Influence of the Consumption of Oxygen from Coalbed Gas Composition from Desorption Canisters
Desorption canisters are routinely employed to quantify coalbed gas contents in coals. If no purging with inert gas or water flooding is used, entrapment of air with ~79 vol. % nitrogen (N2) in canisters during the loading of coal results in air contamination and subsequent overestimates of N2 in desorbed coalbed gas. Coalbed gas does not contain any elemental oxygen (O2) whereas air contamination originally includes ~20 vol. % O2 and has a N2/O2 volume ratio of ~3.95. A correction for atmospheric N2 is often attempted by quantifying O2 in gas and proportionally subtracting atmospheric N2. However, this study shows that O2 is not a conservative proxy for air contamination in desorption canisters. Time-series of gas chromatographic (GC) compositional data from several desorption experiments using high volatile bituminous coals from the Illinois Basin document that O2 is rapidly consumed, especially during the early phase of desorption. After about 2 weeks of desorption, the concentration of O2 has declined to near or below GC detection limits. Irreversible loss of O2 in desorption canisters is caused by biological, chemical, and physical mechanisms. The use of O2 as a proxy for air contamination is justified only within a few hours after loading of desorption canisters, but such rapid measurements preclude meaningful assessment of coalbed gas concentrations. With increasing time and progressive loss of O2, the use of O2 content as a proxy for atmospheric N2 results in exacerbated overestimates of N2 in desorbed coalbed gas and, consequently, erroneous reports on gas quality .
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009