Study on the dissociation conditions of methane hydrate in silty-clayey sediments

Abstract

Natural gas hydrate is widely distributed in silty-clayey sediments, that occur largely in a highly dispersed state. However, the phase equilibria of methane hydrate in the presence of clay and how surface adsorption associated with clay particles affect natural gas hydrate thermodynamics and stability are not fully elucidated and warrant investigation. In this study, the dissociation conditions of methane hydrate in representative silty-clayey sediments (sand, montmorillonite and illite) with a water content of ∼20 % at different clay contents (10, 30, 50, 70 and 100 wt%) were measured using the multi-step heating method. The results indicated that montmorillonite has a greater dissociation temperature depression compared to illite. In the pure montmorillonite system, the temperature depression of methane hydrate is up to -2.24 K at a pressure of 11.69 MPa compared to pure water, however, that for illite is very small, less than -0.5 K. When the montmorillonite content decreases from 100 wt% to 70 wt%, the dissociation temperature of methane hydrate decreases significantly. As the montmorillonite content decreases below 50 wt%, the dissociation temperature of methane hydrate in silty-clayey sediments is almost the same as bulk hydrates, which due to the clay sediments is almost saturated with water. Furthermore, we introduce the Van Genuchten (VG) model to quantify the effect of clay content on water potential (matric suction) in unsaturated clays. Based on the classical van der Waals model, the VG model has successfully been applied to calculate methane hydrate phase equilibria in clay-bearing sediments, with a maximum absolute deviation (AADP) <5 %. It reveals that montmorillonite, characterized by its strong capacity for interlayer water absorption, shows an approximately exponential increase in matric suction as clay content rises. In contrast, the matrix suction of illite is weak, which can be approximately considered as a linear increase with the increase of clay content.