Surface borehole
Surface Borehole
Surface Borehole are a connection between the surface and the minig area by a vertikal borehole. Most used vertical gob wells are developed from the surface in advance of mining. The usual practice is to drill large-diameter (up to 300 mm) vertical gob wells in advance of mining to within 10 to 30 m above the working coal seam.
A special kind of surface degassing borehole are used in eastern countries. This borehole with a diameter up to 2000 mm locate in a raodway were the longwall starts. The exhausted gas mixture contins mostly less than 4% CH4. The volume is up to 2000 m3 / min.
Boreholes for degassing with more than 25 % CH4
This typ of boreholes are often used in gassy mines to extract the gas from the roof of the layer. The extracted gas contains high percentage of methane mostly more than 90 % CH4 before to longwall passes this location. After that the amount of CMM increases but the part of methane in the gas mixturedecreases.
The Scheme of a surface borehole is shown in the picture.
This gob degasification technique involved vertical wells developed from the surface in advance of mining. These holes provide an opportunity to start degassing the coal mining area before digging the caol. This implies that the surface above the mining can be used for such a drilling.
Operators who use vertical gob wells in favourable geologic and reservoir settings have claimed high methane capture efficiencies.
(Hier noch link zu <<vertical gob wells.pdf>> einfügen)
More information you get in http://www.epa.gov/cmop/docs/ggasrecpv.pdf
Ventilation Borehole
Ventilation boreholes are a part of ventilation system of the mine. They are also used for special degassing purposes. Often a great part of the methane is released in the gob, specially near the longwall. The function of the ventilation borehole is creating a direction of depression for the gas in the gob.
