Phase Equilibria of Natural Gas Hydrates in Bulk Brine and Marine Sediments from the South China Sea

Studies on gas hydrate phase equilibrium conditions are reported widely in the literature. However, the data available for natural gas hydrates (NGHs) in real marine conditions are limited. In this study, NGH dissociation conditions in bulk brine and marine sediments were measured by using a microdifferential scanning calorimetry (mu-DSC). The sediments used in the experiment were obtained from the Shenhu area of the South China Sea, and the natural gas, as well as part of the water samples, were prepared according to the field data. Additionally, the Chen-Guo model was improved to calculate the NGH equilibrium conditions in the systems containing electrolytes and porous sediments. The experimental results showed that the NGH is relatively more stable than pure CH4 hydrate. The hydrate phase equilibrium curves shift to the left with the increased salinity in bulk water and sediments. In addition, the hydrate decomposition temperatures in marine sediments are obviously lower than those in bulk water. The average deviations of the improved model are 2.32% and 3.7% for bulk brine and sediments, respectively, which indicates great predictability. We consider that besides pore size as the main factor affecting the hydrate phase equilibrium, the water absorption capacity of marine sediments can also affect their decomposition conditions. The hydrate decomposition enthalpies were calculated by the Clausius-Clapeyron equation, and the results showed that the enthalpies increased with the decreased salinity and the addition of marine sediments. These findings are significant to NGH production safety in real marine sediments.