Verification and Validation Studies of the Addition of Pitzer's Equations to the EQ3/6 Brine Model

摘要

Recent modifications to the EQ3/6 geochemical modeling software package provide a new option which can be used to compute the activity of water and the activity coefficients of solute species in both brines and dilute solutions. This option is based on equations proposed by Pitzer (1973) which allow approximation of mean molal activity coefficients (/gamma//sub /+-//) and osmotic coefficients (/phi/) up to high ionic strengths, and which, together with an appropriate ion-splitting convention, also afford calculation of individual ion activity coefficients (/gamma//sub i/) in brines. Values of /gamma//sub /+-// and /phi/ generated by EQ3/6 for binary and compositionally higher order systems compare favorably with their experimental and hand-calculated counterparts. The addition of Pitzer's equations to EQ3/6 as an optional method for calculating activity coefficients represents a significant improvement over the previous versions of the codes, which were limited to the use of a simple extended form of the Debye-Hueckel equation (the B-dot equation of Helgeson, 1969). Test runs using EQ3/6 to calculate solubility limits in simple binary and ternary systems (NaCl-H/sub 2/O, KCl-H/sub 2/O, and KCl-NaCl-H/sub 2/O) confirm the capability of the codes to accurately predict geochemical equilibria between brines and evaporite minerals in the sample systems. The test runs were sufficiently comprehensive to verify the accuracy of the calculational procedures and to partially validate the capability of the codes to dependably model the geochemical behavior of aqueous electrolyte solutions having ionic strengths as high as /approximately/6 m, subject to the availability of the requisite thermodynamic data and activity coefficient parameters. 24 refs., 6 figs., 8 tabs. (ERA citation 14:005914)