Carbon dioxide (CO2) sequestration in deep un-minable coal seam causes the seam's permeability and strength to be significantly reduced because of CO2 adsorption-induced matrix swelling. This paper reviews this swelling process in coal and its influence on coal's flow and strength properties.
The amount of swelling depends on the properties of both the gas and the coal mass. The swelling caused by CO2 adsorption is higher compared with that caused by CH4 and N2 and greatly depends on the state of the CO2 phase. The super-critical state of CO2 adsorption causes greater swelling compared with the sub-critical state. It has been observed that the swelling rate increases with increasing CO2 pressure; however, high saturation pressures may cause the coal matrix to shrink. The swelling rate reduces with increasing temperature, and the effect of coal rank on swelling still remains unclear. The CO2 adsorption-induced swelling effect causes the coal mass permeability to be significantly reduced, and the reduction is significant for super-critical CO2.The effect of swelling on coal permeability reduces with increasing temperature. The coal matrix swelling and associated polymer structure re-arrangement occur in the coal mass during and after the CO2 injection, resulting in the weakening of the coal mass strength. The strength reduction is much higher for high rank coal compared with low rank coal. It is also observed that the influence of super-critical CO2 on strength is more powerful than that of sub-critical CO2. Although CO2 sequestration in deep coal seams has been carried out in several main coal seams in the USA, Australia and some other countries, none of these projects has achieved the original target of storing large amounts of CO2, possibly because initial analysis did take into account the effects of coal seam property changes because of the adsorption of CO2 during and after injection. Copyright © 2012 John Wiley & Sons, Ltd.