High Temperature and Pressure Equation of State Models for Natural Fluids in the System NaCl-KCl-CaCl2-H2O-CO2-CH4

University Of California-San Diego, La Jolla CA

Investigators

Abstract

Weare and Moller EAR-0126331 This research program will contribute the development of an equation of state (EOS), which can be used to accurately predict thermodynamic properties (e.g., liquid-vapor phase coexistence, enthalpy and free energy) in the NaCl-KCl-CaCl2-H2O-CO2-CH4 (SWG) system over the wide PTX ranges associated with Earth processes. Many important geochemical processes, such as mineral deposition, metamorphism and chemical fractionation via phase separation, are controlled by the thermodymanic behavior of aqueous formation fluids with compositions approximately in this system. Unfortunately, most experimental data available for development of an EOS for this system are confined to much smaller PTX ranges than those encountered in Nature. To overcome these important limitations in data availability, our research approach will use recent advances in the theory of dense fluids and molecular simulation methods to: (1) support the construction of an EOS that not only correctly summarizes data but also reliably extrapolates to desired regions of PTX space (e.g., deep crustal and magma conditions); and (2) supplement the experimental data by direct simulation at the molecular level. The proposed EOS will be based on thermodynamic perturbation theory. In this theoretical method the free energy is written as a sum of contributions from an ideal reference system and from a perturbation correction. In the proposed research program we will concentrate on developing reference systems that optimally represent the behavior of the subsystem under study. Our objective is to lower the complexity and magnitude of the perturbation corrections needed to represent the measured behavior in order to maximize the extrapolation properties of the EOS. Molecular dynamics simulations of model systems will provide useful tools for testing reference system improvements and mixing rules. Special functional behavior (scaling behavior) is required to correctly describe thermodynamic properties in the critical region. We will apply scaling methods to improve critical region predictions and examine how scaling can be generalized to calculate other thermodynamic properties, such as heat content and free energy. In addition, we will develop crossover EOS, which provide accurate predictions both in and away from the critical region. To reproduce thermodynamic properties via simulation at the molecular level, we will continue to develop simulation methods [e.g., first principles ab-initio (AIMD) and classical molecular dynamics (MD), Gibbs Ensemble Monte Carlo (GEMC)]. A main objective of the proposed research is to significantly improve the accuracy of ionic solution simulations by developing better intermolecular effective potential representations of ions in solution. We will test new methods to improve the performance of GEMC methods and prediction in the critical region. AIMD methods will be applied to guide intermolecular potential development and to study important effects on these interactions of system properties such as local polarization. These methods will be used to establish the forces for systems with little data, such as ion-neutral interactions.