Page 318 - Advanced Mine Ventilation

Floor Gas Emissions and Gas Outbursts 295

Figure 17.8 A typical horizontal well drilled from the surface for deep coal.

coal seam (usually 100e200 ft below). The well is logged again to choose the location
where the deviated borehole will start. Assuming that the kickoff point will be at
7500 ft, the bottom of the vertical well is cemented to a depth of 6000 ft. The direc-
tional borehole assembly is lowered in the well and the well is drilled to the target
kickoff point of 7500 ft. Next, the curve is drilled with foam mist to make the vertical
well a horizontal one. The drill cuttings indicate if the well has entered the coal seam.
The rate of angle build is 8e12 degrees per 100 ft. A 2 degrees bent housing is used for
this purpose. Directional control of the well is usually provided by professional direc-
5
tional drillers. The horizontal lateral is drilled with a PCD bit of 8 / 8 -in. diameter and a
mud-driven motor (a Moyno pump in reverse).
In coal seams, all drilling is done with foam, but in shale, they use a 12e14 ppg
mud (12e14 lbs of mud in a gallon of water). The horizontal drilling continues until
the target depth is reached. For a 3000 ft lateral, the target depth would be about
1
11,000 ft. The drill string is tripped out and a 5 / 2 in., 20 lb/ft, P-110 casing is
cemented in the entire well.