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

PANEL REPORT: MODELING AND SIMULATION OF GEOLOGIC SYSTEMScolloids, the catalytic biochemistry of signal transmission in eukaryote cells, and the predictionof the chemistry that occurs under extreme temperature and pressure conditions which may beexperienced in the operation of advanced nuclear power systems.The complexity of the processes of interest (e.g., bond breaking and formation, changes in bondvalence and polarization) requires reliable, predictive simulation methods based on firstprinciples (i.e., the electronic many-body Schrödinger equation). The problem is made moredifficult by the amorphous structure of many species of interest (e.g., solid solutions), thusrequiring treatment of many atoms to provide a complete description of chemical processes. Abinitio calculation of chemical equilibria, speciation and reactions poses a grand challengecomputational problem.Equation-of-State (EOS) models and data. Regardless of the state of understanding andcalculations at the atomic level, to translate these advances into useful applications at themacroscopic level, efficient equations must be available that succinctly summarize experimentaland computational data for use on the macroscopic level (e.g., a rate equation or more commonlyan Equation of State, or EOS). The free energy summarizes all the thermodynamics of thesystem and usually is chosen as the basis for EOS development. The development of EOSrepresentations is dependent on the availability of a succinct representation of thethermodynamics of the system that can capture the variation of the free energy for a wide rangeof intensive variables. There has been considerable research effort in this area. However, thereare very few EOS that represent thermodynamic behavior with the accuracy that is required forproblems such as nuclear waste disposal and CO 2 sequestration. It is likely that the requiredaccuracy may be achieved from models that are tailored to describe specific states (e.g., solids,liquids, gases, polyion colloids, solid solutions), rather than from a quest for a single EOSrepresentation that will describe all physical states.EOS for compressible mixtures. For systems composed of water and gases, the most convenientvariables are usually the temperature, volume (or density) and the composition. The appropriatethermodynamic function on which to base an EOS is then the molar Helmholtz free energy. Allother properties (e.g., the enthalpy) may be derived from this function by the appropriatederivatives. To provide optimal interpolation and extrapolation of mixing properties, thefunctional form of the free energy should be based on a reasonably accurate molecular-leveldescription of the system. The thermodynamic perturbation theory originally introduced by Pople(1954) provides a framework from which such an EOS may be generated. To provide the highlevel of accuracy necessary for quantitative description of thermodynamic data, empiricalcorrections must usually be added to the EOS. This type of approach was successfully applied tothe analysis of mixtures of polar and nonpolar fluids, including soluble solids that maydissociate.Geochemistry in the absence of an aqueous phase. Injection of CO 2 into aqueous environmentswill not only give rise to CO 2 dissolution in the aqueous phase, but also induce partitioning ofwater into the gas (CO 2 -rich) phase. Over time this will produce a complete dry-out of the regionaround the injection well, with potentially important ramifications for chemical interactionsbetween CO 2 and in situ mineral assemblages that may affect important practical aspects, such asthe mechanical stability of the injection well. Currently there is little knowledge of rock-fluidinteractions in the absence of an aqueous phase. Experimental and modeling studies of CO 2 -rockBasic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 59