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such as the need for hidden geologic anomalies, some of which can decrease seam height, as well as maintain adequate ventilation air to the tailgate. Increased gob emissions and floor heaves are areas of concern as well. Premining degasifica- tions is needed, but with that comes a cap- ital expense. "My philosophy is all coal mines are gassy," Thakur said. "If anyone tells you [their] seam's not gassy, don't believe it." He defines the varying levels of gassi- ness as one of three categories: mildly gassy, with less than 100 ft 3 /ton of gas con- tent (less than 600 ft in depth); moderately gassy, with 100-300 ft 3 /ton of gas content (between 600-1,500 ft in depth); and high- ly gassy, with 300-700 ft 3 /ton of gas con- tent (1,500 to 3,000 ft in depth). The central core of the Pittsburgh No. 8 seam is moderately gassy and its outer rim is mildly gassy (see chart to the right). He noted that the mildly gassy mines, those with 75-80 ft 3 /ton, do not need pre- mining degasification, but gob wells are recommended on every panel. As the lev- els become moderate, such as those mines in the Pittsburgh seam's deepest area, pre- mining degas is needed in order to main- tain optimum production and keep gob gas at bay. Highly gassy mines, including those in the Alabama coalfield, are very difficult to degas, but it has been done with suc- cess via vertical drilling and fracking ini- t i a l l y , f o l l o w e d b y i n - m i n e h o r i z o n t a l drilling. "Each longwall panel [may be] 10,000 ft, 500 ft wide, and may have 20-40 gob wells," he said of the region. Each costs about $300,000, and mines must make money to defray the cost — which, by and large, they have done. While there are two ways to go about the process, horizontal drilling from the surface or in-mine horizontal drilling, the former is by far the cheaper option. Thakur, in fact, estimated a 10:1 cost dif- ference, though the impact is less if there is a way to market the gas and offset that expense. Additionally, fracking is not needed in the shallower seams with high permeability. In the Pittsburgh No. 8 seam, the goal is generally not to market or do anything else with the gas; operators are much more concerned with getting the gas out and safely mine the best coal with the best rates. In mines with higher permeability, where development can't be done, devel- opment boreholes are drilled to degas ahead of mining, followed by another drilled across. A typical panel, thus, may have eight or nine settings. In these places, the boreholes will also increase in density as they will have more time to produce, he said. "This strategy has worked very well. We [MEC] have recovered 50% of the gas," an ideal goal for the Pittsburgh No. 8 seam. "The Pittsburgh No. 8 seam is very pecu- liar. You never think you will recover 100% of gas; it will take 1,000 days, three years, to get 65% of this gas. If [a mine does] 50% in a year and a half, they're doing damn good," noting that most in-situ gas being recovered with this goal can be drained within six to 18 months. Post-mining Degasification Premining is not the only concern as it relates to gas in this key region of coal m i n e v e n t i l a t i o n c o n t i n u e d Thakur's definitions of the varying levels of gassiness; the central core of the Pittsburgh No. 8 seam is moderately gassy and its outer rim is mildly gassy. Mildly gassy mines do not need premining degasification, but gob wells are recommended on every panel. As the levels become moderate, such as in the Pittsburgh No. 8 seam's deepest area, premining degas is needed in order to maintain optimum production and keep gob gas at bay. Ventilation layout for a longwall in moderately gassy coal seams. February 2016 www.coalage.com 29