Confined freezing of pore water in gas-saturated sediments under gas pressure associated with cryogenic gas concentration is studied experimentally. The behavior of pore water freezing temperature in pressurized sediments is modeled for different saturating gases: nitrogen, methane, carbon dioxide, and a CH4 + CO2 (1:1) mixture. Natural sand and silt samples from permafrost are frozen in laboratory in a specially designed system which can record temperature change in heated and frozen gas-saturated soil samples under gas pressure specified to be below the level of gas hydrate stability. The freezing point of pore water shows dependence on the gas pressure and composition. The changes of the freezing temperature are notable if the saturating gases are highly soluble (carbon dioxide), but the presence of salt ions reduces the effect. Poorly soluble gases, such as nitrogen and methane, can provide no more than 0.1 °C/MPa reduction of the freezing point unless carbon dioxide is present in the pores, which increases the coefficient to 1.36 °C/MPa. The experimentally estimated effect of gas pressure on the freezing temperature of pore water agrees with the results of thermodynamic calculations based on pore water activity.
|Translated title of the contribution||Effect of gas composition and pressure on pore water freezing point in gas-saturated sediments: An experimental study|
|Number of pages||9|
|Publication status||Published - 2019|