A Model for Thermophysical Properties of Co2-brine Mixtures at Elevated Temperatures and Pressures

A mutual CO2-H2O solubility model was previously reported for application to CO2-enhanced geothermal systems. The ability of this model to predict PVT and caloric properties of the compressed gas phase is investigated. Compressibility factors of pure CO2 and CO2-H2O mixtures can generally be predicted within a few percent of reference and experimental data. Caloric data are also reasonably well reproduced for CO2-H2O gas mixtures at moderate water content. At elevated water contents, more significant deviations are observed between model results and published experimental enthalpies of mixing. However, total enthalpies may still be predicted with reasonable accuracy for applications to CO2-enhanced geothermal systems. Experimental data in the temperature and pressure range of most interest to CO2-enhanced geothermal systems are scarce and would be needed for further model validation. BACKGROUND AND OBJECTIVES Recent theoretical studies have stimulated interest in the potential of using CO2 instead of water as heat transfer fluid in enhanced geothermal systems (EGS) (Brown, 2000; Fouillac et al., 2004; Pruess, 2006, 2008). Evaluating the development and operation of an EGS with CO2 as a working fluid requires a capability to accurately represent the thermophysical properties of CO2-brine mixtures for the entire range of fluid compositions and thermodynamic conditions, from injection to production. Furthermore, in order to assess the behavior of CO2 in natural subsurface environments, and to evaluate potential leakage scenarios, thermophysical properties need to be represented all the way to the land surface. We have previously reported on the development of a phase partitioning model for CO2-brine mixtures that is based on thermodynamic equilibrium principles and a cubic equation of state (Spycher and Pruess, 2010). Our model represents the mutual solubilities of CO2 and NaCl brines largely within experimental uncertainties for temperatures of 12–300C, pressures of 1–600 bar and salinities from 0 to 6 molal NaCl. Here, we investigate the application of this model to the calculation of pressure-volume-temperature (PVT) data and caloric properties, with the objective of applying this model to CO2-EGS studies. MODELING APPROACH AND TESTING Original Solubility Model The solubility model was described in detail by Spycher and Pruess (2010) and references therein. Aspects of the model relevant to the present study are described here. Mutual solubilities are computed using a modified Redlich-Kwong (RK) equation of state (EOS) to compute the fugacity coefficients of CO2 and H2O in the compressed gas phase, coupled with equilibrium constants expressing the partitioning of CO2 in water. The effect of salts on CO2 solubility in saline solutions is also accounted for by a Pitzer activity coefficient model. The EOS parameters, equilibrium constants, and Pitzer interaction parameters were fitted to a large number of experimental mutual solubility data. Below 100°C, the amount of water partitioning into compressed CO2 is very small, which allows neglecting H2O when computing properties of the gas mixture. At temperatures above 100°C, however, a significant amount of water can vaporize in compressed CO2, such that this amount can no longer be ignored when computing thermophysical properties and phase equilibrium. In the present study, focus is given to temperatures above 100°C, as it is most relevant to EGS with CO2 as a working fluid. PVT and caloric properties predicted by the solubility model, without any modifications of its original EOS parameters, are presented below. PVT Properties Compressibility factors (Z=PV/RT) of pure CO2 and H2O were fitted to reference data as part of the original solubility model. For CO2, compressibility factors from Span and Wagner (1996) could be reproduced with a root mean square error (RMSE) about 0.5 % and no absolute deviations > ~2%. For H2O, compressibility factors from Wagner and Pruβ (2002) for the liquid phase could be reproduced with an RMSE < 0.5% and no absolute deviations > ~9%. However, as would be expected with any cubic EOS, the vapor phase data along the saturation curve and the saturation curve itself could not be accurately reproduced. Few PVT data have been reported for CO2-H2O mixtures in the P-T range of interest to CO2-EGS (100–300°C and 100–600 bar). This is in part because, at these temperatures and pressures, CO2H2O mixtures form two immiscible phases over most of the mixture compositional range. In addition, the water content at the dew-point of CO2-H2O mixtures below ~250°C and pressures above 100 bar is typically limited. As a result, experimental PVT properties are not only scarce, but also are mostly limited to pressures below 100 bar. We previously reported a relatively good agreement of model results with the compressed gas phase density data of Fenghour et al. (1996). Comparisons of model results with experimental density values were since extended to include data from Patel et al. (1987), Patel and Eubank (1988) and Zawisza and Maleslnska (1981). Computed compressibility factors show deviations mostly within a few percent from these experimental data sets (Figure 1).