PVTx properties of H2O–H2S fluid mixtures at elevated temperature and pressure based on new experimental data

The volumetric properties of H2O–H2S fluid mixtures have been determined experimentally at temperatures of 150 to 400 °C and pressures up to 240 bar. Using these data and existing equations of state, we have developed a thermodynamic model for H2O–H2S fluid mixtures. This model is based on an asymmetric description of phases, which includes an activity model and a P–T-dependent Henry’s law constant for the liquid, and equations of state with mixing rules for the vapour. The fugacity of the vapour was calculated using the cubic equations of state of Peng and Robinson, 1976 and Patel and Teja, 1982 with density-dependent and composition-dependent mixing rules. Sets of binary interaction parameters for these equations were fitted to the experimental data obtained in this study supplemented by high-temperature PVTx data for H2O–H2S fluid mixtures reported in the literature. The Peng–Robinson equation used in conjunction with density-dependent mixing rules was found to be the most accurate of the available equations in representing the properties of the vapour phase. The errors in the pressure of the homogeneous vapour mixtures estimated using the above equations of state (relative deviation from the experimentally determined pressure) were comparatively low, ∼5% to 8%. However, the errors were significantly higher for the estimated pressure of vapour saturated with liquid, i.e., along the vapour–liquid phase boundary (11–15%), due to the polar nature of H2O and H2S and the resulting highly non-ideal behaviour of the fluid mixtures. The results of this study make it possible to reliably estimate the volumetric properties of aqueous fluids containing H2S at temperatures and pressures up to 400 °C and 240 bar, i.e., for conditions commonly encountered in natural hydrothermal systems.