Gas Adsorbed in Coal Bed Methane Reservoirs

on 23 September 2024. Posted in Reservoir Engineering

In coal bed methane (CBM) reservoirs, the gas stored is primarily methane. Unlike conventional gas reservoirs where gas is stored in pore spaces, in CBM reservoirs, methane is primarily adsorbed onto the surface of the coal within micropores.

Here are Some Gas Adsorbed Concept

Adsorption - The methane in coal seams is mostly stored by a process called adsorption, which is the adhesion of gas molecules to the surface of the coal particles. The gas molecules attach to the coal in its solid matrix, primarily in the micropores.

Methane Source - The methane found in coal beds is primarily generated during the coalification process (the transformation of plant material into coal). It’s often referred to as "biogenic" (produced by microorganisms) or "thermogenic" (produced by heat and pressure over geological time).

Reservoir Characteristics - Coal beds generally have low permeability, meaning gas does not flow easily through them. Therefore, the methane needs to be desorbed (released from the coal surface) before it can be extracted. When the reservoir pressure is lowered, typically through water removal (dewatering), the methane desorbs from the coal and can be collected.

Methane Recovery - The process of extracting methane from these reservoirs involves pumping out the water that often saturates the coal seams. As the pressure reduces, the methane is released (desorbed) and flows into the wellbore, where it is then extracted.

Methane in Coal Beds are Stored in Three Forms

Adsorbed Gas - The majority of methane is stored this way, attached to the surface of the coal.
Free Gas - Some methane exists as free gas in the fractures or cleats of the coal.
Dissolved Gas - A small portion of methane may be dissolved in the water within the coal bed.

Gas Adsorbed in Coal Bed Methane Reservoirs formula
\( G_a \;=\; 1359.7 \; A \; h \; \rho \; V \) (Gas Adsorbed in Coal Bed Methane Reservoirs) \( A \;=\; G_a \;/\; 1359.7 \; h \; \rho \; V \) \( h \;=\; G_a \;/\; 1359.7 \; A \; \rho \; V \) \( \rho \;=\; G_a \;/\; 1359.7 \; A \; h \; V \) \( V \;=\; G_a \;/\; 1359.7 \; A \; h \; \rho \)
Symbol	English	Metric
\( G_a \) = Gas Adsorbed	\(SCF\)	-
\( A \) = Area	\(acres\)	-
\( h \) = Height	\(F \;/\; 100\;ft\)	-
\( \rho \) = Density	\(g\;/\;cc\)	-
\( V \) = Adsorption Gas	\(SCF\;/\;ton\)	-

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Tags: Reservoir

Gas Adsorbed in Coal Bed Methane Reservoirs formula

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