Abandoned Mine Methane
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.
Abandoned mine methane (AMM) can be recovered from previously working but now disused underground coal mines. Although the primary driver for recovery of AMM is energy production, there is also the potential for reducing atmospheric emissions if significant amounts of methane continue to escape from the mine following the completion of mining activities.
Abandoned underground mines are generally found in one of three conditions.
- Sealed – any entrances into the mine (e.g. ventilation shafts, methane drainage wells) have been sealed. The volume of methane trapped in the mine is dependent on the standard of the sealing.
- Vented – old wells and ventilation shafts are left unsealed, allowing air into the mine and methane to escape freely to the atmosphere.
- Flooded – the mine has been flooded by water found within the remaining rock strata following the completion of mining operations.
Well sealed abandoned mines provide a much greater opportunity for methane extraction than flooded or vented mines. The methane release rates of flooded and vented mines can decrease significantly within two years of mine closure (estimated at around 25% of initial release rates), whereas well sealed mines can retain the vast majority of methane reserves over long periods of time. However, in all cases, the highest benefits will be reaped if recovery takes place in the first two years post-closure.
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.
Emissions from Abandoned Mines & Project Potential
Data on methane emissions from abandoned mines has historically been scarce. This is despite widespread understanding that significant volumes of methane are emitted from the ventilation shafts and gas drainage wells of disused mines around the world. Many countries may include these emissions from abandoned mines in national methane emissions inventories from coal mining activity, but they are not always specifically defined.
The unique features of abandoned mines means that the methodologies for quantifying methane emissions differ considerably from those used in the case of working mines. The United States is one of the few countries that has accounted for methane emissions from abandoned underground mines, with around 9% (13.5 billion cubic feet) of all coal related methane emissions coming from this source between 1998 and 2006.
AMM mitigation potential may largely be determined by the current state of a region’s coal mining industry. Those regions in which the coal mining industry has declined significantly in recent times are likely to have greater opportunities for AMM recovery. These opportunities will, of course, be subject to the conditions of the abandoned mines and the time elapsed since closure.