Page 209 - Advances In Productive, Safe, and Responsible Coal Mining

188 Advances in Productive, Safe, and Responsible Coal Mining

and offset investment costs. International studies determined that 30%–40% of all coal
mines produce suitable CMM that can be effectively used for power generation with
gas engines. Therefore, drainage systems can drain and produce commercial CMM as
well as reduce and prevent health and safety risks.
The purpose of methane drainage is to capture CMM in high purity form. It is fre-
quently performed in one of two ways. First, in advance of mining, boreholes are
drilled into the unmined seam (Fig. 10.3) and in situ CMM is collected. Second, after
coal exploitation, boreholes are drilled into the gob area (Fig. 10.4) and liberated
CMM is collected. The former is called a “predrainage system” meaning that
CMM is drained ahead of mining. The latter is called a “postdrainage system” mean-
ing that CMM is drained after mining. Pre- and postdrainage systems can be per-
formed by both in-seam horizontal or crossmeasure boreholes drilled underground
from active workings or by vertical wells drilled from the surface.
As shown in Fig. 10.4A, it is possible to combine both pre- and postdrainage
approaches. In this scenario, the drainage system consists of drilling underground hor-
izontal or crossmeasure boreholes from the mine workings into the unmined coal seam
or into gob areas. As horizontal boreholes degasify the unmined coal seam,
crossmeasure boreholes may also be drilled to effectively degasify the fractured rock
strata above the coal seam in the gob area. Underground boreholes are typically
10–100m (33–330ft) in length, and within a single mine several hundred boreholes

Cross-measure boreholes

Horizontal boreholes

Direction of mining

Gate road

Mining equipment

Fig. 10.3 Schematic of predrainage system using horizontal or crossmeasure boreholes [11].