Kinetic study of fulvic acid on methane hydrate formation and decomposition in clay-rich silty sediments

Abstract

Humic substances and salts are widely distributed in marine sediments, a phenomenon that undoubtedly increases the complexity of recovering methane from hydrate. In this work, kinetic experiments were carried out to determine the role of fulvic acid (FA) on methane hydrate formation and decomposition in marine sediments. We used the mixed clay-rich silty with pore water, containing 3.0 wt% NaCl, to simulate the sediments, with experimental pressure (12.7–14.6 MPa) and temperature (8.6–16.2 °C) conditions adopted from the hydrate reservoir of Shenhu area, South China Sea (SHCS). Results revealed that FA has favorable/unfavorable effects on methane hydrate formation in the sediments, in a concentration-dependent manner. FA solutions, at concentrations of 6.0 and 10.0 wt%, inhibited the formation of methane hydrate, because the high-water holding capacity itself could reduce water activity. Conversely, 2.0 wt% FA solution accelerated methane hydrate formation by arranging surrounding water molecules with H-bonds. Interestingly, it was found that the addition of FA could significantly weaken NaCl's inhibitory effect on methane hydrate formation, while water conversion ratios increased ∼2/3-fold relative to NaCl without FA solution containing systems. These results suggested that presence of FA in marine sediments promoted hydrate reformation during reservoir production. Besides, we found that there is a synergistic effect between FA and NaCl in the process of methane decomposition.