Coalbed methane (CBM) is a valuable source of energy in the USA and Australia and is increasing in popularity in China. When either pure CO2, N2 or raw flue gas, is injected into a CBM well to enhance recovery of methane whilst storing CO2 underground it is known as enhanced coalbed methane (ECBM). The interest in ECBM is due to its potential for increased revenue and environmental benefits:
• methane recovery from existing wells can increase from below 50% to over 95%
• CO2 can be stored in the methane-depleted coal seams
• revenue could be obtained from both increased gas production as well as from greenhouse gas funding mechanisms.
In a standard CBM well, methane production slows over time after the initial pressure in the coal seam is released. The injection of gas into the well helps to flush out remaining methane from the seam. It also competes with methane for sorption sites on the surface of the coal itself. Nitrogen can be used, but CO2 is far more effective at releasing methane –around two CO2 molecules replace every one methane molecule released. Thus, in theory, there is significant storage potential for CO2 in deep coal seams. But, in practice the sorption of CO2 onto the coal causes the coal to swell by up to 7–8%. This reduces the gas flow within the seams and, eventually leads to the complete cessation of gas flow in many cases – injectivity reduces to zero and no more gas can be recovered.
Various projects have tried to counteract this effect by performing injection and production in a cyclical ‘huff and puff’ manner or by staging the injection, starting with nitrogen and then moving to CO2 afterwards. Some projects have succeeded by using raw flue gas as the injection gas. This has worked to an extent, but offers little benefit in terms of actual volumes of carbon storage.
The potential for CO2 storage is probably the main draw for investment in ECBM. However, CO2 injection is not simple, even if coal swelling were not an issue. For example, the CO2 gas must be delivered to the site in a processed form and injected into the seam under pressure. This is possible but expensive. In fact, CO2 processing costs are around 42 $/t, which means ECBM projects are simply not economically viable. Even if these costs were halved, projects would still struggle without some form of financing mechanism, such as CO2 credits under the UNFCCC (United Nations Framework Convention on Climate Change). In fact, the only ECBM project which has not been completely abandoned is a MOST (Ministry of Science and Technology, China) project in the Quinshui basin, China. Around 100 wells are planned for a significant ECBM site. The project economics appear to reply almost entirely on guaranteed funding for CO2 capture which will offset the additional gas processing costs.
The report Potential for enhanced coalbed methane recovery by Dr Lesley Sloss at the IEA Clean Coal Centre looks at the challenges involved in ECBM – both technological and economic – and the results from the various projects worldwide. It reviews each project in turn giving details of the parties involved, the success in terms of both enhanced methane recovery and total CO2 stored, and goes on to explain why each has either stalled or halted completely.
“ECBM has so far proven to be a bit of a pipe dream – offering a win-win situation in theory but proving to be too much of a technical challenge in practice. Even if funding does become available to overcome the financial hurdles, each project has the potential to cause further challenges in terms of additional unique, site specific technical hurdles” says Dr Sloss.
She concludes that for the moment, several barriers remain to the further development of ECBM:
• demonstration of the technology at a commercial level;
• practical experience with counteracting swelling and injectivity issues;
• best practice guidance for new developers and investors;
• high natural gas prices.
• financial incentives for flue gas and CO2 storage.
The report Potential for enhanced coalbed methane recovery by Dr L L Sloss CCC/252, ISBN 978-92-9029-574-7, 41 pp, May 2015 is available for download from the IEA Clean Coal Centre Bookshop http://bookshop.iea-coal.org.uk/site/uk/clean-coal-technology-research-reports. Residents of member countries and employees of sponsoring organisations can download the report at no charge after a one-off registration.
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