Enhanced coalbed methane (ECBM) recovery processes rely on the sorption of a stripping or displacing gas to recover the sorbed methane. Both the adsorption and desorption of methane and the injected gases are important to the success of the process. Oxygen, which can chemisorb to coal, is expected to show different adsorption than desorption behavior. However, adsorption and desorption isotherms for mixtures of physisorbed gases such as nitrogen and methane appear similar in form to those for oxygen. Moreover, results from a field ECBM recovery project also suggest that in the mixed state physisorbed gases may show different behavior during desorption than during adsorption. Because reservoir simulators typically use a reversible equilibrium sorption model such as the Langmuir isotherm, the presence of hysteresis due to sorption irreversibilities could affect the reliability of the simulations.

Constant composition expansion and constant volume depletion sorption results were analyzed. This analysis demonstrates that although mixtures of physisorbed ECBM recovery gases (no oxygen) become enriched in the more strongly held component during desorption, the process is one of reversible equilibrium sorption and isotherm models such as the extended Langmuir isotherm effectively capture this adsorption/desorption equilibrium. No additional changes to the simulator are needed because of this compositional effect. However, for oxygen-bearing gases modification of the Langmuir isotherm will be needed to quantitatively represent adsorption/desorption behavior during an ECBM recovery process.

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