Dissociation behaviors of methane hydrate in marine sediments from South China Sea under constant pressure

Abstract

The dissociation behaviors of methane hydrate under constant pressure in the marine sediments from the South China Sea were experimental studied. The experiments were carried out in a high-pressure reactor with the effective volume of 416 cm3. The marine sediments with mean pore diameter of 12.178 nm and total pore volume of 4.997×102 mL/g, surface area of 16.412 m2/g were used for the experiments. The mass fraction of water in the marine sediments is 40%. The methane hydrate was formed at the initial pressure of 14.4 MPa and dissociated by depressurization through the gas release from the reactor to the supply vessel with the volume of 1091 cm3. The moles of the gas released were calculated by the pressure change in the supply vessel. The dissociation experiments were carried out at the temperature range of 274.15–281.15 K and the dissociation pressure range of 3.0–5.0 MPa. The experimental results show that the hydrate begins to dissociate during the depressurization period, resulting in the significant decrease of the temperature in the reactor to be below the melting-point due to that the hydrate dissociation is an endothermal process. The lowest temperature during the hydrate dissociation in marine sediments is significantly lower than that in silica sands. The methane hydrate can dissociate when the pressure is higher than the equilibrium dissociation pressure for bulk methane hydrate. The dissociation rate of methane hydrate, the final gas dissociated from the hydrate and the final hydrate dissociation ratio increase with the decrease of dissociation pressure. For the experiments with different experimental temperature, the hydrate dissociation rate increases with the increase of the experimental temperature, and the final gas dissociated from hydrate is affected by the initial hydrate saturation in the marine sediments. The hydrate can dissociate completely when the dissociation pressure is lower than the equilibrium hydrate dissociation pressure corresponding to the experimental temperature for the bulk hydrate.