Abstract
Coalbed gases from 11 wells producing from the Springfield and Seelyville Coal Members (Pennsylvanian) were analyzed for composition and carbon and hydrogen stable isotope ratios in four sampling events to investigate short-term variation trends. Nine wells in the Seelyville Coal Member produce coalbed gases from the virgin seam, whereas two wells in the Springfield Coal Member produce gas from mine voids. Methane dominates gas composition in all wells, and its content ranges from ~94% to almost 98%, with ethane typically accounting for less than 0.01%. Carbon dioxide content in most samples is below 1%, whereas N2 content ranges from less than 2% to 4.8%. Methane δ13C values range from −55.3‰ to −61.1‰, and δ2H values range from −201‰ to −219‰. Isotopic values of methane and C1/(C2 + C3) ratios indicate a biogenic origin along the CO2-reduction pathway, consistent with previous studies in this area. Our results demonstrate that gas properties may change significantly during a period of one year of production history. Compositional trends (e.g., C1/(C2 + C3), CH4/CO2 ratios) are specific for each well and often irregular. These changes result from a combined influence of numerous factors and, therefore, are difficult to predict.
Highlights
The Illinois Basin contains Pennsylvanian coals (Figure 1) that are of economic importance mostly because they are used to generate electricity
All coalbed methane (CBM) wells are vertical; nine wells produce gas from the Seelyville Coal, whereas two wells reach into the Springfield Coal nine wells produce gas from the Seelyville Coal, whereas two wells reach into the Springfield Coal (Jackson 1 and Creed 1) and produce gas from mine voids
CBM production from Seelyville coal derives from a depth of 152 to 183 m (500–600 ft)
Summary
The Illinois Basin contains Pennsylvanian coals (Figure 1) that are of economic importance mostly because they are used to generate electricity. The coals are of high-volatile C to B bituminous rank [1,2], with higher rank (high-volatile A bituminous) present only in the southeastern part of the basin in western Kentucky. The latter is related to intense tectonic activity and a higher geothermal gradient along the Rough Creek and Pennyrile fault systems [3]. With this relatively low rank, the coal has never reached the window of significant thermogenic gas generation. Inoculation of coalbeds with microbial consortia and the onset of biodegradation and methanogenesis likely occurred during
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