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Alternative sources of energy and fuel are hot topics among the environmental and business communities. Solar energy, wind farms, hybrid vehicles are all part of today's vernacular. Another alternative energy source considered to be an "unconventional" natural gas is coalbed methane (CBM). This natural gas from coal can be used in the same way as natural gas from conventional reservoirs such as sandstone and shale. The CBM reservoir is unique because it is a dual-porosity system that does not rely on conventional trapping mechanisms such as a fault or impermeable rock. Most of the gas is bound in primary porosity by adsorption in a low permeability coal matrix (attached to coal's large internal surface area) and is retained by hydraulic pressure. After reservoir pressure is reduced by dewatering in most cases, gas is desorbed and can flow to the production well through secondary porosity of the coal's natural fracture system. As with conventional gas wells, hydraulic fracturing (fracing) is used to enhance CBM production in virgin coal seams by enlarging the natural fracture system and increasing the fracture network in and around the coal zone.

Coalbeds are self-sourcing media because they are gas generators and storage reservoirs. This storage of gas is so efficient that the gas-generating potential of the CBM reservoir can exceed its storage capacity. Coalbeds can contain as much as seven times the volume of conventional gas reservoirs in shallow, low pressure formations. CBM is generally 98 to 99 percent pure methane and typically requires only dehydration to achieve pipeline quality. These characteristics contribute to the economic attractiveness of CBM, in addition to low geological risk and finding cost since coal deposits are easily identified, shallow, and relatively quick to complete. According to Milici (e-mail communication October 26, 2011) and Milici (2004), there is an estimated 2.5 trillion cubic feet (TCF) of technically recoverable CBM in Pennsylvania.

 

CBM Basin Map

          CBM basin map courtesy of U.S. Energy Information Administration (2007)

 

In the western United States, CBM is one of the most important and valuable natural resources. In Pennsylvania, CBM has developed into an established part of the total energy mix that rivals conventional natural gas in composition and heating value. CBM is extracted in many areas to improve mine safety and it is produced from abandoned mines and from coals too thin or deep to be mined. The most common use of CBM is in natural gas pipelines for domestic, commercial, and industrial fuel. Some of the other more common uses are boiler fuel in gas turbines to generate electricity at cogeneration plants; chemical feedstock for methanol, ammonia, and other chemicals; raw material for the production of liquefied natural gas (LNG) and gas products; and cofiring, in which natural gas and coal are burned together to use coals that are marginally compliant with clean air laws.

Despite the hurdle of ownership of the gas based on severed mineral rights, interest in developing this resource continues in Pennsylvania because of established/negotiated ownership of acreage, improved technology, better understanding of coal as a natural gas reservoir, higher gas prices, and the need to develop domestic energy sources. When CBM became more visible in the eyes of landowners, a public hearing was held in Blairsville, PA on February 28, 2003. The League of Women Voters of Indiana County held a meeting on this same subject on May 13, 2003. Toni Markowski, other representatives from state government, law, the coal industry, the oil and gas industry, and several landowners presented testimonies. The issues were landowners’ concerns about gas rights and disturbances to the land surface due to CBM operations. Two more CBM forums for property owners and residents were held in the spring of 2007 to provide general information and address concerns. This all led to new legislation regarding CBM development in Pennsylvania as in News Highlights. Please see additional information at Production Statistics, Reports and Data, and CBM and Related Links.     

References

Milici, R.C. (2004), Assessment of Appalachian basin oil and gas resources: Carboniferous coal-bed gas total petroleum system, U.S. Geological Survey Open-File Report 2004-1272, 98 p.

U.S. Energy Information Administration (2007), U.S. Coalbed Methane: Past, Present, and Future, Panel 2 of 2, November 2007. Comments or questions to EIA Reserves & Production Division: Bob King or Gary Long.