Handbook for Methane Control in Mining - AMMSA

94• Estimated cost for moderately gassy coal seams, 100–300 ft 3 /ton (see Table 6–2):Premining degasification: The longwall panel should be drilled horizontally at 1,000-ft intervals,and development boreholes should be drilled to degas development sections. Total in-mine drillingfootage for a typical panel may total 25,000 ft.Postmining degasification: In moderately gassy coal seams, a proposed longwall panel may needfive to six gob wells. The diameter and size of exhaust fans will depend on local conditions.The total cost is approximately $1.55 million, or $0.50/ton.• Estimated cost for highly gassy coal seams over 300 ft 3 /ton (see Table 6–2):Premining degasification: Highly gassy coal seams must be drained several years ahead of miningwith vertical frac wells (wells that have been hydraulically fractured). These frac wells canbe placed at about a 20-acre spacing. Frac wells drilled about 5 years ahead of mining can drainnearly 50% of the in situ gas prior to mining, but this may not be sufficient. Additional degasificationwith in-mine horizontal drilling can raise the gas drained to nearly 70%. Horizontalboreholes are drilled 200–300 ft apart to a depth of 900 ft. Assuming a 200-ft interval, nearly45,000 ft of horizontal drilling and about 15 vertical frac wells may be needed to properly degasthe panel.Postmining degasification: Because of very high gas emissions from the gob, the first gob wellmust be installed within 50–100 ft from the setup entry. Subsequent gob wells may be drilled ata 6- to 15-acre spacing, depending on the rate of mining and the gas emission per acre of gob.In Virginia and Alabama, which have some highly gassy coal seams, gob wells are generally9–12 inches in diameter. Powerful exhaust fans capable of a suction of 5–10 inches of mercuryare needed to capture up to 70% of gob gas emissions.The total cost of degasifying a longwall panel in a highly gassy coal seam is approximately$11 million, or $3.52/ton. Coal seam degasification is needed for mine safety and high productivity,but in highly gassy mines it becomes quite expensive. In these mines, the processing andmarketing of coal mine methane becomes necessary to defray the cost.REFERENCESCreedy DP, Saghafi A, Lama R [1997]. Gas control in underground coal mining. IEACR/91.London: IEA Coal Research.Davidson RM, Sloss LL, Clarke LB [1995]. Coalbed methane extraction. IEACR/76. London:IEA Coal Research.Davis JG, Krickovic S [1973]. Gob degasification research: a case history. In: Methane controlin eastern U.S. coal mines. Proceedings of the Symposium of the Bureau of Mines/IndustryTechnology Transfer Seminar, Morgantown, WV, May 30–31, 1973. Washington, DC: U.S.Department of the Interior, Bureau of Mines, IC 8621, pp. 62–72.