Factors affecting methane emission and their implications regarding the selection of control techniques.
Proceedings of a conference on the underground mining environment, October 27-29, 1971, University of Missouri, Rolla, Missouri. Rolla, Missouri. Christiansen CR, ed. Rolla, MO: University of Missouri, 1972 Oct; :229-240
The best thing that can be said of the methane problem is that most of the gas produced during the coalification stage is gone. Tens of thousands of cubic feet of methane are produced in those reactions that transform vegetation into a ton of coal, and on the average only a few hundred cubic feet of methane remain. Even this amount creates enormous problems. A ton of coal is approximately 25 cubic feet; this means about 10 cubic feet of gas for 1 cubic foot of coal. It has been said that mining of deep coal seams is like digging out a low-grade gas well. Coal adsorbs gas like a low-grade activated charcoal. Because of its basic graphitic structure it contains an extensive network of fine pores, called micropores. These micropores are extremely small; diameters have been estimated in the 5 to 10 angstrom range. As a result, coal, like charcoal, has a large internal surface. One hundred square meters per gram is a typical figure for coal. Most of the gas in coal is adsorbed on this internal surface under pressure. These pressures can range from just a few psi upwards. Gas pressures as high as 650 psi have been measured in U.S. coalbeds, and the Russians have seen over a thousand. It has been known for a long time that the pressure, and consequently the amount of gas, correlates with the depth of the coalbed more than with anything else, and that shallow mines have less gas. Of course there are other factors: for instance, the coalbed around an older mine has been draining into the mine for some time and therefore is at lower pressure and contains less gas. Factors like this natural drainage are the subject of our studies. Unfortunately, high gas pressure is not the only problem: Deep coalbeds that contain a lot of gas under high pressures also tend to have a very low permeability. In fact, permeabilities of coal seams can be lower by a factor of 100 or 1,000 than the permeability of rock in which there are gas wells. The reason for this is that flow through gas reservoir rocks is to a large extent an intergranular flow; in a sandstone, for instance, gas flows in the interstices between the sand grains. Coal does not have this kind of granular structure, and the flow of gas through coal is primarily through cracks in the coal itself. Now, if you compress a lump of sandstone, you do not change the intergranular porosity more than 5 to 10 percent because of the way the sand grains pack. The result is that permeability of a sandstone will not change more than 50 percent with an applied pressure. However, if a material contains flat cracks, like coal, an external pressure, such as that created by the weight of the overburden, causes these cracks to close tightly. Under a thousand feet of overburden, the permeability is perhaps a hundred to a thousand times lower than it would be were that coal at the surface. It can be as low as a few tenths of a millidarcy. This combination of high pressures and low permeabilities means that some remarkably high pressure gradients can exist. For instance, at a deep mine in the Pocahontas No. 3 coalbed, one of our drill crews drilled a 20 foot horizontal hole into an active face. All but the last foot or so of the hole was sealed with expandable packers, and the pressure at the end of the hole was measured. It was over 250 psi. It does not take very long for a continuous miner to advance 20 feet--perhaps an hour--and it is not difficult under these circumstances to see why methane is a problem. Fortunately, this is an extreme case, and relatively few mines have problems this severe.
Coal; Coal mining; Coal processing; Mining industry; Underground mines; Underground mining; Methane control
Maurice Deul, Supervisory Geologist, U.S. Department of the Interior Bureau of Mines, Pittsburgh Mining and Safety Researc h Center Pittsburgh, Pennsylvania
OP; Conference/Symposia Proceedings
Proceedings of a conference on the underground mining environment, October 27-29, 1971, University of Missouri, Rolla, Missouri