CoalBed Methane (as taken from HOGC 10Q 14-Nov-2003)

Natural gas normally consists of 80% or more methane with the balance comprising such hydrocarbons as butane, ethane and propane. In some cases it may contain minute quantities of highly poisonous hydrogen sulfide, referred to as "sour gas". Coalbed methane is, generally, a sweet gas consisting of 95% methane and thus is normally of pipeline quality. Coalbed methane is considered an unconventional natural gas resource because it does not rely on 'conventional' trapping mechanisms, such as a fault or anticline, or stratigraphic traps. Instead coalbed methane is "adsorbed" or attached to the molecular structure of the coals - an efficient storage mechanism as coal bed methane coals can contain as much as seven times the amount of gas typically stored in a conventional natural gas reservoir such as sandstone or shale. The adsorbed coalbed methane is kept in place as a result of a pressure equilibrium often from the presence of water. Thus the production of coalbed methane in many cases requires the dewatering of the coals to be exploited. This process usually requires the drilling of adjacent wells and from 6 to 36 months to complete. Coalbed methane production typically has a low rate of production decline and an economic life typically from 10 to 20 years. 

The principal sources of coal bed methane are either biogenic, producing a dry gas which is generated from bacteria in organic matter, typically at depths less than 1000 feet, or thermogenic, which is a deeper wet gas, formed when organic matter is broken down by temperature and pressure. 

The three main factors that determine whether or not gas can be economically recovered from coal beds are: the gas content of the coals; the permeability or flow characteristics of the coals; and, the thickness of the coalbeds. Gas content is measured in terms of standard cubic feet (Scf) per ton and varies widely from 430 Scf per ton in the deep (2,000 to 3,500 feet) San Juan, New Mexico thermogenic coals, and only 60 Scf per ton for the shallow (300 to 700 feet deep) Powder River, Wyoming biogenic coals. The San Juan coals are considered to have the industry's highest permeability. Relatively high permeability, which can affect the ability of gas to easily travel to the borehole, is an important factor for the success of a coalbed methane well, but is not absolutely required. The thickness of coalbeds from which coalbed methane is economically being produced varies from as little as a few feet in some areas of the gas rich (300 Scf) Raton Basin to as much as 75 net feet of coalbed thickness at the relatively gas poor Powder River. 

TOP