Methane trapped, or occluded, in coal beds is a virtually untapped source of clean, sulfur-free, pipeline-quality energy that today constitutes an economically viable exploration and development objective. Drilling and completing wither vertical or horizontal boreholes, utilizing essentially conventional technology, with some modifications, especially in the completion of such wells, can accomplish the production of gas from coal beds. In addition, many coalbed methane wells require initial dewatering of the coal by means of a variety of pumping arrangements. Most of the wells drilled for a coal degasification require reservoir stimulation, usually through casing perforations, employing various combinations of hydraulic fracturing, such as sand-water, sand-foam, or sand-gel, in order to increase permeability to gas.
The production of gas from coal beds, unlike in-situ gasification techniques, is non-destructive to the coal, except possibly by the enlargement of already present cleats, or fractures, in the coal as a result of hydraulic fracturing. Furthermore, coalbed methane is high in heating value, generally between approximately 900 and 1,050 Btu/scf; conversely, gas produced from the in-situ combustion of coal is low in heat content, typically in the range of 150 to 300 Btu/scf.
The coalification process, by which organic matter (e.g. wood, peat) is converted to coal, generates very large quantities of methane, both biogenic (I.e., formed by bacterial action in the early stages of diagenesis) and thermogenic (formed by thermal reactions during the phase of catagenesis). This gas is stored in high concentrations in both the coal and the associated sediments after the gas-expulsion point in the coal has been attained. Volume-for-volume, high-rank coals are capable of storing several times as much gas as are porous sandstone reservoirs under similar conditions of pressure, due to the extremely high internal surface areas of coal - - as high as 1.5 million ftï¿½/lb (or about 3,300 ftï¿½/g) (Cervik, 1969).
Until recently, methane from coal beds has been considered an 'unconventional' resource because of the unique properties of coal, which constitutes both a source and a reservoir of natural gas. Coal is an extremely complex substance, one that still is incompletely understood. According to Van Krevelen (1961), coal has many attributes: it is a fuel, organic sediment, a rock, a collection of plant debris, an organic chemical substance, a solid colloid, and a chemical reactant. It is not surprising, then, that such aspects as the reservoir behaviour of coal are so difficult to predict and to model, unlike the more conventional sandstone or carbonate rock reservoirs. In geologic basins such as the San Juan of Colorado and New Mexico and Black Warrior of Alabama, the experience of certain petroleum industry operators has progressed to the point where coalbed methane now may be considered a conventional resource, albeit one possessing a number of unsolved problems, particularly in the area of production.
An important characteristic of coalbed methane wells, in particular those that initially produce water along with the gas, is an increase in gas production with time and a corresponding decrease in water production. This gas production incline may persist for several years before the inevitable decline occurs. Based on limited, but well-documented, production histories, it appears that typical coalbed methane wells will be long-lived.
When optimum conditions of rank, gas saturation, reservoir temperature, coal permeability, and other critical factors are present, a high rate of success should be experienced in the development of a coalbed methane field or pool. The ubiquitous nature of methane in coal beds generally is independent of structural position in a basin - I.e., whether anticlinal or synclinal - except as structural deformation may affect the permeability of a coalbed reservoir.
The resource base of coalbed methane in the United States alone is indeed immense. Preliminary estimates, based on very incomplete data on only the 48 conterminous states, place this resource in the range of 400 to 800 or more trillion cubic feet (Tcf) in-place. Estimates for individual basins vary from a few Tcf to more than 80 Tcf.
Nikols, D.J., Treasure, S., Stuhec, S. and Goulet, D.E. (1990): Coal bed methane in Alberta - What's it all about? Seminar proceedings; Alberta Research Council, ARC/AGS Information Series 108, 265 p.