Development and application of reservoir models for the evaluation and optimization of longwall methane control systems

Comprehensive assessments of the need for additional methane control capacity beyond ventilation often require both an empirical and theoretical approach for an adequate or timely control of increased methane emission levels. Thus, the prediction of methane emissions and optimization of methane control systems prior to starting a new mining operation will be a major improvement towards eliminating the explosions in the underground workplace. During longwall mining, the caving of immediate strata and stress relief create horizontal fractures along bedding planes and vertical fractures in the strata overlying the caved zone. These fractures provide an extensive pathway for gas migration from the surrounding coalbeds and other gas bearing strata into the longwall mining environment (Fig. 1). The thickness of the fractured zone can vary up to 100 times the height of the mined coalbed (Palchik 2003). The fractured and caved rock mass left behind the advancing longwall face is generally referred to collectively as “gob” (Fig. 1). The methane that originates and accumulates in the gob above the mined-out longwall panel is the main source of potential gas emissions during longwall mining. Gob gas extraction in the Northeastern U.S. is almost exclusively accomplished using ventholes that are drilled from the surface to within a short distance [typically 10-15 m (30-45 ft)] of the coalbed being mined (Diamond 1994). Commonly, the bottom section of the well casing [generally about 60 m (200 ft)] is slotted. The gob gas ventholes generally become productive only when the mining-induced fractures are created as mining advances under the venthole (Diamond 1994).