Page 309 - Advanced Mine Ventilation

286 Advanced Mine Ventilation

fracture and keeps it open for gas to escape to surface. Several coal seams/gas-bearing
horizons can be fracked from a single well.
Under ideal conditions, 60%e70% of the gas in the coal seams can be removed by
vertical wells if drilled and produced for 5e10 years in advance of mining. Reservoir
pressures can be reduced by 80% prior to mining. This technique is ideally suited to
deep, very gassy coal seams with low to medium permeability (1e10 md). Its effec-
tiveness may be reduced if the coal seam is deeper than 4000 ft where the coal
seam permeability is generally less than one microdarcy.

17.1.1.2 Horizontal Boreholes Drilled from Surface
It is a relatively new technique where a vertical well is drilled from surface, but it is
deviated by 90 degrees to make it horizontal and intersect a number of coal seams
that needs to be degassed. Fig. 17.2 shows the general layout of a typical drilling
scheme.
The horizontal extension can be 3000e10,000 ft depending on the depth of the coal
seam and the type of drill rig used. The borehole can be lined with slotted liners to
guard against accidental closure due to ground movement. Gas production even prior
to mining could be quite brisk if the coal seams are less than 2000 ft deep and have
medium to high permeability. Gas production is likely to be much higher when the
coal seam overlying these horizontal boreholes is mined out and the underlying coal
seam heaves and experiences an order of magnitude improvement in permeability.

Figure 17.2 Horizontal
boreholes in multiple seams
drilled from surface.