[hal-01076995] Electrolyte CPA equation of state for very high temperature and pressure reservoir and basins applications

In this work an electrolyte version of the Cubic Plus Association eCPA equation of state has been adapted to systems containing CH4 CO2 H2O and NaCl up to 5 molal at pressures up to 200 MPa and temperatures up to 773 K for salt-free systems and 573 K for salt-containing systems Its purpose is to represent the phase behavior including salting-out effect and critical point and the phase densities in a range of temperature and pressure encountered in deep reservoirs and basins The goal of the parameterization proposed is not to reach a very high accuracy for phase equilibrium and volumetric properties but rather to develop a semi-predictive approach to model the phase and volumetric behavior of this system while allowing an easy extension to other compounds Without salt predictions for pure component vapor pressures and liquid molar volumes present an average absolute deviation AAD lower than 3% compared to experimental reference values The pure component molar volumes out of saturation show an AAD lower than 4% The highest deviations in densities are observed as expected in the vicinity of the critical coordinates of pure water and this effect increases when gases or salts are added to the system For each binary system CH4 + CO2 CH4 +H2O and CO2+H2O binary interaction parameters have been fitted to correctly represent the shape of the fluid phase envelopes including all critical points in the entire temperature and pressure range considered 219 K to 633 K and up to 250 MPa The methane concentration in both phases of the CH4 + CO2 binary system is represented with an AAD lower than 9% The methane solubility in water is represented within 16% and 8% for the methane content of the vapor The CO2 solubility in water is within 26% while the CO2 in the vapor phase shows an average deviation of 12% All molar volumes are represented with an AAD lower than 3% The few VLE experimental data for the CH4 + CO2+H2O ternary system are fairly well predicted with the model without extra parameter which confirm the ability of the eCPA equation of state to be extended to multi-component systems In the presence of salts gas + ion binary interaction parameters have been fitted and all phase equilibrium are qualitatively correctly described and more specifically the salting out effect The solubility of methane or CO2 in brines up to 5 molal is represented with an AAD of 33% in a large temperature and pressure range up to 673 K and 150 MPa It should be noticed that for high temperatures experimental data are relatively scarce and not always consistent No data exist for water content of the vapor phase in these conditions The new eCPA model can be easily extended to other components including ions to better represent real fluid behavior in very deep reservoir conditions



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