METHOD OF RAPID ESTIMATION OF THE EFFECT OF THE HYDRATE-FORMING GAS PRESSURE ON NONCLATHRATED WATER CONTENT IN SOILS
V.A. Istomin1,2, D.V. Sergeeva2, E.M. Chuvilin2, B.A. Bukhanov2, N.S. Sokolova2
1Gazprom VNIIGAZ LLC, St. Petersburg, Russia
2Skolkovo Institute of Science and Technology, Center for Petroleum Science and Engineering, Moscow, Russia
Keywords: gas hydrates, sediments, pore water, phase equilibria, nonclathrated water, unfrozen water, ice, thermodynamic calculations
Abstract
Natural gas hydrates exist in porous media under high pressure and low temperature conditions, including permafrost. The development of methods for calculating the phase equilibria in hydrate-bearing soils and sediments is of great interest. This includes the equilibrium content of nonclathrated water, as pore water in equilibrium with pore hydrate and under given thermobaric conditions. Nonclathrated water is similar to unfrozen water in frozen soil. The current study focuses on thermodynamic relationships for calculating nonclathrated water content in soil under certain thermobaric conditions based on experimental data of pore water activity and soil water content. It has been shown that at a constant temperature the nonclathrated water content decreases sharply according to a power law as gas pressure increases. The results of thermodynamic calculations are in agreement with direct measurements of nonclathrated water in soil systems using the contact method. Thus, at temperatures below 0°C, the nonclathrated water content decreases by more than two times in kaolinite clay and in sandy clayey soils with an increase in methane pressure from 2.3 to 11 MPa. The obtained relationships allow us to recalculate the nonclathrated water content after transition from one hydrate-forming gas to another, as well as calculate nonclathrated water content using the unfrozen water content curve at different temperatures. Developed thermodynamic approach can be applied to various hydrate-forming gases and their mixtures.
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