Effect of Fulvic Acid and Sodium Chloride on the Phase Equilibrium of Methane Hydrate in Mixed Sand-Clay Sediment

摘要

The phase equilibrium conditions for methane hydrate in mixed sand-clay sediment with the presence of fulvic acid (FA) and sodium chloride (NaCl) were measured using the multistep heating method. The experiments were conducted at the temperatures from 281.45 to 290.65 K and pressures from 6.33 to 17.39 MPa, respectively. The mixed sand-clay porous media and aqueous solutions containing 2.0, 6.0, and 10.0 wt % FA and 3.0 wt % NaCl were used. The experimental results indicate that the phase equilibrium curve of methane hydrate in mixed porous media shifts to the left side as compared to that in bulk solutions because of the capillary force and hydrophilic surfaces of porous media. FA is a thermodynamic inhibitor for methane hydrate formation as the -COO- and short alkyl substituent of FA could compete with hydrate for water molecules, and the inhibition effect of FA on methane hydrate formation is weaker as compared to 3.0 wt % NaCl under all studied concentration conditions. The phase equilibrium pressures of methane hydrate in mixed porous media with the presence of 2.0 wt % FA + 3.0 wt % NaCl,6.0 wt % FA + 3.0 wt % NaCl, 10.0 wt % FA + 3.0 wt % NaCl are all higher than that with only 3.0 wt % NaCl present, and the pressure increases with the increase in FA concentration. The existence of FA solutions and FA + NaCl solutions in mixed porous media does not affect the methane hydrate structure.