CBM

The BC Government Ministry of Energy, Mines and Petroleum Resources provides a good overview of the coal bed methane in its booklet “Coal Bed Methane: Energy for Our Future” which can be found at the following link: http://www.em.gov.bc.ca/subwebs/coalbedgas/

Coal bed methane is a rapidly expanding sector of the global energy market which comprises approximately 8-10% of U.S. natural gas production. Significant CBM development is also occurring in Canada, Australia, and China.

CBM is natural gas generated in and resident in coal seams by thermal and/or biogenic means. The gas can be produced economically by drilling conventional style vertical and horizontal gas wells and employing special completion techniques which are specific to this type of reservoir. CBM is virtually identical to the gas produced from conventional sandstone reservoirs with the exception that carbon dioxide is often present with the methane. After extraction of the carbon dioxide, CBM is a pure natural gas which can be incorporated into the gas markets via existing pipeline systems and at equivalent prices.

CBM reservoir characteristics differ fundamentally from those of conventional petroleum reservoirs. In CBM reservoirs, gas molecules are attached (adsorbed) to the coal matrix by Van der Waals forces. Coal is unusual as a reservoir because it is both the source rock and the reservoir for the gas. Gas is stored in an adsorbed state in coal, and thus for a given reservoir pressure much more gas can be stored in a coal seam than in a comparable sandstone reservoir. As the pressure in the coal seam is reduced, the gas molecules detach from the coal surface and diffuse through the matrix until they reach a natural fracture called a cleat. Coals contain small (typically, several per centimeter), regularly spaced, naturally occurring fractures called face and butt cleats. The gas molecules then flow through the natural fracture system to the wellbore. As the reservoir pressure drops from the critical desorption pressure to the abandonment pressure, the amount of gas that the coal can store also decreases. This difference in storage capacities represents the amount of gas that can desorb and become available for production.

The composition and geological history of a coal seam will determine whether it is saturated with gas or whether it exists in some state of undersaturation. In an undersaturated coal seam only water is produced initially, with gas production being delayed until reservoir pressure has declined to the point of saturation (critical gas desorption pressure). In most coal reservoirs, the cleat system is filled with water. The water maintains the reservoir pressure that holds the gas in the adsorbed state in the coals. Typically, water must be produced from coal seams to reduce the reservoir pressure and release the gas.