What is coalbed methane?
Coalbed methane is natural gas or methane (CH4) that occurs in coal beds and has been generated during the conversion of plant material to coal (the process known as coalification). Coalbed methane produced from low rank subbituminous coal in the Powder River Coal Field, Wyoming, is composed almost entirely of methane, with a minor amount (1.5 to 2%) of carbon dioxide (CO2). Coalbed methane produced in other areas of the U.S. from higher rank bituminous coal may contain minor amounts (less than 3% each) of CO2 and nitrogen (N2), very minor to trace amounts of higher hydrocarbons (ethane, propane, butane, etc.), and sometimes a trace of hydrogen sulfide (H2S) (Rightmire, 1984).
Unlike conventional natural gas deposits, coalbed methane does not have condensates (natural gas liquids) associated with it. This can have a significant impact on the profitability of projects, as condensate is a very valuable product.
Methane produced from a typical coalbed methane well has a heating value of about 1000±25 British Thermal Units (Btus) per standard cubic foot. One million Btus (the energy equivalent of 1000 cubic feet of methane or one MCF) approximate the energy consumed by a person in the U.S. in about 1.2 days. A million Btu's of fossil fuel can generate about 100 kilowatt-hours of electricity at an electric utility. (TOP of page)
How does coalbed methane form?
During coalification, plant material that accumulated and was preserved in ancient swamps and bogs at rates fast enough to prevent decay (oxidation) begins to compact upon burial. The material is first converted to peat as much of the water in the original material is expelled. As the temperature increases with further burial, ever-increasing ranks of coal form, starting with lignite, followed by subbituminous coal and bituminous coal. If the heat (and pressure) is great enough, anthracite (the highest rank of coal) forms. Biogenic methane (that attributed to bacterial activity) is first to form. When the temperature exceeds that in which bacteria can live, thermogenic methane (that attributed to heating) forms.
At these different stages of coalification, various hydrocarbons (called volatile matter, including methane), along with carbon dioxide, nitrogen, and water, are released. Increased temperatures throughout burial drive off volatile matter. The coalification process can stop at any time, depending on geologic conditions, leaving what we see today as varying ranks of coal. Much of the methane generated by the coalification process escapes to the surface or migrates into adjacent reservoir or other rocks, but a portion is trapped within the coal itself, primarily adsorbed on or absorbed within micropores of the coal. (TOP of page)
What are the two types of coalbed methane?
During the early stages of coalification, 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 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°F (Figure 1), and after a sufficient amount of time, most of the biogenic methane has been generated, about two-thirds of the original moisture has been expelled, and the coal attains an approximate rank of subbituminous (Rightmire, 1984). As the temperature increases above 122°F through increased burial or increased geothermal gradient, thermogenic processes begin and additional water, carbon dioxide, and nitrogen are generated as coalification proceeds to approximately the rank of high volatile bituminous (Rightmire, 1984). Maximum generation of carbon dioxide, with little methane generation occurs at about 210°F. Generation of thermogenic methane begins in the higher ranks of the high volatile bituminous coals, and at about 250°F, generation of methane exceeds generation of carbon dioxide. Maximum generation of methane from coal occurs at about 300°F. With even higher temperatures and higher rank coals, methane is still generated, but at somewhat lower volumes (Rightmire, 1984). (TOP of page)
How does coalbed methane occur in the coal?
Because coal beds serve as both the source rocks and the reservoir rocks, gas storage in coal beds is more complex than in most conventional reservoirs (e.g., carbonate and sandstone). Although coalbed methane can (and does) migrate to non-coal reservoir rocks, once the gas leaves the coal beds it is no longer considered coalbed methane. Gas reservoirs composed of coal contain unique properties for gas storage that are not present in other reservoirs.
Coalbed methane can be stored in four ways:
- As free gas within the micropores (pores with a diameter of less than .0025 inches) and cleats (sets of natural fractures in the coal);
- As dissolved gas in water within the coal;
- As adsorbed gas held by molecular attraction on surfaces of macerals (organic constituents that comprise the coal mass), micropores, and cleats in the coal;
- As absorbed gas within the molecular structure of the coal molecules.
The amount of methane present within a particular volume of coal is very large. Coals at shallower depths with good cleat development contain significant amounts of free and dissolved gas while the percentage of adsorbed methane generally increases with increasing pressure (depth) and coal rank.(TOP of page)
Benefits of Capturing and Using Coal Mine Methane
Historically, coalbed methane has been considered a nuisance in the coal mining industry. Once a mine is built, and coal is extracted, the methane contained in the seam usually leaks out into the coal mine itself. This poses a safety threat, as too high a concentration of methane in the well create dangerous conditions for coal miners. In the past, the methane that accumulated in a coal mine was intentionally vented into the atmosphere.
There are numerous benefits to capturing and using coal mine methane, including:
- Reducing greenhouse gas emissions
- Conserving a local source of valuable, clean-burning energy
- Enhancing mine safety by reducing in-mine concentrations of methane
- Providing revenue to the mine
How is Coal Seam Methane recovered?
A range of technologies are available to recover methane from coal and these can be broken down into three categories.
- Coal Bed Methane (CBM)
- Coal Mine Methane (CMM)
- Abandoned Mine Methane (AMM)
Coal Bed Methane (CBM)
Methane recovered from un-mined coal seams. The coal seams may be mined in the future but this is largely dependent upon geological factors such as coal depth and quality.
Methane from unmined coal seams is recovered through drainage systems constructed by drilling a series of vertical or horizontal wells directly into the seam. Coal seams are usually saturated with water and the water’s hydrostatic pressure keeps the methane adsorbed on the coal. To obtain methane from coal, water is pumped from a well, reducing pressure and causing methane to desorb and begin to flow from the coal. The coal must be very permeable to allow the gas to flow in large quantity through the coal to the producing well.
The choice of vertical or horizontal wells is dependent on the geology of the coal seam. In the case of seams at shallow depths, vertical wells have been traditionally used. These vertical systems often use layers of fracture wells, which drain the methane from fractures in the coal seam produced as result of the increased pressure created during the dewatering process. At these shallow depths, the combination of high permeability and low pressure make the vertical systems ideal as extra methane flow enhancement is not required and the structure of the vertical and fracture wells remains stable.
At greater depths, the structure of the vertical and fracture wells may not be able to withstand the higher pressure levels and extra flow enhancement may be required to produce the methane. This is often true in cases of VCBM recovery due to the depths at which the coal is found. In these instances, horizontal drilling techniques may be used for increased accuracy and flexibility. Within these horizontal systems, flow enhancement techniques such as extra hydraulic fracturing - where water is pumped into the seam at high pressure - may be deployed to further facilitate the release of the methane from coals seams.
Although horizontal systems can recover much higher volumes of methane from coal seams at extreme depths than a vertical system possibly could, recovery efficiency is relatively low and heavily dependent on the overall length of the drill through the coal seam. Horizontal systems are still in their infancy and over time there may be increased movement towards their use as the technologies mature and efficiencies are improved.
Coal Mine Methane (CMM)
Methane recovered during mining activities as the coal is in the process of being extracted and thus emitting significant quantities of the gas.
Recovery techniques for CMM vary for each of the two stages of emissions.
- Methane released from the worked coal face can be diluted and removed by large ventilation systems designed to move vast quantities of air through the mine. These systems dilute methane within the mine to concentrations below the explosive range of 5-15%, with a target for methane concentrations under 1%. The ventilation systems move the diluted methane out of the working areas of the mine into shafts leading to the surface. The methane removed from working mines via this technique is known as Ventilation Air Methane (VAM).
The VAM is released through the ventilation shafts and can then be destroyed or captured for utilisation rather than allowing it to be released directly into the atmosphere, as may have occurred in the past. VAM has the lowest concentration levels of all forms of recoverable methane from coal seams because of its high exposure to air; often displaying levels of 0.05-0.8%.
- To pre-empt the release of gob gas from post mining collapse, it is possible for vertical gob wells to be drilled directly into the coal seam’s surrounding strata before mining activities pass through that section. These pre-drilled wells can then remove the gob gas once the collapse takes place, thus avoiding the release of methane directly into the mine. The gob gas can then be destroyed or captured for utilisation via the wells, rather than allowing it to be released directly into the atmosphere. As gob gas is exposed to significantly lower volumes of air than VAM, it displays much higher methane concentration levels - typically between 35-75%.
Abandoned Mine Methane (AMM)
Methane recovered from mines that have been abandoned following the completion of mining operations. Significant amounts of methane may remain trapped in the mine or may continue to be emitted from openings.
Recovery & Utilisation
Recovery techniques for AMM are largely determined by the existing infrastructure at a site. In the case of sealed mines, vertical and horizontal well drilling similar to that deployed in CBM recovery can be used. These wells may often seek to recover gob gas from post mining collapse and may already be present as a result of methane recovery activity that took place at the site prior to and during mining.
With vented mines, recovery can take place via pre-existing ventilation shafts similar to those from which ventilation air methane is drained in working mines. This option can often present a low-cost method of methane recovery as much of the required infrastructure may already exist. This contrasts to AMM recovery from flooded mines in which major water drainage must take place before methane can be retrieved. The extra effort involved in dewatering the site can be extremely costly and time consuming and therefore may make AMM recovery from flooded mines an unattractive option.
AMM provides a good recoverable source of medium to high quality methane and therefore has strong potential as a substitute for conventional natural gas in pipelines and power generation systems. The UK, US and Germany have been leaders in the development of AMM projects and huge potential also exists in China and the Czech Republic.
The quality of AMM improves with the depth of the coal seam. The state in which the abandoned mine has been left can also have a significant effect on the timeframe available for effective methane recovery.