The increase in value of energy resources has generated serious interest in so-called "unconventional energy resource developments", Coal and coal gas recovery schemes have existed for some years, but the application of petroleum technology to the recovery of coal gas on a commercial scale is relatively new. The methane gas produced from coal seams, once considered a menace to coal mining operations, can be recovered and produced using technology currently available.
It is the intent of this paper to demonstrate that recovery of this unconventional gas resource is both practical and economic. The generalized design parameters and the important considerations involved in the design of such a system are discussed. In addition, specific references are made to the considerations relevant to the Brookwood, Alabama Coal Degasification Project, a gas production project which is currently demonstrating the viability of this technology.
The emission of methane gas from coal seams has been a persistent safety and economic problem since the beginning of the coal mining industry. In order to operate successfully, sub-surface mines have traditionally been engineered to vent as much methane as possible. While the need for venting increases mine capital and operating cost, this has been viewed as preferential to the alternative of reducing coal extraction rates or, in the extreme, to mine closure.
Recent innovations to reduce gas concentrations, via vertical well bores, in advance of active mining operations have created a new method of coal degasification and a new source of a clean fuel.
The virtually pure methane gas given off during coal mining operations, if gathered and conserved carefully, can either be used to reduce mine fuel costs, or alternatively, can be sold to gas distribution firms at a profit. The application of conventional petroleum recovery technology to the recovery of the methane gas has proven to be a viable method of not only reducing gas emissions into the mining operation, but also of collecting a valuable product for sale.
Methane gas can be found in coal beds either as free gas residing in the coal seam fractures or as a thin layer of gas molecules adsorbed onto the walls of minutely sized pores within the coal structure. The amount of gas stored in a particular coal seam is a function of both the degree of fracturing of the coal and of the properties of the coal seam itself. The amount of gas which can be stored within some selected coal types at varying pressure levels has been reported by Kissell1. Figure 1 demonstrates that certain coal seams, if fully saturated at critical seam pressure, will release up to 15 cubic meters of methane gas per tonne (500 cubic feet per ton). This gas volume is in addition to that gas which may be stored in the coal fractures.
This figure also demonstrates an interesting natural law regarding gas production rates expected from gas wells drilled into coal seams.