Thermodynamic Modeling Of The Vapor-liquid Equilibrium And Pvtx Properties Of The H₂S-H₂O And H₂S-H₂O-NaCl Fluid Mixtures

Sour natural gas reservoirs containing H₂S have been frequently reported,and the variations of H₂S solubility in the water-based drilling fluid can lead to the quick changes of annulus pressure.Recently,CO₂ capture and sequestration?CCS?has become a technological feasible method to reduce the amount of CO₂ emissions to atmosphere.Deep saline formations have been regarded as the best storage option for CCS for their high potential capacity,and the co-injection of both CO₂ and H₂S into deep saline formations will reduce the costs of CCS.Therefore,it is indispensable to know the vapor-liquid equilibrium?VLE?and pressure-volume-temperature-composi-tion?PVTx?properties of the H₂S-H₂O and H₂S-H₂O-NaCl system to predict the related geochemical behaviors and dynamic mechanics and assess the safety and cost benefit before the co-sequestration of CO₂ and H₂S in deep saline formations.In this work,an equation of state?EOS?explicit in Helmholtz free energy has been developed to calculate the VLE and PVTx properties of the H₂S-H₂O fluid mixture.This EOS,where five mixing parameters are used,is based on the highly accurate EOSs of pure H₂S and H₂O fluids,and contains a simple departure function.Compared to reliable experimental data available,the average absolute deviations of H₂S solubility in liquid phase,water content in vapor phase and liquid density of the H₂S-H₂O system are 3.88%,5.03%and 0.20%,respectively.Based on the EOS of the H₂S-H₂O system developed here and the Pitzer activity coefficient of H₂S in aqueous NaCl solution from previous study,the H₂S solubility in aqueous NaCl solution can be predicted with an average absolute deviation of 7.52%.The water content of vapor phase in the H₂S-H₂O-NaCl system can be reproduced within experimental uncertainties by a fugacity-activity method.The PVTx properties of the H₂S-H₂O-NaCl mixture can be predicted with the Helmholtz free energy EOSs of binary H₂S-H₂O and H₂O-NaCl fluid mixtures without using additional mixing parameters.The developed thermodynamic models can accurately reproduce the VLE and PVTx properties of the H₂S-H₂O and H₂S-H₂O-NaCl fluid mixtures covering a valid temperature-pressure-salinity range of 273.15-473.15 K,0-400 bar and 0-6molality of NaCl.