Coal bed methane extraction

Coalbed methane extraction (CBM extraction) is a method for extracting methane from a coal deposit.

Methane adsorbed into a solid coal matrix (coal macerals) will be released if the coal seam is depressurised. Methane may be extracted by drilling wells into the coal seam. The goal is to decrease the water pressure by pumping water from the well. The decrease in pressure allows methane to desorb from the coal and flow as a gas up the well to the surface. Methane is then compressed and piped to market.

The objective is to avoid putting methane into the water line, but allow it to flow up the backside of the well (casing) to the compressor station. If the water level is pumped too low during dewatering, methane may travel up the tubing into the water line causing the well to become "gassy". Although methane may be recovered in a water-gas separator at the surface, pumping water and gas is inefficient and can cause pump wear and breakdown.

Tens of thousands of methane wells have been drilled, and extensive support facilities such as roads, pipelines, and compressors have been installed for CBM extraction in the Powder River Basin of northeast Wyoming and southeast Montana and now in India at West Bengal- Ranigunj, Panagarh etc. Seven percent of the natural gas (methane) currently produced in the United States comes from CBM extraction. Methane from coalbed reservoirs can be recovered economically, but disposal of water is an environmental concern.

There are also sites in Central Scotland at Letham Moss.

Most gas in coal is stored on the internal surfaces of organic matter. Because of its large internal surface area, coal stores 6 to 7 times more gas than the equivalent rock volume of a conventional gas reservoir. Gas content generally increases with coal rank, with depth of burial of the coal bed, and with reservoir pressure. Fractures, or cleats, within coal beds are usually filled with water. Deeper coal beds contain less water, but that water is more saline. Removing water from the coal bed reduces pressure and releases methane. Large amounts of water, sometimes saline brine, are produced from coalbed methane wells. The greatest water volumes are produced during the early stages of production. Environmentally acceptable disposal of brine is a major cost factor for economic methane production. Fresh water may be discharged on the surface, but brine is usually injected into rock at a depth where the salinity of the injected brine is less than connate fluids of the host rock. Evaporation of water for recovery of potentially salable solid residues might be feasible in regions having high evaporation rates.

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