CBNG is the gas found in coal deposits. It consists mostly of methane but may also contain trace amounts of carbon dioxide and/or nitrogen. Most coalbeds are permeated with methane, and a cubic foot of coal can contain six or seven times the volume of natural gas that exists in a cubic foot of a conventional sandstone reservoir.

Within coal seams, methane is present on the surface of the solid material. Hydrostatic pressure causes the methane to adhere to the coal surface via a phenomenon termed adsorption. Whenever reservoir pressure is reduced, the methane desorbs off of coal surfaces, diffuses through the matrix material, and then flows through a system of natural fractures (cleats) and into a well for delivery to the surface. CBNG is the same as the natural gas in our transmission and distribution pipelines; it is used for space heating and power generation, as a feedstock for chemical production, and in manufacturing processes.

Coalbed natural gas is either biogenic or thermogenic in origin. Biogenic methane is generated from bacteria in organic matter and is typically a dry gas. It is generally found at depths of less than 1,000 feet from the earth’s surface in low-rank coals (those coals with a lower carbon content). Thermogenic methane forms when heat and pressure transform organic matter in coal into methane. This type of methane is typically a wet gas and frequently contains trace amounts of water vapor, carbon dioxide, nitrogen, and possibly hydrogen sulfide. It is generally found at greater depths, in higher-rank coals.
 

The contiguous United States is estimated to have CBNG in-place resources of 700 trillion cubic feet (Tcf), of which 100 Tcf may be economically recoverable. The most prolific basins exist in the western United States, but eastern areas of the Nation also have notable reserves of CBNG. Other areas that have significant CBNG potential include Alaska and the Illinois Basin.
 

United States Coal Bed Natural Gas Resources

U.S. CBNG proved reserves and production have grown nearly every year since 1989. In 2003, CBNG accounted for 18.7 Tcf, or 10%, of the Nation’s proved dry gas reserves, with 1.6 Tcf being produced, or >8% of dry gas production. CBNG produced in Colorado, New Mexico, and Wyoming totaled nearly 1.3 Tcf during 2003, which represents 80% of total CBNG production. Other notable producing areas include the Central Appalachian and Warrior basins in the eastern United States and the Uinta and Raton basins in the Rocky Mountain region. The majority of future CBNG production is expected from western basins.

Coalbed natural gas can be recovered from underground mines before, during, or after mining operations. Significant volumes of CBNG also are extracted from “unminable” coal seams that are relatively deep or thin, of poor or inconsistent quality, or represent difficult mining conditions. Ninety percent of the country’s coal resource is unminable but represents a vast potential source of natural gas.

Vertical and horizontal wells, including multi-laterals, are used to develop CBNG resources. For the most part, the quality of a seam’s cleat system (high-conductivity flow paths) will dictate the type of well completion and stimulation employed. In high-permeability settings, flow enhancement may not be required. In other situations, hydraulic fracturing and cavitation stimulations are used.

Hydraulic fracturing is a process in which fluids, primarily water, are pumped at high pressure, creating a crack through which natural gas can flow easily into a well. The cavitation method involves enlarging the original wellbore and linking the wellbore with the cleat system within one or more coal seams. In all cases, water initially must be pumped out of the coals (dewatering) in order to reduce the reservoir pressure and allow the methane to desorb.

Using a different production process, CBNG can be recovered after removing coal from an underground mine. After mining, the resulting void needs supports to hold up the strata above it. When these supports are removed, the roof and walls collapse, and the former mine fills with debris. (This also occurs when coal is longwall-mined.)

This debris is referred to as the “gob.” Methane existing within the debris is referred to as “gob gas,” and is released into the mine as it collapses. Gob gas is initially of high-quality, usually 30-80% methane; however, over time its quality declines as the methane mixes with air. Lower-quality gas cannot be injected into pipelines, so gob gas is well-suited for applications that allow for variation in methane content, including power production and heating.

Although development of CBNG resources has been quite successful, the industry continues to face many issues. These issues are varied, some highly contentious, and include access to resources, permitting, exhaustive environmental planning, litigation, produced-water management, natural gas markets and capital formation, and the need for advanced technologies.

Another important environmental issue for CBNG developers stands as a positive. The release of methane into the atmosphere, either through natural seeps, ventilation during mining, or via other means, has environmental consequences. Methane is a potent greenhouse gas, with 21 times the global warming potential of carbon dioxide. In fact, coal mining accounts for about 10% of U.S. methane emissions. Therefore, recovery of CBNG mitigates a large source of methane emissions and allows for economic use of the energy source.

DOE is a longtime supporter of CBNG research and development. The agency has been instrumental in providing the fundamental knowledge base to industry. This foundation includes assessing the resource, identifying geologic areas of favorable production, establishing efficient recovery schemes, demonstrating advanced drilling and completion technologies, and supporting capture and use of diluted natural gas streams.

Currently, NETL supports a CBNG research effort in the Illinois Basin as well as a portfolio of R&D projects targeting the water management issues associated with CBNG development. DOE also is active in CBNG-related regulatory, policy, and marketing issues, in collaboration with multiple Federal and State agencies.

CBNG-related research at DOE also focuses on the potential for enhanced gas recovery and carbon sequestration as an integrated operation. The idea is to sequester CO₂ in unminable coalbeds, which have an enormous capacity for CO₂ storage. The CO₂ would be injected via wells drilled into the coal, and pressure from the CO₂ would displace the methane out of the coal. CO_{2 storage is feasible because coal preferentially adsorbs CO2 at twice the volume that it stores methane. The net result would be less CO2 in the atmosphere and additional recovery of sorely needed natural gas. Although technical and economic hurdles still exist, continued, successful research can provide a new solution to our energy and environmental concerns.}