Carbon dioxide sequestration in coal seams with enhanced coal-bed methane recovery (CO2-ECBM) is considered as a promising option for permanent carbon dioxide storage. As one of the most important factors to the success of the CO2-ECBM process, adsorption of methane and CO2 on coal helps to assess the amount of recoverable methane as well as the storage capacity of CO2 of the targeted coal seam. In this work, the methane and CO2 adsorption isotherms were measured with a volumetric technique at temperatures of 35, 50, and 65 °C and pressures up to 16 and 12 MPa (CO2 adsorption at 35 °C is limited below 6 MPa), respectively. Four coals of various rank exploited from four main coal seams in China were tested. The isotherms fit well to the Ono-Kondo lattice model, which confirms the applicability of this model in describing the adsorption behaviors of methane and CO2 on coal under the supercritical conditions. In addition, the experimental results show that the excess adsorption of CO2 reaches the maximum level between 7 and 9 MPa, while the excess adsorption of methane exhibits a less pronounced maximum. The maximum adsorption capacities of the coals for methane and CO2 decrease slightly with temperature increase. Additionally, the maximum adsorption capacities of methane and CO2 are also dependent on coal rank (indicated by vitrinite reflectance coefficient, R o max) and present a U-shaped trend with coal rank. The preferential adsorption ratio of CO2 to methane on a basis of absolute adsorption obtained from Ono-Kondo lattice model is in the range of 1.13−3.52 under test conditions. This ratio of bituminous coals (R o max ranging from 0.47% to 1.35%) decreases significantly with increasing pressure; however, the pressure dependence of the preferential adsorption ratio is less pronounced for the anthracite (R o max = 4.06%). The preferential adsorption ratio decreases with an increase in coal rank with the only exception of anthracite.

The secondary biogenic gas is an important original type of the coalbed methane (CBM) in China. Based on the analyses of sedimentary and burial history of the Permian coal-bearing strata, combined with thermal history and gas generation process of coals, the CBM reservoirforming dynamic system with mixed secondary biogenic and thermogenic gases in the Huainan Coalfield is subdivided into four evolutionary stages as follows: (i) shallowly-buried peat and early biogenic gas stage; (ii) deeply buried coal seams and thermogenic gas stage; (iii) exhumation of coal-bearing strata and adsorbed gas lost stage; and (iv) re-buried coal-bearing strata and secondary biogenic gas supplement stage. The Huainan CBM reservoir-forming model has the features of the basin-centered gas accumulation. The evolution of the reservoir-forming dynamic system proves that the thermogenic gas is not the main gas source for the Huainan CBM reservoir. Only the secondary biogenic gases as an additional source replenish into the coal bed after basin-uplift, erosional unroofing and subsequent scattering of thermogenic gases. Then this kind of mixed CBM reservoirs can be formed under suitable conditions.

The displacement behavior of methane adsorbed on coals by CO2 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 CO2 as well as the amount of methane recovery. The results show that the adsorption of CO2 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 CO2 on coal without initial methane saturation have been conducted. It is found that, for the coal sample without methane saturation, the CO2 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 (CO2–ECBM), it would be better to inject CO2 after partial desorption of methane by depressurizing in order to improve CO2 storage. The selectivity ratio of CO2 and effects of operation parameters such as pressure and temperature have been discussed. On the basis of the results, a recommended depth of CO2 sequestration in coal seams has been put forward.

Based on the adsorption potential theory, this paper processes and analyzes the isothermal adsorption data at different temperatures of four coal samples with different ranks. The research results indicate that the adsorption characteristic curve of methane adsorbed on the coal sample is a single curve, and the mathematical expression of the methane adsorption capacity, temperature and pressure is achieved from the characteristic curve. According to the calculating routine and method presented, the adsorption capacity at other equilibrium conditions can be calculated from the adsorption data of one temperature. The results of this paper are a significant contribution to the research on the storage mechanism of the coalbed methane.