Water–methane interface broadening by low-dose methanol addition highlights the need for true surface sampling on the atomistic level.

The amount of energy in the form of natural gas hydrates is huge and likely substantially more than twice the amount of worldwide conventional fossil fuel. Various ways to produce these hydrates have been proposed over the latest five decades. Most of these hydrate production methods have been based on evaluation of hydrate stability limits rather than thermodynamic consideration and calculations. Typical examples are pressure reduction and thermal stimulation. In this work we discuss some of these proposed methods and use residual thermodynamics for all phases, including the hydrate phase, to evaluate free energy changes related to the changes in independent thermodynamic variables. Pressures, temperatures and composition of all relevant phases which participate in hydrate phase transitions are independent thermodynamic variables. Chemical potential and free energies are thermodynamic responses that determine whether the desired phase transitions are feasible or not. The associated heat needed is related to the first law of thermodynamics and enthalpies. It is argued that the pressure reduction method may not be feasible since the possible thermal gradients from the surroundings are basically low temperature heat that is unable to break water hydrogen bonds in the hydrate–water interface efficiently. Injecting carbon dioxide, on the other hand, leads to formation of new hydrate which generates excess heat compared to the enthalpy needed to dissociate the in situ CH₄ hydrate. But the rapid formation of new CO₂ hydrate that can block the pores, and also the low permeability of pure CO₂ in aquifers, are motivations for adding N₂. Optimum mole fractions of N₂ based on thermodynamic considerations are discussed. On average, less than 30 mole% N₂ can be efficient and feasible. Thermal stimulation using steam or hot water is not economically feasible. Adding massive amounts of methanol or other thermodynamic inhibitors is also technically efficient but far from economically feasible.

It was shown earlier that adsorption isotherms on microporous adsorbents at temperatures above the critical values can be described using the Dubinin-Radushkevich and Tolmachev-Aranovich equations when saturated adsorptive vapor pressures used as standard pressures at temperatures below the critical values are replaced by standard pressures P*(T) found by linear extrapolation of dependences of the logarithm of the saturated adsorptive vapor pressure on the inverse temperature. This was confirmed using the example of description of experimental adsorption isotherms of krypton, xenon, oxygen, and nitrogen on microporous active carbons AUK at the temperatures above the critical value.