Mining regions that have had extensive activity, such as the Appalachian coal fields in the U.S., have incomplete archives and cannot accurately specify the location of old mining workings. In this region, auger mining, or drift mining from outcrops has been extensive and modern mining operations must navigate away from these potentially water-filled workings. MSHA guidelines require that mines developing near old works drill at frequent intervals from developments to ensure that the new workings are driven sufficiently away from the abandoned ones.
In-seam methane drainage and exploration drilling requires maintaining the borehole in the seam. Plotting the profile of the borehole from downhole surveys and driller logs, provides the mining operation with an indication of coal seam structure. Any discontinuities intercepted during drilling, such as faults, palaeochannels, intrusions, etc., are detected by monitoring drilling fluid pressures, changes in thrust, vibration, rate of penetration, and inspection of cuttings. Because of the high placement accuracy, directional drilling can determine the thickness of coal seams and normal fault displacements with reasonable precision.
Directional drilling technology provides gassy mining operations a new range of methane drainage possibilities. Long, directionally steered in-seam boreholes can effectively reduce insitu gas contents of large coal volumes in advance of mining in low to high permeability coals. With longhole drilling capabilities, drilling can effectively drain coals in the mining, underlying or overlying level.
Horizontal gob boreholes are directionally drilled over the mining seam in advance of the longwall face and placed vertically in what will be the fracture zone when the gob forms, and typically along the lower pressure side of the gob in the horizontal plane. Various parameters affect the performance of horizontal gob boreholes: (i) diameter and length; (ii) vertical and horizontal placement relative to the working seam and mine ventilation system; (iii) borehole integrity following undermining; and (iv) borehole production issues related to vacuum and dewatering.