Research Progress on the Effect of Salts on the Stability of Methane Hydrate

The thickness of the methane hydrate stable zone is mainly controlled by the stability of methane hydrate. The temperature, pressure, pore water salinity, and gas component affect the thickness of the hydrate stable zone. The formation of methane hydrate is closely related to the formation of water. Various salt ions in formation water (Cl^-, Na⁺, Mg²⁺, SO₄^2-, Ca²⁺, and transition metals Fe, Mn, Cu, Co, Ni) can affect the formation and decomposition conditions of natural gas hydrate. Therefore, studying the effect of salts on the stability of methane hydrates is helpful to better understand the reservoir-forming conditions of methane hydrate. The effects of chloride, sulfate and carbonate on the stability of methane hydrate are evaluated in this paper. With the increase of salinity, the phase equilibrium curve of methane hydrate migrates to low temperature and high pressure under the same salinity condition. The inhibitory effects of different salts and anion and cation on methane hydrate are summarized. Under conditions of the same concentration and different types of salts, when the concentration of salt ranges from 1 to 1.5 mol/L, the inhibition effect of salt on methane hydrate is MgCl₂ > CaCl₂ > NaCl > KCl. When the concentration of salt is larger than 1.5 mol/L, the inhibitory effect of CaCl₂ on methane hydrate is stronger. The inhibitory effect of anions on methane hydrate is controversial, and the inhibitory effect of Mg²⁺ on methane hydrate is the strongest. From available research results, there is still a gap between the existing data and the actual geological conditions. It is necessary to strengthen the detailed study on chloride-sulfate-carbonate-methane-water systems under actual experimental conditions. It is proposed that the combination of the high-voltage visual reaction chamber with Micro Laser Raman Spectroscopy is likely to accurately obtain the stable temperature and pressure conditions of gas hydrate formation, clarify the inhibitory effect of cations and anions, and understand how salts and ion affect the formation of hydrate and its stability, which will provide reference for the future exploration and development of hydrate.