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

Abstract

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.

Experimental Study on the Dissociation Equilibrium of (CH4 + CO2 + N2) Hydrates in the Mixed Sediments

“…Their results revealed that the hydrate dissociation equilibrium temperature for the kaolin clays lowered at most 1.5 K than that for bulk hydrate. Lv et al 39 tested the stability condition of CH 4 hydrates in the mixed sediments of quartz sand, montmorillonite, and superfine kaolin with a mass ratio of 7.5:1.5:1 in the presence of 3.0 wt % NaCl solution. They reported that the average temperature shift is 1.98 K compared to that of the pure water system.…”

Improving Methane Hydrate Formation Kinetics and Gas Storage Capacity with a Promoter

“…Recently, soil organic matter, such as humic acid, 26 fulvic acid (FA), 27 and fatty acid, 28,29 was reported to show a good promotion effect on hydrate formation. 30,31 Sagidullin et al 26 investigated CH 4 hydrate formation in the 0.1 wt % humic acid solution, reported that about 75% of water transformed into hydrate within 15−20 min, and believed that humic acid can act as a good KHP.…”

“…Recently, soil organic matter, such as humic acid, fulvic acid (FA), and fatty acid, , was reported to show a good promotion effect on hydrate formation. , Sagidullin et al investigated CH 4 hydrate formation in the 0.1 wt % humic acid solution, reported that about 75% of water transformed into hydrate within 15–20 min, and believed that humic acid can act as a good KHP. Liu et al explored CO 2 hydrate formation in the presence of natural organic matter and found that the hydrate formation kinetics was strengthened, and this effect became more distinct for sulfur-containing acid-soluble organic matter.…”

Experiment and Observation of Methane Hydrate Formation in Silty Clay Sediments of the South China Sea with 50–85% Water Saturation

“…Jang and Santamarina 30 studied the process of CH 4 hydrate formation in clay medium and found that gas transport by convection could form a higher hydrate saturation, with the value reaching 20%, and that it took a longer time for hydrate formation in clay than in sandy medium. Lv et al 31 explored the phase equilibrium data of the hydrate formation process in sandy clay and concluded that the phase equilibrium data shifted to a lower temperature and higher pressure than in the bulk solution. That is the reason why it took longer to form the hydrate in silty clay sediments.…”