Handbook for Methane Control in Mining - AMMSA

104obvious solution to this problem would be to increase the face ventilation, if not already at thepractical limit.If past experience indicates that excess emissionsmay be encountered in a developing section, themine operator should consider an undergroundhorizontal borehole methane drainage system orvertical methane drainage boreholes drilled fromthe surface to remove gas prior to mining. Mineoperators should always be aware of regional jointand fracture trends to facilitate prediction andremediation of abnormal emissions associated withjoints and fractures.Clay veins. Clay veins or clastic dikes (Figure 7–6) are sedimentary intrusions, usually fromoverlying strata, that invade the coal in a vertical or near-vertical orientation. Their appearancein cross-section is not unlike an igneous dike, hence their name. Clay veins tend to be systematicin occurrence and are often related to differential compaction/diagenetic processes, but can alsobe influenced by tectonic processes.These features, their characteristics, and modes of formation have been widely documented inU.S. coal mines [Chase and Ulery 1987]. Clay veins, which are generally composed of veryfine-grain sediment or clay,are virtually impermeablebarriers to gas migration incoalbeds. Therefore, theytend to have a “damming”effect on gas flow whenapproached in a developingsection. When a continuousminer or longwall shearerpenetrates a clay vein withgas trapped behind it, theabnormally high emissionsmay cause production interruptionsdue to methaneconcentrations above thelegal limit, and in a worstcasescenario, an explosivemethane/air mixture couldignite.Figure 7–6.—Typical clay vein acting as a barrier to gas migration.