Stable Isotope Systematics Of Coalbed Gas During Desorption And Production

The stable carbon isotope ratios of coalbed methane (CBM) demonstrate diagnostic changes that systematically vary with production and desorption times. These shifts can provide decisive, predictive information on the behaviour and potential performance of CBM operations. Samples from producing CBM wells show a general depletion in C-13-methane with increasing production times and corresponding shifts in delta C-13-CH4 up to 35.8 parts per thousand. Samples from canister desorption experiments show mostly enrichment in C-13 for methane with increasing desorption time and isotope shifts of up to 43.4 parts per thousand. Also, C-13-depletion was observed in some samples with isotope shifts of up to 32.1 parts per thousand. Overall, the magnitudes of the observed isotope shifts vary considerably between different sample sets, but also within samples from the same source. The delta C-13-CH4 values do not have the anticipated signature of methane generated from coal. This indicates that secondary processes, including desorption and diffusion, can influence the values. It is also challenging to deconvolute these various secondary processes because their molecular and isotope effects can have similar directions and/or magnitudes. In some instances, significant alteration of CBM gases has to be considered as a combination of secondary alteration effects.