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AAPG Bulletin
Abstract
AAPG Bulletin, V.
2004. The American Association of Petroleum Geologists. All rights reserved.
Differential transport of CO2 and CH4 in coalbed aquifers: Implications for coalbed gas distribution and composition
Xiaojun Cui,1 R. Marc Bustin,2 Gregory Dipple3
1Department of Earth 
Ocean Sciences, University of British Columbia, Vancouver, Canada; [email protected]
2Department of Earth Ocean Sciences, University of British Columbia, Vancouver, Canada; [email protected]
3Department of Earth Ocean Sciences, University of British Columbia, Vancouver, Canada; [email protected]
AUTHORS
Xiaojun Cui received a B.S. degree in geology from China University of Geosciences, Wuhan, an M.S. degree in geodynamics from Beijing University, and a Ph.D. in geological sciences from the University of Missouri, Columbia. Currently a postdoctoral fellow at the University of British Columbia, his research applies computer modeling to fluid flow in various geological systems.
R. Marc Bustin is a professor of petroleum and coal geology in the Department of Earth Ocean Sciences at the University of British Columbia, president of R. Marc Bustin Earth Science Consultants, and a principal of Coalbed Methane Solutions Ltd. Bustin's professional experience includes employment by Mobil Oil Canada, Gulf Canada Resources, Elf-Aquitaine (France), the Commonwealth Scientific and Industrial Research Organization (France), and the Centre National de la Recherche Scientifique (Australia). Bustin's research and consultancy is in the area of unconventional gas reservoirs, gas shales, and coalbed methane.
Greg Dipple is a geochemist and an associate professor at the University of British Columbia with research interests in reaction and transport during water-rock interactions and CO2 sequestration.
ACKNOWLEDGMENTS
This research was funded by the National Sciences and Engineering Research Council of Canada. We thank J. Lorenz, W. B. Ayers, S. E. Laubach, M. A. Smith, and an anonymous reviewer for their helpful reviews and comments to improve this work.
ABSTRACT
Coalbed gas content and composition are critical for the successful exploration and production of coalbed methane. We investigated the differential transport of a preadsorbed CO2-CH4 gas mixture along a coalbed aquifer by groundwater flow or across a coal bed by upward diffusion through pore water. Consistent analytical approximations and numerical solutions were obtained for typical cases. The results suggest that differential transport of CH4 and CO2 in a water-saturated coal seam is mainly controlled by their adsorption equilibrium in coals and solubilities in water. Although CO2 is about 20 times more soluble than CH4 in water at temperatures lower than 50
C, the transport of CO2 from a coal seam is only several times more efficient than that of CH4 because of the stronger adsorption of CO2 than CH4 in coals. Preferential transport of CO2 over CH4 by groundwater advection can substantially accumulate more gas rich in CO2 at downstream or near discharge zones if gas-trapping structures exist, and it may cause low total gas content of CH4 near recharge zones, which may explain the CO2 and CH4 distribution in the San Juan and Powder River basins and the undersaturation of gas in many other coals. The separation and redistribution of coalbed gas by transport processes may also be complicated by late-stage biogenic gas generation with infiltration of meteoric water with nutrients and gas-producing microbes into the coal beds as suggested for some basins. Overall, hydrogeologic systems strongly influence the distribution of coalbed gas content and composition in coal seams.
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