Page 397 - Advanced Mine Ventilation

366 Advanced Mine Ventilation

Figure 22.2 Degasification scheme for a highly gassy coal seam.

Frac wells are typically drilled at a 20-acre spacing, 5 to 10 years ahead of mining,
and can drain nearly 50% of the gas contained in coal. Additional degasification is
generally necessary to remove another 20 to 30% of the in situ gas contents. The
secondary degasification is done with horizontal drilling across the longwall panel
at 100e200 ft intervals. These boreholes can be extended beyond the longwall panel
to intersect and degas the next set of development headings. Additional details of
these techniques can be found in the literature [4] as well as Chapters 15 to 17 in
this book.

22.2 Ventilation

Even after the coal seam is properly degassed, development headings and longwall
faces must be properly ventilated to prevent dangerous accumulations of methane.
Methane concentrations in mine air are restricted to 1% by law everywhere except
in the bleeders where the methane concentration can go up to 2%. A minimum velocity
of 60 ft/min and a minimum ventilation air quantity of 3000 CFM at the face in the
development heading is also required by the Federal laws. In coal seams containing
3
less than 100 ft /t of methane, the abovementioned requirements may be sufficient,
but for gassier coal seams a different criterion must be used to calculate ventilation
air quantities and prevent the layering of methane in roof areas. Methane layering in
mine airways, especially near the working faces, can occur if there are high methane
emissions and ventilation air velocity may not be high enough to create good turbulent
mixing. These gas layers can be easily ignited by hot cutting tools resulting in
frictional ignitions.