COAL BED METHANE

Coal bed methane (CBM) also called Coal bed gas, is a form of natural gas which has no colour or smell. It is a byproduct of coal mining operations and is extracted from coal beds.

It is well known from its occurrence in underground coal mining, where it presents a serious safety risk; its presence makes the mining operation difficult and costly but it is a very clean fuel when burnt. The CBM is in a near-liquid state, lining the inside of pores within the coal.

Fractures (or cleats) of coal beds permeate to fill with water and this water presence decreases with the increased depth of a coal bed and increases the salinity. Removing water from a coal bed is essential to reduce partial pressure from coal so that it can release the gas.

Fracture permeability acts as the major channel for the gas to flow, so the higher the permeability, the higher the gas production. The permeability of fractured reservoirs changes with the amount of stress applied to them. Coal displays a stress sensitive permeability and this process plays an important role during stimulation and production operations.

Gas contained in coal bed methane is mainly methane and trace quantities of ethane, nitrogen, carbon dioxide and few other gases. The CBM content of coal increases with burial depth, the rank of the coal and with reservoir pressure. Intrinsic properties of coal as found in nature determine the amount of gas that can be recovered.

coal bed methane extraction

THE PROCESS OF EXTRACTING THE GAS

To extract the gas, a steel-encased hole is drilled vertically or horizontally into the coal seam. As the pressure within the coal seam declines, due to the hole to the surface or the pumping of small amounts of water from the coalbed, both gas and 'produced water' escape to the surface through tubes.

The gas is then sent to a compressor station and into natural gas pipelines. The water is reinjected into isolated formations or after cleansing through various alternave methods, released into streams or used for irrigation. The water typically contains dissolved solids such as sodium bicarbonate and chloride.

As production occurs from a coal reservoir, the changes in pressure are believed to cause changes in the porosity and permeability of the coal. This is commonly known as matrix shrinkage or swelling. As the gas is desorbed, the pressure exerted by the gas inside the pores decreases, causing them to shrink in size and so restrict gas flow through the coal. As the pores shrink, the overall matrix shrinks as well, which may eventually increase the space the gas can travel through (the cleats), increasing gas flow.

Coal bed methane is used the same as traditional natural gas though its production is very different from conventional means and it can be used as an energy source that is environmentally more acceptable than mining and the combustion of coal. It can partly replace coal as a fossil energy source, and it sometimes occurs where other conventional resources of oil and gas are not sufficient or present.

Ground Water Practitioners and Coal Bed Methane

Coal bed methane extraction
Coal bed permeability testing
Notching of casing and cement
Complete fitting for methane and water extraction as well as seperation of methane from water
Purification of brine water
Grouting of casing
Dewatering with locally produced product
Flaring of gas
Gas analysis
Power generation
Electircal (HD and LT - overhead and underground) and Pipeline reticulation

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