Displacement Behavior of Methane Adsorbed on Coal by CO 2 Injection

The displacement behavior of methane adsorbed on coals by CO 2 injection has been experimentally studied. With the assumption of the additive property of the adsorbed-phase volume, a novel data processing method is established to obtain the absolute adsorption amount of methane and CO 2 as well as the amount of methane recovery. The results show that the adsorption of CO 2 on low-rank Bulianta coal initially saturated with methane is higher than that on high-rank Qinshui coal. For comparison, the adsorption isotherms of pure methane and CO 2 on coal without initial methane saturation have been conducted. It is found that, for the coal sample without methane saturation, the CO 2 adsorption capacity of high-rank Qinshui coal is superior to that of low-rank Bulianta coal. A conceptual model is proposed to give the explanation. It is indicated that for carbon dioxide sequestration on coal with enhanced coal bed methane recovery (CO 2 –ECBM), it would be better to inject CO 2 after partial desorption of methane by depressurizing in order to improve CO 2 storage. The selectivity ratio of CO 2 and effects of operation parameters such as pressure and temperature have been discussed. On the basis of the results, a recommended depth of CO 2 sequestration in coal seams has been put forward.