When plant material such as roots, bark, and wood are decompose in wet areas they become peat deposits. As the peat is buried deeper, under layers of sand and mud, it forms coal through the coalification process. The first stage is brown coal, then bituminous coal, and eventually hard, anthracite coal.
As the coal is formed, the decomposing organic material produces methane gas, as well as nitrogen, carbon
dioxide, and other gases. The burial process puts pressure on the coal, which keeps much of the gas in the coal. Like natural gas from conventional sources, CBM is “sweet” not “sour” as it doesn’t contain hydrogen sulphide. CBM is an unconventional gas as the gas is contained in difficult-to-produce reservoirs, which require special completion, stimulation and/or production techniques to achieve economic production. The coal remains in place after the CBM is removed.
CBM is extracted by drilling a well into a coal seam applying similar techniques used for other natural gas wells. The sides of the well are "cased" with cemented steel pipe. Usually, small holes, called perforations, are then made in the wall of the casing to let the CBM flow through into the well bore and up the casing to the surface. In some cases the wells are drilled horizontally and the coal seams are often stimulated or "fractured" with Hydraulic Fracturingto make the CBM flow more freely. Standard drilling and extraction technology is used or adapted as conditions require.
Potential in Alberta
The Alberta Geological Survey estimates there may be up to 500 Trillion cubic feet of natural gas in Alberta's coals. It is not known how much of this gas may be economic to produce.
The majority of CBM development is taking place in the dry Horseshoe Canyon.
In 2012, nearly all coalbed methane wells drilled in Alberta have targeted the thinner coal seams in the Horseshoe Canyon (ultimate gas in place 179 Tcf) and Belly River coal zones along the Calgary-Red Deer corridor. Wells targeting these seams tend to produce gas with little or no water, with production referred to as "dry CBM". The first commercial production of CBM in Alberta was from these coals, and they constitute the majority of CBM reserves booked. The depth range of these coals is 200 to 800 m.
The remaining CBM wells drilled have targeted the deeper Mannville coals (ultimate gas in place 321 Tcf). These coals tend to be thicker, deeper, and more continuous with substantial saline (salt) water production. The depth range of these coals is 900 to 1,500 m.
Most CBM wells in the Horseshoe Canyon Formation are vertically drilled wells, whereas most wells in the Mannville Group are horizontal wells.
The Energy Resources Conservation Board/Alberta Geological Survey completed a report on the Ardley-Paskapoo entitled "Preliminary Investigation of Potential, Natural Hydraulic Pathways between the Scollard and Paskapoo Formations in Alberta: Implications for Coalbed Methane Production"
, it supports improved understanding of aquifer systems.