Coalbed Natural Gas
Coalbed methane (CBM) is natural gas found in coal beds and used for a variety of purposes that range from domestic, commercial, industrial to electrical power generation. Other gases that may exist in coal gas deposits in trace amounts are ethane, propane, butane, carbon dioxide and nitrogen.
One cubic foot of methane gas has a heating capacity of approximately 1000 Btus (British thermal units.) Natural gas is typically measured in units of one thousand cubic feet (MCF). In the United States, one MCF of methane gas generates enough energy to match the energy consumed by one person for 1.2 days.
Coalbed methane (CH4) forms in one of two ways. During the earliest stage of coalification (the process that turns plant detritus into coal), biogenic methane is generated as a by-product of bacterial respiration. Aerobic bacteria (those that use oxygen in respiration) first metabolize any free oxygen left in the plant remains and the surrounding sediments. In fresh water environments, methane production begins immediately after the oxygen is depleted (Rice and Claypool, 1981). Species of anaerobic bacteria (those that don’t use oxygen) then reduce carbon dioxide and produce methane through anaerobic respiration (Rice and Claypool, 1981). When a coal’s temperature underground reaches about 122 degrees Fahrenheit, and after a sufficient amount of time, most of the biogenic methane has been generated. Also at this time nearly two thirds of the moisture has been expelled and the coal has reached a rank of subbituminous (Rightmire, 1984).
After the coal’s temperature has exceeded 122 degrees Fahrenheit due to the geothermal gradient and excessive burial, thermogenic processes begin to generate additional carbon dioxide, nitrogen, methane and water. At this point the amount of hydrocarbons or volatile matter has increased and the coal has reached a rank of bituminous (Rightmire, 1984). After the temperature exceeds 210 degrees Fahrenheit carbon dioxide production increases with little production of methane. The thermogenic production of methane does not exceed the production of carbon dioxide in high volatile high ranks of coal until the temperature is about 250 degrees Fahrenheit. The maximum generation of methane in bituminous coals occurs at around 300 degrees Fahrenheit (Rightmire, 1984).
Environments rich in plant material such as in swamps, estuaries and marshes that exist today also existed in the past. In the area we now call Wyoming, during the Eocene epoch (54 to 33 million years ago (mya)) and the Paleocene epoch (65 to 54 mya) swamps and marshes were prolific. It was during this time that the majority of the coal deposits that we, as a nation, are exploiting today were formed. The areas around the state where coal and related CBM resources exist are known as coalfields. In the Powder River, Bighorn, Green River, Hams Fork, and the Hanna coalfields work has already begun to tap the CBM riches of Wyoming. The coal resources of Wyoming currently being developed for CBM are predominately subbituminous and occur in the Wasatch (Eocene) and the Fort Union (Paleocene) formations. The earliest CBM development in the state was during the late 1970’s but it wasn’t until the mid 1990’s when the quest for CBM boomed. In the late 1980’s there were fewer than 20 wells and today there are more than 13,600 producing and shut in CBM wells across the state. The most active development is currently in the Powder River Coal Field.
In the Powder River Coal Field, coalbed methane wells are completed open hole. Using this method, casing is set to the top of the target coalbed and the underlying target zone is under-reamed and cleaned out with a fresh-water flush. A down-hole submersible pump is then used to move water up the tubing; the gas then separates from the water and flows up the annulus.
The natural gas and the water that are produced at individual wells are piped to a metering facility, where the amount of production from each well is recorded. The methane then flows to a compressor station where the gas is compressed and then shipped via pipeline. The water produced is diverted to a central discharge point at a drainage or impoundment. Some of the produced water is reinjected into nearby aquifers.
A Modified schematic diagram of a Coalbed Methane well showing an open hole completion technique.
Diagram was furnished by the Wyoming State Engineers Office.
Below is an example of the Coalbed Methane Map showing CBM well locations and density, pipelines and coal lease locations in the Powder River Basin. The Wyoming State Geological Survey’s Coal and Oil & Gas sections produce the map as a cooperative project. The map is updated on the first of September and on the first of March. This information is available either as a plotted map at 1:250,000 or digitally on CD.
Rice, DD and Claypool, GE., 1981, Generation, accumulation, and resource potential of biogenic gas: American Association of Petroleum Geologists Bulletin, v. 65, no. 1, p. 5-25.
Rightmire, CT., 1984., Coalbed methane resources, in Rightmire, C.T., Eddy, G.E., and Kirr, J.N., editors, Coalbed methane resources of the United States: American Association of Petroleum Geologists Explorer, v. 21, no.4 (April, 2000), p. 16, 18-20, 22-23.