American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the SPE 4th Annual Eastern Regional Meeting to be held in Pittsburgh, Pa., Nov. 2–3, 1967. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.
Gas occurs in coalbeds in an adsorbed and a free gas state. Adsorbed gas is stored in the micropore structure and its transport is governed by Fick's law. The free gas occurs in the fracture system and flows according to Darcy's law. These two modes of mass transport are interdependent. Production decline curves are of the constant percentage decline type and, thus, show no indications of flow characteristics peculiar to coal-gas reservoirs. The effectiveness of surface boreholes as a degasification scheme depends upon both good fracture permeability and a high fracture density. Conventional methods of reservoir engineering analysis are not applicable to coalbeds.
Mining of deep coalbeds (1,500 to 2,000 ft.) and production of the associated gas is analogous to a gas well with an expanding well bore radius. Gas production of 10 to 15 MMscfd is not uncommon from deep mines. State and Federal codes require that each unit vol. of gas be diluted with approximately 100 vol. of air. Methane-air mixtures in the range 5 to 15 pct. methane are explosive.
Mining deeper coalbeds requires smaller mine openings and these mines tend to be more gassy. Thus, the use of ventilation (dilution) as a means of controlling methane concentrations at active face areas and in the returns is limited severely. Degasification of coalbeds prior to mining and methods of controlling the flow during mining are necessary. Before effective methods of control and degasification can be developed, the laws governing mass transport through coalbeds must be understood.
The purpose of this paper is to present the fundamental concepts governing the transport of gases through a coalbed, and to define one of the environmental problems related to rapid, economical, and safe mining of coal deposits.
Coal is formed from plant substances which mere preserved in various states of degradation in a favorable environment, and later altered by chemical and physical processes. However, there is no universal agreement on what chemical and physical changes take place in the transformation of plant substances to coal.