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Abstract

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AAPG Bulletin, V. 86, No. 2 (February 2002), P. 201-212.

Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

Controls on methane sorption capacity of Indian coals

Chikatamarla Laxminarayana,1 Peter J. Crosdale2

1Power and Energy Division, 342, Yojana Bhavan, Sansad Marg, New Delhi, 110001, India; email: [email protected]
2Coalseam Gas Research Institute, School of Earth Sciences, James Cook University, Townsville, QLD 4811, Australia; email: [email protected]

AUTHORS

Chikatamarla Laxminarayana is currently working on the acid gas sequestration project with Marc Bustin at the University of British Columbia (UBC), Canada. He is a deputy adviser (energy policy) in the Planning Commission, Government of India, New Delhi and presently on study leave for a postdoctoral fellowship with UBC. He has more than 20 years of experience in technical aspects, management, policy planning, development, and construction of coal projects and coalbed methane research. He has a bachelor's degree in mining engineering (distinction) from Osmania University, Hyderabad, a master's degree in rock mechanics (distinction) from the Indian Institute of Technology, Delhi, India, and a Ph.D. in the coalbed methane area from James Cook University, Australia.

Peter Crosdale is currently director of the Coalseam Gas Research Institute, James Cook University, Australia. He has a bachelor's degree (honors) in geology from the University of Newcastle, Australia, and completed a Ph.D. in coal depositional environments at the University of Auckland, New Zealand. He has more than 18 years of experience in coal geology, coal petrology, and coalbed methane in both academic and industry environments. He is also editor for the International Committee of Coal and Organic Petrology (ICCP).

ACKNOWLEDGMENTS

We thank the Planning Commission and Ministry of Coal, Government of India, for permitting us to carry out the field work in India. D. N. Prasad, Deputy Adviser (Coal), Planning Commission; S. K. Bhattacharya, Deputy Chief of Geology, Central Coalfields Limited; and A. Manohar Rao, Deputy Chief Mining Engineer, Singareni Collieries Co. Ltd. are gratefully acknowledged for their coordination in the field work.

ABSTRACT

Details of methane sorption properties of some Indian coals ranging in rank from high-volatile bituminous C (0.62% Ro max) to medium-volatile bituminous coal (1.46% Ro max) were investigated to determine the major factors influencing gas adsorption capacity and desorption rate. Variables studied included moisture content, mineral-matter content, rank, and coal type (maceral composition).

Adsorption isotherm analysis of dry coals showed that adsorption capacity followed a second-order polynomial trend with rank. Equilibrium moist samples showed a linear increase in adsorption capacity with rank and had a significantly reduced adsorption capacity compared to the dry coals, with the reduction being related to the moisture content. Mineral matter acted as a simple diluent to the gas adsorption capacity of the coals and was found to be nonadsorbent. Adsorption capacity (moist) was reduced by 0.32 cm3/g (10 ft3 gas/t) for every 1% increase in the ash yield.

Bright and dull coal lithotypes showed strong separation in their adsorption capacities on an as analyzed basis, with the bright coals adsorbing greater quantities of gas. On a dmmf basis, however, no relationship was observed between coal type and gas storage capacity. Comparison of maceral composition with adsorption capacity (dry) confirmed this observation.

Effective diffusivity (De) of methane through the coal was seen to be affected by coal type and rank. Effective diffusivity, De, decreased as rank increased, which is related to the increasing microporous nature of the coal. Bulk coals tested had 2-3 times larger effective diffusivities than bright coals, and dull coals had intermediate rates. The larger De values for the bulk coals may be related to the presence of collodetrinite coupled with mineral matter, which acts as higher permeability pathways for the gas compared with the high-ash dull coals dominated by inertinite macerals.

Differences were noted with similar-age (Permian) Australian coals, where methane adsorption capacities were larger and coal type had a stronger influence on adsorption capacity.

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