To quantify and rank gas wettability of coal as a key parameter affecting the extent of CO 2 sequestration in coal and CH 4 recovery from coal, we developed a contact angle measuring system based on a captive gas bubble technique. We used this system to study the gas wetting properties of an Australian coal from the Sydney Basin. Gas bubbles were generated and captivated beneath a coal sample within a distilled water-filled (pH 5.7) pressurised cell. Because of the use of distilled water, and the continuous dissolution and shrinkage of the gas bubble in water during measurement, the contact angles measured correspond to a 'transient receding' contact angle. To take into account the mixed-gas nature (CO 2 , CH 4 , and to a lesser extent N 2 ) of coal seam gas in the basin, we evaluated the relative wettability of coal by CH 4 , CO 2 and N 2 gases in the presence of water. Measurements were taken at various pressures of up to 15 MPa for CH 4 and N 2 , and up to 6 MPa for CO 2 at a constant temperature of 22°C. Overall, our results show that CO 2 wets coal more extensively than CH 4 , which in turn wets coal slightly more than N 2 . Moreover, the contact angle reduces as the pressure increases, and becomes < 90°at various pressures depending on the gas type. In other words, all three gases wet coal better than water under sufficiently high pressure.

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SummaryThe subsurface represents a largely unexplored frontier in microbiology. Here, coal seams present something of an oasis for microbial life, providing moisture, warmth, and abundant fossilized organic material. Microbes in coal seams are thought to syntrophically mobilize fossilized carbon from the geosphere to the biosphere. Despite the environmental and economic importance of this process, little is known about the microbial ecology of coal seams. In the current study, ecological succession and spatial niche partitioning are explored in three coal seam microbial communities. Scanning electron microscopic visualization and 16S rRNA sequencing track changes in microbial communities over time, revealing distinct attached and planktonic communities displaying patterns of ecological succession. Attachment to the coal surface is biofilm mediated on Surat coal, whereas microbes on Sydney and Gunnedah coal show different attachment processes. This study demonstrates that coal seam microbial communities undergo spatial niche partitioning during periods of succession as microbes colonize coal environments.