Coal Bed Methane
Methane recovery from un-mined coal seams is often referred to as Coal Bed Methane (CBM). This includes the recovery of methane prior to mining taking place. Virgin Coal Bed Methane (VCBM) describes the recovery of methane from seams in which the coal will remain unmined.
Methane is recovered from un-mined coal seams for two primary reasons:
- It may be necessary to drain the seam of as much methane as possible before mining takes place. This reduces the risk of explosion and mitigates methane emissions to the atmosphere once the process of extracting the coal begins.
- The methane may be recovered for its energy production potential, regardless of whether the coal will actually be extracted.
The potential for future mining operations is largely dependent on the accessibility of the coal seams. Coal found at extremely deep depths is often not considered feasible for extraction because of practical, safety and economic considerations. In such cases, methane recovery activity is purely for the purpose of energy generation and does not have safety or climate change benefits (as the methane would not have been emitted).
Methane from unmined coal seams is recovered through drainage systems constructed by drilling a series of vertical or horizontal wells directly into the seam. Water must first be drawn from the coal seam in order to reduce pressure and release the methane from its adsorbed state on the surface of the coal and the surrounding rock strata. Once dewatering has taken place and the pressure has been reduced, the released methane can escape more easily to the surface via the wells.
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.
CBM generally provides the highest concentration of methane recoverable from coal seams due to the lack of exposure to air from mining. Concentration levels of methane recovered via these techniques can often exceed 95%, making the gas suitable for use as a direct replacement for conventional natural gas in pipeline networks. This gas can then be pumped directly to homes and businesses for use in cooking and heating.
Natural gas pipeline networks need to be easily accessible for the addition of the coal seam methane to be economic and practical. Existing pipeline networks can be extended to reach CBM projects if the distances to be covered and geographical features make the project economically feasible.
The high quality of the gas recovered from unmined coal seams also renders it suitable for replacing or supplementing conventional natural gas in power generation systems, such as gas turbines and gas engine systems. This utilisation option increases in viability the closer the generator is located to the methane recovery site.
Recovered CBM can also be stored in gas canisters for local distribution as a domestic fuel and is also storable in compressed liquid form for utilisation as vehicle fuel.
Global Resource Base & Potential for Utilisation
The largest CBM resource bases lie in the former Soviet Union, Canada, China, Australia and the United States. However, much of the world’s CBM recovery potential remains untapped. In 2006 it was estimated that of global resources totalling 143 trillion cubic metres, only 1 trillion cubic metres was actually recovered from reserves. This is due to a lack of incentive in some countries to fully exploit the resource base, particularly in parts of the former Soviet Union where conventional natural gas is abundant.
|Country||Estimated CBM Resource Base (trillion cubic metres)|
|Canada||17 to 92|
|Russia||17 to 80|
|China||30 to 35|
|Australia||8 to 14|
|USA||4 to 11|
Source: IEA CCC 2005
The United States has demonstrated a strong drive to utilise its resource base. Exploitation in Canada has been somewhat slower than in the US, but is expected to increase with the development of new exploration and extraction technologies.
The potential for supplementing significant proportions of natural gas supply with CBM is also growing in China, where demand for natural gas is set to outstrip domestic production by 2010 and CBM offers an alternative supply.