Hydrate formation and dissociation characteristics in clayey silt sediment

Abstract

The geological occurrence of natural gas hydrates in clayey silt sediments has been extensively confirmed. Compared with sandy sediments, clayey silt is characterized by a large specific surface area, high capillary pressure, and high bound water content, which leads to distinct characteristics of hydrate formation and dissociation. The formation and dissociation of methane hydrates in clayey silt sediments with illite or montmorillonite were experimentally investigated. It was determined that an increase in the water content of silt and illite-silt sediments from ~9.47% to ~28.41% caused a decrease in the water-to-hydrate conversion ratio owing to the formation of a hydrate film between water and CH4 gas. However, in montmorillonite-silt sediments, the water-to-hydrate conversion rate increased between 9.47% and 18.94% of water saturation and decreased between 18.94% and 28.41% of water saturation under the combined effects of weakly bound water content and capillary pressure. In the hydrate dissociation experiment, the temperature of sediments with ~9.47% water content decreased more, which was caused by the smaller specific heat capacity of sediments with lower water content. Meanwhile, the temperature drops below the freezing point during the hydrate dissociation process, which is caused by the strong inhibition of hydrate phase equilibrium by montmorillonite. This study demonstrates the effects of clay type and water content on hydrate formation and dissociation, which is helpful for understanding the production behavior of clayey silt hydrate reservoirs.