Effect of pore water on the depressurization of gas hydrate in clayey silt sediments

Abstract

Marine gas hydrate always occurs in clayey silt sediments partially or fully saturated with water. Pore water is involved in gas hydrate formation and decomposition, which plays an important role in gas recovery from hydrate during depressurization. In this work, the depressurization of methane hydrate in the clayey silt sediments was experimentally investigated in a one-dimensional reactor. The variation of gas production and pressure gradient under the gas-rich and water-rich condition were studied. It was found that the gas production in water-rich environment was much lower than that in gas-rich environment as the movement of hydrate dissociating front is slower in water-rich environment. The permeability of the clayey silt sediments was measured before hydrate formation and after hydrate decomposition, which indicated that there was a large depression in the permeability of the clayey silt sediments after hydrate decomposition, which could be attributed to the hydration swelling of clay minerals with the expansion of diffuse double layer caused by the release of water with lower salinity from gas hydrate dissociation. And the permeability depression of the clayey silt sediments caused by hydrate decomposition was more obvious in water-rich environment, reaching up to ∼50%, and increased with the content of clay minerals. In addition, inorganic salt-based clay stabilizer solution was pre-injected into the clayey silt gas hydrate-bearing sediments before depressurization to mitigate the formation damage during gas hydrate production. The results showed the permeability of the sediments pre-saturated with NH4Cl was maintained at ∼75% of the initial values after gas hydrate decomposition as the exchange of Na+ with NH4+ in sediments was proved to decrease the bound water in montmorillonite interlayers by NMR analysis and inhibit the hydration swelling of clay particles.