Effect Of Marine Sediment Capillary Pressure On The Occurrence Of Natural Gas Hydrate

Gas hydrates form in marine sediments under suitable pressure and temperature when the dissolved methane concentration exceeds the local methane solubility in aqueous–hydrate systems.The capillary pressure can exert effects on the methane solubility in aqueous-hydrate system,inhibiting gas hydrate formation in sediments pore space.Based on the mass conservation of methane and sulfate,considering the effect of capillary pressure on the methane solubility of aqueous-hydrate system,a top occurrence of gas hydrate model was established and applied to two sites of SH7 in South China Sea and ODP 994 in Black Ridge.Three different scenarios were run to illustrate the influence of capillary pressure on the top occurrence of hydrate.The top occurrence of gas hydrate are calculated to 148 mbsf and 169 mbsf for SH7 and ODP 994,respectively,neglecting capillary pressure.The corresponding results are 152 mbsf and 176 mbsf,assuming a homogeneous pore radius of with pore radius assumed to be that in hydrate occurrence zone.Both of the results were shallower than those identified by negative chloride anomies.According to the lithological distribution at the two sites,pore radiuses were assumed to much lower above 155 and 183 mbsf at SH7 and ODP 997,where the methane solubility of aqueous-hydrate system would be higher.The calculated top occurrences of gas hydrate are 155 and 183 mbsf at the two sites,coinciding with their lithological interface depth.Our results show that capillary pressure exert effect on the top occurrence of gas hydrate by increasing the methane solubility of aqueous–hydrate system.In addition,the discontinues capillary pressure due to the lithological interface could deviate the distribution of methane solubility of aqueous–hydrate system and affect the depth of the top occurrence of gas hydrate.The gas hydrates formed in the sediment pore space would reduce the pore size and increase the methane solubility in the aqueous-hydrate system.As a result,a higher methane concentration is needed to further form gas hydrate in sediments pore.Based on the mass conservation of methane,the dissolved methane concentration profiles at SH7 site were simulated for two scenarios.In the first series of simulations,methane is assumed to be transported in dissolved phase,and the result dissolved methane is relatively low and the gas hydrate saturation can only reach about 0-10%,which is much lower than that obtained from chloride anomies.Methane transport in gas phase can produce a methane concentration high enough to form massive gas hydrate with high saturation.Therefore,the methane could be supplied in gas phase at station SH7.