Surface boreholes are a connection between the surface and the mining area via a vertical borehole. In most cases, used vertical gob wells are developed from the surface before mining. The usual practice is to drill large-diameter (up to 300 mm) vertical gob wells prior to mining to 10 to 30 m above the working coal seam.
A special kind of surface degassing borehole is used in Eastern countries. This borehole with a diameter up to 2000 mm is located in a roadway were the longwall starts. The exhausted gas mixture contains mostly less than 4% CH4. The volume is up to 2000 m³ / min.
Boreholes for degassing with more than 25 % CH4
This type of boreholes is often used in gassy mines to extract the gas from the roof of the layer. The extracted gas contains a high percentage of methane mostly, more than 90 % CH4 before the longwall passes this borehole. Subsequently the amount of CMM increases, while the part of methane in the gas mixture decreases.
The scheme of a surface borehole is shown in the picture.
This gob degasification technique involved vertical wells developed from the surface before mining activity. These holes provide an opportunity to start degassing the coal mining area before digging the coal. This implies that the surface above the mining can be used for such a drilling.
Operators that use vertical gob wells in favourable geologic and reservoir settings have claimed high methane capture efficiencies.
More information are available in http://www.epa.gov/cmop/docs/ggasrecpv.pdf
Ventilation boreholes are part of the 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, especially near the longwall face. The function of the ventilation borehole is creating a direction of depression for the gas in the gob.
Because of large leaks between the gob and the evacuated part of the gallery behind it, part of the ventilation air is guided to this borehole. The exhausted air-gas-mixturer should have a methane concentration of less than 4%, so that no explosive mixtures can develop. There is a danger that higher concentrations are achieved when a high amount of gas is released in the gob.