In the enhanced coalbed methane production (ECBM) option involving injection of CO2 in a coal-gas reservoir, the injected CO2 induces incremental swelling of the coal matrix. This is believed to result in a significant reduction in coal permeability, and thus the CO2 injectivity. This paper discusses the results of a study where swelling of the coal matrix was first measured with injection of methane and CO2. This was followed by injecting CO2 in the methane saturated sample, and measuring the incremental swelling for increasing CO2 concentration. The measured swelling with individual gases was then used to model the incremental swelling when CO2 is injected in a CBM reservoir.

The experimental results showed that the amount of swelling produced by CO2 is more than three times that produced by methane. The single-gas swelling data was fitted to a model based on the Langmuir Sorption Theory, and the agreement between the modeled and experimental data was excellent. Finally, the Extended Langmuir (EL) Theory was used to compute the incremental swelling for increasing concentration of CO2. However, the measured and modeled incremental swelling with increasing concentration of CO2 did not match very well. These results are in agreement with the reported finding that the EL Theory fails to accurately predict the sorption behavior of methane/CO2 mixtures. However, the technique serves as a means to estimate incremental swelling until improved methodology is developed to do so.

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