Abstract
Natural gas hydrates (primarily methane hydrates) are considered to be an important and promising unconventional source of hydrocarbons. Most natural gas hydrate accumulations exist in pore space and are associated with reservoir rocks. Therefore, gas hydrate studies in porous media are of particular interest, as well as, the phase equilibria of pore hydrates, including the determination of equilibrium pore water content (nonclathrated water). Nonclathrated water is analogous to unfrozen water in permafrost soils and has a significant effect on the properties of hydrate-bearing reservoirs. Nonclathrated water content in hydrate-saturated porous media will depend on many factors: pressure, temperature, gas composition, the mineralization of pore water, etc. In this paper, the study is mostly focused on the effect of hydrate-forming gas pressure on nonclathrated water content in hydrate-bearing soils. To solve this problem, simple thermodynamic equations were proposed which require data on pore water activity (or unfrozen water content). Additionally, it is possible to recalculate the nonclathrated water content data from one hydrate-forming gas to another using the proposed thermodynamic equations. The comparison showed a sufficiently good agreement between the calculated nonclathrated water content and its direct measurements for investigated soils. The discrepancy was ~0.15 wt% and was comparable to the accuracy of direct measurements. It was established that the effect of gas pressure on nonclathrated water content is highly nonlinear. For example, the most pronounced effect of gas pressure on nonclathrated water content is observed in the range from equilibrium pressure to 6.0 MPa. The developed thermodynamic technique can be used for different hydrate-forming gases such as methane, ethane, propane, nitrogen, carbon dioxide, various gas mixtures, and natural gases.
Highlights
The phase equilibrium problem of gas hydrates in porous media has a long history.Russian scientists first drew attention to this problem in the 1960s of the 20th century during the analysis of hydrate conditions in the oil and gas basins of the Siberian permafrost
To understand the effect of porous media, a new parameter was added to the thermodynamic model of hydrate existence, which described as pore water in a single capillary of a given radius
A new value a appears in the definition of b, and it is essential that a be a. Such dependence was obtained [44] for unfrozen water calculations function of temperature, i.e., a corresponding to the equilibrium of ice–unfrozen water: in the soil under consideration
Summary
The phase equilibrium problem of gas hydrates in porous media has a long history. Russian scientists first drew attention to this problem in the 1960s of the 20th century during the analysis of hydrate conditions in the oil and gas basins of the Siberian permafrost. Nonclathrated water is liquid water in a sample of a porous medium (a soil or sediment system) at pressure P, which is in thermodynamic equilibrium with a hydrate-forming gas and a gas hydrate in a bulk phase. The amount of nonclathrated water in the soil sample decreases with an increase in the pressure of the hydrate-forming gas (part of the pore water transformed into a hydrate phase). The pressure range from Peq to pressure, corresponding to ice ), calculated using the equilibrium of the gas hydrate with ice (this pressure note by Peq relations of type (12), refers to the zone of nonclathrated water metastability (i.e., to a hypothetical situation, as if ice in a given system did not exist). At P > Peq , value a < 1, and when gas pressure P increases, pore water activity a and equilibrium water content W in the sample decrease.
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