Coal bed methane extraction

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


Basic principles

The methane is adsorbed into the solid coal matrix (coal macerals) and is released when the coal seam is depressurised. To economically retrieve reserves of methane, wells are drilled into the coal seam, the seam is dewatered, then the methane is extracted from the seam, compressed and piped to market. 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.

While dewatering is occurring, the operator should make sure that the pumpjack is not running too long. If the water level is pumped too low, this will allow the gas to travel up the tubing into the water line, causing the well to become "gassy". The main objective is not to put the gas in the water line, but to allow it to flow up the backside of the well (casing) and into the pipeline, where it can be transported to the compressor station and delivered to the customer for sales. Once the gas goes up the tubing, it is usually recovered in a water-gas separator at the surface. However, pumping water and gas is inefficient and can cause pump wear and breakdown.

Areas with coal bed methane extraction

This process has resulted in the drilling of tens of thousands of gas wells, and extensive support facilities such as roads, pipelines and compressors.

The use of this method is currently expanding in the Powder River Basin of northeast Wyoming and southeast Montana.

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.

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 coalbed, and with reservoir pressure. Fractures, or cleats, that permeate coalbeds are usually filled with water; the deeper the coalbed, the less water is present, but the more saline it becomes. In order for gas to be released from the coal, its partial pressure must be reduced, and this is accomplished by removing water from the coalbed. Large amounts of water, sometimes saline, are produced from coalbed methane wells, especially in the early stages of production. While economic quantities of methane can be produced, water disposal options that are environmentally acceptable and yet economically feasible, are a concern. Water may be discharged on the surface if it is relatively fresh, but often it is injected into rock at a depth where the quality of the injected water is less than that of the host rock. Another alternative, not yet attempted, is to evaporate the water and collect the potentially saleable solid residues; this scheme might be feasible in regions having high evaporation rates.

Measuring the gas content of coal

Coalbed gas content measurements are commonly used in mine safety as well as coalbed methane resource assessment and recovery applications. Gas content determination techniques generally fall into two categories: (1) direct methods which actually measure the volume of gas released from a coal sample sealed into a desorption canister and (2) indirect methods based on empirical correlations, or laboratory derived sorption isotherm gas storage capacity data.

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