Abstract
In marine sediments, seawater influences the phase behavior of natural gas hydrate. As a porous medium, the water distribution and physical properties of montmorillonite are influenced by the salt ions in seawater. In this work, the bound-water content in, and crystal structure of, montmorillonite is measured to investigate the effect of salt ions on the water distribution in montmorillonite. It can be determined from the results that the bound-water content in montmorillonite decreases as the salt-ion concentration increases. Salt ions affect the intercalation of water molecules in montmorillonite, and they then inhibit the expansion effect of montmorillonite. Next, the phase behaviors of methane hydrate in montmorillonite with NaCl solution are investigated using high-pressure micro-differential scanning calorimetry. The phase behavior of hydrate in montmorillonite with NaCl solution is discussed. In montmorillonite with NaCl solution, the phase equilibrium temperatures and the conversion rate of methane hydrate both decrease with increasing NaCl concentration. The results show that methane hydrate in montmorillonite is influenced not only by the phase-equilibrium effect of salt ions, but also by the formation effect of the salt ions on the bound-water content in montmorillonite.
Highlights
The hydrate formation behavior in porous sediments plays a pivotal part in reservoir research and gas hydrate drilling
The sample preparation method of water content experiment is shown in Section 2.2, and the concentrations of solution are shown in Table 1. 75 ◦ C is the boundary point of the free water and bound water [36]
The montmorillonite samples that were fully hydrated for 48 h were dried in an oven at 75 ◦ C to remove the free water
Summary
The hydrate formation behavior in porous sediments plays a pivotal part in reservoir research and gas hydrate drilling. The phase equilibrium conditions of hydrate are influenced by capillary forces and fluid properties in porous sediments [1,2,3,4]. The phase behavior of hydrate in quartz sand and other porous media have been studied through experiments and simulations [5,6,7,8,9]. Gas hydrate formation and decomposition is a kinetics process of multi-phase and multi-element interaction. Owing to the complex properties and diverse conditions of natural sediments, it is difficult to directly study the conditions for hydrate formation in it. Montmorillonite is an important component of natural gas hydrate reservoirs
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