Coalbed methane, as its name indicates, is derived from coal, which acts in both the role of source rock and reservoir for methane. During the geological formation of coal under high temperature and pressure, volatile substances such as water and methane are liberated. Some gas may escape the coal. However, if the pressure of the formation is sufficient, significant quantities of methane are retained in the pressurized coal matrix in an adsorbed form. Because of its large surface area, coal can store up to six or seven times more natural gas than the equivalent rock volume of a conventional reservoir. Gas content generally increases with depth of burial of the coalbed and reservoir pressure. Fractures (or cleats as they are called in this case) that permeate the coalbeds are usually filled with water. The deeper the coalbed, the less water is present, but the more saline it becomes. In order to produce methane, wells are drilled through the coal and the pressure is reduced by pumping out water. This allows the methane to desorb and pass into the gaseous phase, so that it can be produced in a conventional manner and transported by pipeline. While economic recovery of methane is possible, water disposal from coalbeds exploitation raises environmental concerns. Coalbed accumulations are widespread and characterized by their large sizes. Because of the heterogeneous nature of coalbeds, the production rates are, however, highly variable even within a small area. The USGS estimates worldwide resources at up to 210 trillion cubic meters, but this number is uncertain because of the scarcity of basic data on coalbed resources and their gas content. The largest resources are located in regions rich in coal such as the United States,
China, Russia, Australia, Germany, and Canada. Numerous pilot projects are exploiting these resources, and many countries have active coalbed methane wells, but the United States is currently the only country where commercial production on a large scale is taking place. Production from this source now represents about 10% of the methane production in the United States.
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