Maksim Zhmaev

1. Natural gas in the shallow crust can exist in either free or hydrate (clathrate) form. Gas hydrates are ice-like, crystalline water-based solids that form under specific temperature and pressure...

Permafrost stores large amounts of natural gas in free and hydrate (clathrate) forms. Intrapermafrost gas hydrates were revealed in frozen core samples recovered from test, exploration, and production wells in the Arctic oil and gas fields. Being similar to ice in many respects, gas hydrates can be mute for geophysical surveys. Meanwhile, laboratory experiments show that the presence of a gas hydrate component in pore moisture affects considerably some parameters of frozen sediments, as well as processes in evolving permafrost. Hydrate-bearing permafrost has a lower thermal conductivity and permeability but a higher geomechanic strength than hydrate-free frozen ground. Gas hydrates also influence some geophysical variables, such as electrical resistivity which becomes higher at higher hydrate contents. Accumulation of gas hydrates changes the relative percentages of pore moisture components (ice-hydrate-liquid water) in systems comprising ice, free gas, and gas hydrates, which has bearing on the physical properties of permafrost. The presence of gas hydrates increases the strength and resistivity of permafrost and, on the other hand, tends to reduce its permeability and the rates of heat and mass transfer.

Formation and dissociation of pore gas hydrates in permafrost can change its properties, including fluid flow capacity. Permeability is one of the most significant parameters in the study of hydrate-containing rocks, especially in the case of gas burial or extraction. Gas permeability variations in frozen sand partially saturated with CO₂ or CH₄ hydrates are studied experimentally at a constant negative temperature of −5 °C, as well as during freezing–thawing cycles. The gas permeability behavior is controlled by the formation and dissociation of pore gas hydrates in frozen sand samples. The samples with an initial ice saturation of 40 to 60% become at least half as permeable, as 40% of pore ice converts to hydrate. The dissociation process of accumulated hydrates was modeled by both depressurizing methane or CO₂ to atmospheric pressure and by stepwise injection of gaseous nitrogen up to 3 MPa into a frozen sample. In sand samples, with a decrease in gas pressure and without subsequent injection of nitrogen, a decrease in pore hydrate dissociation due to self-preservation was noted, which is reflected by a deceleration of gas permeability. Nitrogen injection did not lead to a decrease in the rate of dissociation in the frozen hydrate-containing sample, respectively, as there was no decrease in the rate of gas permeability.