Introduction
Sorption is the attachment of gas molecules to the solid surface of the coal matrix. They can be loosely classified as Chemical and Physical Adsorption:
Physical Adsorption:
Physical sorption is due to van der Waals forces and is relatively weak while chemical sorption is due to formation of chemical bonds and hence is much stronger. In coal reservoirs gas in the sorbed state is held by physical forces. However, in microporous materials because of the large surface area, the physical attraction of the fluid for the solid is substantial.
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Force of attraction: Van der Waal’s
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Usually takes place at lower temperatures, and decreases with increasing temperature. See also Temperature Effects
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Reversible
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Can form multi-molecular layers
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Does not require any activation energy
Physical sorption is reversible.
Chemical Adsorption:
Because of the physical attraction, the molecular density of the fluid near the pore walls is significant and is assumed to approach the liquid density of methane. Because the molecular density of a gas in the sorbed state is substantially greater than that of free gas due to compression alone, more gas can exist in the adsorbed state than in the free gas state up to saturation of the coal.
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Forces of Attraction: Chemical Bond Forces
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Takes place at higher temperatures
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Reversible
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Forms mono-molecular layers
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Requires activation energy
In general, coals typically contain more gas per unit volume than sandstone reservoirs. Sorption is a very effective method of storing gas.
References
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Bustin, R. M., Geology & Some Engineering Aspects of Coalbed Methane, 2001 CBM Solutions
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John Sedile, Fundamentals of Coalbed Methane: Reservoir Engineering, 2021 PennWell Corporation,
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Zhang, Pengfei, Adsorption and Desorption Isotherms, Sept 3, 2016, KE Research Group