Phase Equilibrium Condition of Methane Hydrates in Clayey-Silty Sediments: Effects of Clay and Water Content

Over 90% of natural gas hydrates (NGHs) on Earth occur in fine-grained clayey-silty sediments, which also serve as ideal sites for hydrate-based CO2 geological sequestration. However, hydrate phase equilibria in such sediments remain poorly understood, hindering advancements of NGH exploitation and carbon sequestration technologies. Here, we conducted a series of experiments to investigate the phase behavior of methane hydrates in representative clayey-silty sediments (montmorillonite and silt) with varying clay and water contents and measured the hydrate dissociation conditions via a stepwise heating method. The results demonstrate that the hydrate dissociation temperature depression increases exponentially with rising clay content and decreasing water content. This leads to a more pronounced dissociation temperature shift in silty clays (clay content >50 wt %) than in clayey silts (clay content <50 wt %). Specifically, at a water content of 20 wt %, the hydrate dissociation temperature depression in silty clays (80 wt % montmorillonite and 20 wt % silt) is as high as 1.5 K on average relative to bulk hydrates, whereas that in clayey silts (20 wt % montmorillonite and 80 wt % silt) remains below 0.3 K. Furthermore, compared to clayey silts, the hydrate dissociation temperature depression in silty clays exhibits a stronger dependence on water content. These findings highlight the pivotal role of clay and water content in regulating hydrate stability within geological systems.