamics simulations of binary, ternary and complex mixturesof simple molecules in the C-O-H-N-S system. Scattering and spectroscopic methods can beused to identify local clustering and short-range ordering that affect deviations from ideal mixingBasic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 161