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Significance Statement
Where the effects of pressure, composition, temperature, composition and water are to be considered, the main components involved in the production of natural gas hydrates are water and natural gas, at conditions of low temperatures and high pressures. Such natural gas hydrates usually form in marine sediments at depths below 300m or in soil beneath permafrost at shallow depths of within 250m. Permafrost is mainly subject to high pressures from crystallization of pore water and from loads applied to gas pockets in frozen sediments i.e. crystallization factor and baric factor. Pioneering scholarly works have estimated, based on available estimates of natural gas hydrate resources, that the amount of methane sequestered in hydrates is in orders of magnitude greater than in conventional and unconventional gas reservoirs summed up as one.
Several techniques exist for methane recovery from hydrates reservoirs namely: thermal stimulation, depressurization, carbon dioxide replacement, inhibitor injection or a combination of these procedures. Normally, methane production from hydrate reservoirs is often simulated with reference to the thermal conductivity of pure methane hydrate, however, this assumption neglects the specificity of hydrate reservoirs thereby entraining grave inaccuracies during methane recovery, specifically for permafrost that contains hydrates and ice.
Evgeny Chuvilin at Skolkovo Institute of Science and Technology in collaboration with Boris Bukhanov at Moscow State University investigated the thermal conductivity variations in gas-saturated sediments exposed to hydrate formations at various conditions. Their aim was to model hydrate and ice formation in gas saturated sediments for empirical study of thermal conductivity and its implications in simulation of methane recovery from natural gas hydrate reservoirs and the respective technologies applied. Their research work is now published in Energy & Fuels.
The research team commenced their empirical procedure by utilizing a specially designed gas hydrate system that maintained a high gas pressure and a steady-state thermal regime. The team then used a built-in unit for thermal conductivity measurements. The recorded measurements were then applied to natural samples of fine sand and silty sand collected from gas emanation sites in permafrost and to synthetic sand and sand-clay blends.
Chuvilin and Bukhanov chiefly observed that the thermal conductivities of the hydrates could only either increase or decrease depending on the hydrate formation conditions. They noted that the thermal conductivity increased if the gas hydrates were formed at positive temperatures but decreased during hydrate formation in frozen samples. The researchers also observed that freezing and thawing of hydrate bearing sediments above the equilibrium pressure reduced their thermal conductivity as a result of additional hydrate formation.
A novel empirical technique was also suggested to study the thermal conductivity of hydrate- water-, gas- and ice-bearing sediments under varying temperatures and pressures at gas pressure up to 7 mega Pascal. The observed thermal conductivity behavior results from pore space changes associated with additional hydrate formation, as explained by aid of the models used. These models can be utilized for reference in geo-mechanical and thermal simulations of gas hydrate reservoirs, bearing in mind the conditions of pore hydrate formation, with implications for methane recovery.
Reference
Evgeny Chuvilin, Boris Bukhanov. Effect of Hydrate Formation Conditions on Thermal Conductivity of Gas-Saturated Sediments. Energy Fuels 2017, volume 31, pages 5246−5254.