Computation of Gas-Liquid Equilibria in High-Saline Geothermal Fluids

Publications Rights Reserved, U.S. Government Publications Rights Reserved, U.S. Government This paper was prepared for the 1977 SPE-AIME International Symposium on Oilfield and Geothermal Chemistry, held in La Jolla, California, June 27–28, 1977. Permission to copy is restricted to an abstract of not more than 300 words, Illustrations may not be copied. The abstract should contain conspicuous acknowledgement of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal, provided agreement to give proper credit is made. Discussion of this paper provided agreement to give proper credit is made. Discussion of this paper is invited. Abstract The Lawrence Livermore Laboratory is actively engaged in the development of energy extraction processes for high-temperature, hypersaline brines, such as are found extensively in the Salton Sea Geothermal Field of California. A computer-assisted chemical modeling study of equilibria in flowing brine systems has been undertaken as part of the LLL program. Equilibria between liquid and volatile components of two-phase fluids have profound effects on such important phenomena profound effects on such important phenomena as corrosion, scale formation, environmental quality, and the economics of energy extraction. The principal reactions considered are those involving volatiles naturally present in the fluid, including NH3, H2S and CO2. A computer code is described which performs stepwise equilibrium computations at performs stepwise equilibrium computations at 1 deg. C intervals for a unit mass of fluid, between reservoir and reinjection temperatures. The data base used is the quality-temperature function for the brine, the chemical composition, and equilibrium constants for the solubility reactions of gases and dissociation of their aqueous species. Chemical changes that take place during cooling and expansion are illustrated by distribution diagrams. Although the computations were developed for application to the high-saline resource, they are not fluid or system-specific, and should be of general usefulness where a chemical composition and temperature history can be specified. Introduction The objective of this study was to construct a predictive model for a flowing, high salinity geothermal fluid. It is intended to provide a theoretical basis for understanding experimental results, optimizing process control, and predicting the effects of changes in brine chemistry. The properties to be estimated include the amount and composition of gas released, pH changes, and concentrations of scale-forming species. Such a system is chemically very complex and cannot be treated adequately by hand calculations. It should, in principle, be modeled with the help of large, general purpose geochemical computer codes, which purpose geochemical computer codes, which can simultaneously consider all the many coexisting equilibria, However, several considerations led to the writing of a relatively simple code for this specific modeling purpose. It is, in essence, a computer-assisted calculation of many of the more important individual equilibria.