Stable Isotope Systematics of Coalbed Gas during Desorption and Production

Abstract

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 13 C-methane with increasing production times and corresponding shifts in δ 13 C-CH 4 up to 35.8. Samples from canister desorption experiments show mostly enrichment in 13 C for methane with increasing desorption time and isotope shifts of up to 43.4. Also, 13 C-depletion was observed in some samples with isotope shifts of up to 32.1. Overall, the magnitudes of the observed isotope shifts vary considerably between different sample sets, but also within samples from the same source. The δ 13 C-CH 4 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.

Evaluation of the Gas Content in Archived Shale Samples: A Carbon Isotope Study

“…In this way, the calculated isotope fractionation was similar to the results of the coal canister method [12]. Determined isotope enrichment ε = −3% for the methane release neared the −2% value for the methane desorption from coal [12,22] and the experimental values from Reference [9] (−1.6 to −2.8% ). The study referred to in Reference [8] reported values from −3.3 to −1.3 % (−4 % as a maximum for coal and mudstone).…”

“…Results from diffusion modeling in Reference [11] were also similar. However, the secondary process as desorption could have had a significant effect on isotope fractionation, as was observed during coal bed gas production experiments [22]. We supposed that such secondary processes modified our ethane isotope fractionation, which was higher than literature values.…”

“…Higher gaseous hydrocarbons (ethane, propane, and butane) contribute only in units of percentage. Moreover, methane is less retarded by adsorption/desorption phenomena in pore transport, in comparison with higher hydrocarbons [9,22]. As the first approximation in the calculation of the gas loss, we suppose that methane is the only gas lost from the rock.…”

Large Hydrogen Isotope Fractionation Distinguishes Nitrogenase-Derived Methane from Other Methane Sources

“…Individual data points from this study are shown as diamonds ( n = 31). The observed ranges for fermentative (green) and hydrogenotrophic (blue) mcr -based methanogenesis pathways and geological methane sources (red) were taken from reference 93 , although we note that these boundaries are not absolute (see, e.g., reference 37 ).…”

Measurement report: Methane (CH<sub>4</sub>) sources in Krakow, Poland: insights from isotope analysis

“…The heterogeneity of isotopic signatures from coal mining activities in the USCB reflects the geological complexity of the area. Secondary processes (desorption, diffusion or oxidation) also influence the CH 4 isotopic composition, and depend on external parameters such as physical characteristics of the coal reservoirs and the soil layers (Niemann and Whiticar (2017)). These represent additional difficulties as regards the isotopic characterisation of coal associated CH 4 emissions.…”