Background

Production of natural gas from both shale formations and coal deposits is rapidly developing as a major energy supply option in regions including North America, Europe and Australasia. Significant exploitation of these resources could affect CO2 geological storage potential.

Coal deposits have long been regarded as a potential CO2 storage option, in association with coal bed methane (CBM) production. Coal deposits used for enhanced coalbed methane (ECBM) are typically those that are too deep or too thin to be currently economically mined.

All coal deposits have varying amounts of methane adsorbed onto the pore surfaces. The methane may initially be recovered through dewatering and depressurisation i.e. as coal bed methane (CBM). Additional recovery and/or storage can then take place by injection of CO2 into the formation. The CO2 can be preferentially adsorbed onto the surface of the coal, thereby trapping the CO2 in the coal deposit or trapped in the coals cleat system. As it is now likely that additional recovery/storage operations using CO2 will take place after the initial dewatering and depressurisation, the integrity of the coal seam will need to be considered and whether it is still a receptor for CO2. This approach to coal seam CO2 storage may also overcome the injectivity problems encountered with the pilot CO2-ECBM projects. The cost of storage of CO2 may be offset by the production of methane. However this approach to coal seam storage is new and it is not known whether the CBM production process leaves behind a reservoir that is suitable for CO2 storage.
Shale formations constitute the most common, low-permeability caprocks that could prevent migration of buoyant CO2 from underlying storage units, particularly deep saline aquifers.

Organic-rich shales may also form potential storage units for CO2 based on trapping through adsorption on organic material (similar to coals), although this has not been demonstrated on a field scale. Lately, oil and gas companies combined horizontal drilling and rock fracturing technologies to produce oil and gas from shales, particularly in North America. Whilst these technologies open up the possibility of using shale formations as actual storage media for CO2 by increasing permeability and injectivity, the same technology may compromise the integrity of shale caprocks in some basins.

Advanced Resources International (ARI), a company based in the USA was commissioned by IEAGHG to undertake this study.

Key Messages

• Exploitation of gas from both shale and coal will leave the formations with increased permeability and injectivity and therefore with increased potential to store CO2.

• Large scale demonstration has yet to take place to confirm CO2 storage capability and capacity for both shale and coal. Though demonstration projects are more advanced for coal with several small scale projects injecting CO2 into wet coal seams (where there have been some injectivity problems related to coal swelling) and one project into an already dewatered coal seam.

• Overlap between potential shale gas exploration and potential storage reservoirs in deep saline formations may be considerable geographically, but much less so in 3D. Therefore use of both resources should be possible if well managed, though this will need considered on a case by case basis.

• There are still some uncertainties regarding CO2 storage in shale and coal, and knowledge gaps where further research is needed have been identified as part of this study.