The promoting effect of natural sand on methane hydrate formation: Grain sizes and mineral composition

• Strong variations in the promotion of methane hydrate formation in natural sand.
• Surface and mineralogical changes such as iron-oxy-hydroxide coating affect formation.
• Involved processes include accelerated hydrate nucleation, changes in dissolution rates or bulk water volume.

Investigations regarding the effects of particle size on gas hydrate growth are commonly carried out using glass beads, silica powder, and various clay minerals. Comparisons of different grain sizes of sands are less common as are natural samples. Since the highest gas hydrate saturations and possible energy resource targets are associated with natural sandy sediments, the presented experiments address the possible effects of different grain sizes from a natural sand sample on methane hydrate formation. Influences on the kinetics of hydrate formation in natural deposits are of particular interest in hydrate production or carbon dioxide sequestration in methane hydrates. In both cases an undesired secondary hydrate formation may occur on short time scales, usually under non-equilibrium conditions.The static small-volume experiments are carried out far within the methane hydrate stability field (7 MPa/274 K) using five different grain size classes of a natural quartz sand: <125 μm, < 250 μm, 250–500 μm, 500–1000 μm, and 1000–2000 μm. A constant volume of water or salt water is added to saturate the sediment sample and pressure is built up using methane gas. Pressure and temperature are continuously recorded and a glass window allows for microscopic observation and Raman spectroscopy to verify methane hydrate formation.For the chosen experimental set-up there is a strong particle size effect on the kinetics of methane hydrate formation. A high concentration of fine sands with a grain size <125 μm led to explicitly faster gas hydrate formation compared to coarser sand or a small fraction of fine particles diluted in a sample of coarser sand grains. Our article discusses possible causes in conjunction with changes of the mineral surface. These include accelerated hydrate nucleation or growth due to the surface area (1) or mineral composition (2) and changes in dissolution kinetics (3) caused by enhanced dissolution rates or changes in the ratio of bound to bulk water volumes. The addition of salt significantly increased induction times but did not supplant the particle size effect.

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