Coalbed methane potential in the Appalachian states of Pennsylvania,West Virginia, Maryland, Ohio, Virginia, Kentucky, and Tennessee--An overview
Paul C. Lyons
Open-File Report 96-735
Cleats in Appalachian coals
Natural fractures in coal (cleats) are the principal conduits for the transfer of methane from coal reservoirs (Diamond et al., 1988; Close, 1993; Law, 1993; Rice et al., 1993; Rogers, 1994). Face and butt cleats are the primary and secondary cleat systems in coal, respectively, and these are a function of regional structure, coal rank, coal lithotype, bed thickness, and other factors. Diamond et al. (1988) suggested that closer fracture spacing results in higher permeability of coal beds for CBM. Conversely, Law (1993) reported that the spacing of face and butt cleats are similar and, therefore, the well-known permeability anisotropy of these cleat systems is due to connectivity and not cleat spacing (see also Jones et al., 1984). The permeability of face and butt cleats in the San Juan basin are generally different (Young, 1992), averaging about 12-20 md and 4-5 md, respectively. The greater permeability of face cleats is supported by stimulation experiments using fluorescent paint (Diamond, 1987).
In the central and northern Appalachian basin, face and butt cleats are perpendicular and parallel, respectively, to fold axes (McCulloch et al., 1974). Kelafant and Boyer (1988) reported two dominant cleat trends in the central Appalachian basin--a northeast-southwest set and a north-south set (see also Colton et al, 1981). For the Pocahontas No. 3 coal bed in Buchanan County, Virginia, the face and butt cleats strike N 18o W and N67o E , respectively. In Wise County, Virginia, Law (1993) reported similar cleat spacings of 1.02-1.32 cm for face and butt cleats.
In the northern Appalachian basin, the face cleat of the Pittsburgh coal bed rotates from N 80o W in northwestern West Virginia to N 57o W in southwestern Pennsylvania, following a shift in the axial trend (McCulloch et al., 1974). This set of face cleats corresponds to the regional system of N70-800W face cleats mapped by Kulander et al. (1980). Cleat spacings of 0.5-9.7 cm were reported by Law (1993) in the northern Appalachian basin. McCulloch et al.(1974) and Kulander et al. (1980) reported that horizontal drill holes perpendicular to the face cleats yielded much higher gas yields (up to ten times) as compared with drill holes perpendicular to butt cleats, thus suggesting that face cleats are the primary conduit for CBM. In the Anthracite region of eastern Pennsylvania, Law (1993) reported that cleat systems are poorly developed and mineral-filled, and this will undoubtedly be a major factor in preventing CBM development in that region.
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