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Abstract
Despite the significance of biogenic methane generation in coal beds, there has never been a systematic long-term evaluation of the ecological response to biostimulation for enhanced methanogenesis in situ. Biostimulation tests in a gas-free coal seam were analysed over 1.5 years encompassing methane production, cell abundance, planktonic and surface associated community composition and chemical parameters of the coal formation water. Evidence is presented that sulfate reducing bacteria are energy limited whilst methanogenic archaea are nutrient limited. Methane production was highest in a nutrient amended well after an oxic preincubation phase to enhance coal biofragmentation (calcium peroxide amendment). Compound-specific isotope analyses indicated the predominance of acetoclastic methanogenesis. Acetoclastic methanogenic archaea of the Methanosaeta and Methanosarcina genera increased with methane concentration. Acetate was the main precursor for methanogenesis, however more acetate was consumed than methane produced in an acetate amended well. DNA stable isotope probing showed incorporation of 13C-labelled acetate into methanogenic archaea, Geobacter species and sulfate reducing bacteria. Community characterisation of coal surfaces confirmed that methanogenic archaea make up a substantial proportion of coal associated biofilm communities. Ultimately, methane production from a gas-free subbituminous coal seam was stimulated despite high concentrations of sulfate and sulfate-reducing bacteria in the coal formation water. These findings provide a new conceptual framework for understanding the coal reservoir biosphere.
Highlights
Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Gas produced by biological processes has a large role to play in meeting rising energy needs worldwide [1]
The oxygenic treatment involved the addition of 100 mg calcium peroxide (Sigma Aldrich, Australia) contained in a fine mesh inside a permeable PVC canister, connected to the bottom of a larger permeable PVC canister containing coal pieces of the size of ~3 cm3 per coal piece taken from the original coal cores and placed in the formation water such that coal pieces could be retrieved for microbial community characterisation and the peroxide treatment could be terminated by removal of the connected canister (Fig. 1)
Composition and geochemical characteristics of the coal formation water and the coal from all 4 wells used at the trial site in the Western Coalfields of NSW, Australia were analysed (Table 1, Table S1)
Summary
Gas produced by biological processes (biogas) has a large role to play in meeting rising energy needs worldwide [1]. Coal-bed methane (CBM) is a relatively untapped energy source representing up to 20% of the world’s biogas resources [2,3,4] and is contained in actively mined and abandoned coal reservoirs globally [5]. Only incremental improvements of in situ biogas yields from coal have been observed [5, 8]. Mixed biogenic (δ13C value less than −60‰) and thermogenic (δ13C value greater than −50‰) signatures [10, 11] have been observed in many coal reservoirs worldwide [5]. The yield of biogenic methane is dependent on the degree of coal biodegradation
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