A Model for the Calculation of the Bulk Thermodynamic Properties of Geothermal Fluids

Abstract

Models have been developed for predicting the bulk thermodynamic properties of complex brines found in geothermal systems The vapor pressure may be calculated from the boiling and freezing points of the brine, and the density may be calculated from the sum of the densities of the binary components. These calculated parameters can be used to erect a P-V-T-X grid from which the thermodynamic properties may be extracted. If the brine has a freezing point, boiling point, and density similar to a solution with known thermodynamic properties, then the thermodynamic properties of the reference solution can be used to describe the properties of the brine. NaC1 solutions are thermodynamically well known in the P-T range of geothermal interest. Furthermore, NaC1 is one of the dominant constituents of most geothermal fluids. Experimental studies confirm that most synthetic brines in the system Na-K-Ca-Mg-C1-SO4-Br, the boiling point, freezing point, and densities are equal to those of NaC1 reference solutions. From the freezing point of the brine, the equivalent NaC1 concentration can be determined and the thermodynamic properties of the geothermal brine calculated.