**3.3 CO2 injection into unconventional reservoirs**

The two main unconventional formations where CO2 injection is promising are coal seams and shale gas reservoirs. CO2 enhanced coalbed methane recovery (CO2- ECBM) has the potential to store large volumes of CO2 in deep unmineable coal seams while improving the efficiency of coal bed methane recovery [39, 40]. The injected CO2 displaces methane and remain sequestered in the coal seams as CO2 is preferentially adsorbed onto coal seams, thus releasing the coal bed methane which can then be produced as free gas [41]. Based on the simple assumption that, for

every CH4 molecule, two molecules of CO2 can be stored, IEA-GHG [42] estimated that about 220 GT of CO2 could be stored in deep unmineable coal formations worldwide.

The potential to store CO2 in organic-rich gas shales is attracting increasing interest, especially in countries that have extensive shale deposits [43–45]. Although still in early-stage research, CO2 injection into organic-rich gas shales could provide dual benefits: an economic benefit from the incremental recovery of adsorbed methane, and an environmental benefit of secure CO2 storage.
