Hydrate phase equilibrium in the most important foundation for other hydrate, especially for marine environment condition. In this study, a three-phase liquid water-hydrate-vapor equilibrium conditions for methane hydrate in aqueous solutions of NaCl + CaCl2/MgCl2/SrCl2 mixture with a salinity of 34.5 in the temperature and pressure range of 283.65 – 288.85 K and 8.84 – 13.2 MPa have reported using a temperature search method. The ion categories, ion concentrations, temperature and pressure range were determined based on the situ conditions of haima cold seep. And the in situ concentrations of Mg2+, Ca2+, Sr2+ in haima cold seep were 1000 mg/L, 300 mg/L, 8 mg/L, respectively. Comparison of ions in the same ionic conditions with different ionic concentrations at the same salinity revealed that methane hydrate phase equilibrium in high salinity containing Ca2+, Mg2+ ion is mainly affected by salinity while the influence of cation concentration can be ignored, however, it is clearly observed that the inhibitory effect of Sr2+ on hydrate phase equilibrium increases with increasing cation concentration at a salinity of 34.5. This may be because Sr2+ has the smallest electron density and the smallest electrostatic force to interact with water molecules when intruding into the hydrogen bonding network within the water molecule, and has the weakest inhibitory effect on the hydrate. The inhibition effect of Sr2+ concentration on hydrate was more significant than Ca2+, Mg2+ at the same salinity.
Keywords gas hydrate, hydrate inhibitors, phase equilibrium, seawater ions