Thermodynamic effect of surfactants on the inhibition of methane hydrate formation in different oil-water system

In the production of natural gas hydrates, the methane gas released by hydrate decomposition and the in-situ free gas present in sediments are inclined to form or re-form hydrates under a low-temperature high-pressure condition, leading to blockages in high-pressure wellbores or pipelines near the mudline and posing safety hazards. Surfactants can be used as effective hydrate inhibitors to prevent the reformation of hydrates and subsequent blockages. In this paper, we studied the inhibitory effects of low-dose cationic surfactant (dodecyl dimethyl benzyl ammonium chloride) on methane hydrate formation and decomposition processes in different oil-water systems by high pressure microcalorimeter. The systems of TBME (metrabutylmethyl diethyl ether)-H₂O, MCH (methylcyclohexane)-H₂O, CP (cyclopentane)-H₂O , and pure water were selected and tested. Results show that low-dose cationic surfactant has little impact on the amount of methane hydrate formation in pure water. However, it did influence the hydrate structure, favoring the formation of type I methane hydrates. In contrast, low-dose cationic surfactants show significant inhibitory effects on both the formation and decomposition process of methane hydrate and methane mixed hydrate in TBME-H₂O, MCH-H₂O, and CP-H₂O oil-water separation systems. The introduction of surfactants could reduce the solubility of methane in oil phase, and substantially decrease the amount of hydrate formation. Consequently, TBME-H₂O and MCH-H₂O systems became more likely to form pure methane hydrate, and CP-H₂O system is more inclined to form hydrate with more complex structures. Furthermore, the anti-agglomeration of cationic surfactant could prevent large molecules from being captured in the cage structure, thereby inhibiting the formation of H-type hydrates.