April 8, 2013
Coal is a carbon-rich rock derived from plant material (peat) that accumulated in swamps and was subsequently buried by ongoing geological processes. With increasing depth of burial, the plant material undergoes coalification, releasing volatile matter (water, carbon dioxide, light hydrocarbons, including methane) as it begins to transform into coal. With ongoing coalification, the coal becomes progressively enriched in carbon and continues to expel volatile matter. Generation of methane and other hydrocarbons is a result of thermal maturation in coals, and begins around the sub-bituminous "A" to high volatile bituminous “C” rank stage, see coal rank classification chart, with amounts of methane generated increasing significantly throughout the medium to low volatile bituminous coal ranks.
Shallow coals of the Plains (i.e., Ardley, Horseshoe Canyon) have a low rank and have not generated large quantities of methane. Deeper coals (i.e., Mannville) have experienced a greater degree of coalification (high rank) and have produced and retained greater quantities of methane than shallower coals.
Some coalbed methane (CBM) may have been produced by methanogenic bacterial action. Biogenic gas may be produced at any time throughout the coalification process if proper conditions are present.
Gas is stored in coal as an adsorbed component on or within the coal matrix and as free gas within the micropore structure or cleats within a coal bed. The gas is held in place mainly by reservoir pressure; reducing the reservoir pressure allows gas to be released from the coal.
Mountain building processes in the western parts of Alberta and in British Columbia (Rocky Mountains) have produced great compressional stress in the coal and rocks. This stress contributed to the formation of a series of tight, closely-spaced small fractures in the coal beds, referred to as 'cleat' (see image below and also coal cleat 1, coal cleat 2, coal cleat 3, coal cleat 4, coal cleat 5 and coal cleat 6).
Coal cleat is important in enhancing CBM potential, as it is these interconnected fracture networks (permeability) that allow fluids (water and gas) to move through the coals and into a wellbore for production. The widths or openings of cleats are generally wider near the surface, but with increasing depth the weight of the overlying rock compresses, or closes the cleats, reducing their width and connectivity (reducing reservoir permeability). Cleats may be enhanced in areas of faulting, fracturing or where coal seams may be draped over sandstone channels. Coalbed methane companies are always looking for areas of enhanced permeability to improve their exploration success potential.
The rank of coal in Alberta ranges from very low (lignite), to high (anthracite). Coal near the surface in the Plains is generally of sub-bituminous rank, with lignite occurring in the north and northeast part of the Plains, and high volatile bituminous C in the northwest and southwest areas of the Plains. Coals near the surface in the Foothills generally are of high volatile bituminous C, with minor amounts of high volatile bituminous B and A – rank coals in some areas. In the Mountains area west of the Foothills, coal ranks of medium and low volatile bituminous rank occur, with local occurrences of anthracite near the Canmore area. Low rank, shallow Plains coals are used mainly for heating and power generation (thermal coal). Higher-rank coals of the Mountains and Foothills are suitable for use in the steel making industry (metallurgical or coking coal).
Coal rank increases with burial depth. In the Plains, coal rank increases toward the west, as seams dip and become progressively deeper toward the mountains. Coalbed methane content increases with increasing rank, so that with greater seam depths, gas contents are expected to increase. With increasing depth also comes increasing overburden pressure, which may restrict permeability by closing coal cleats. A challenge in CBM exploration is finding coals with ranks suitable for gas generation, but not so deep as to restrict permeability.
To date, Horseshoe Canyon coals with relatively low gas contents, but with favourable cleating/fracturing, are being successfully exploited for CBM production in the south-central Plains. Mannville (and correlative) coals are showing potentially favourable amounts of cleating/fracturing and high gas contents in some locations, and are undergoing evaluation in the north-central to central Plains. Kootenay coals, which also contain high gas contents, are being evaluated in the southern Foothills. Ardley coals have moderate gas contents and, in general, show less amounts of cleating than many of the other coal zones in Alberta, although in the Pembina area production potential is being evaluated.