Modified assessment method of CO2 geologic storage capacity in deep coal and its application in the Zhengzhuang Block, Qinshui Basin

As an important part of CCUS (carbon capture, utilization and storage), CO2 geologic storage in deep coal seams has been proposed as a method to reduce CO2 emissions into the atmosphere and achieve carbon neutrality in the immediate future. This study, taking No. 3 coal in the Zhengzhuang Block as an example, was based on the CO2 adsorption isotherms experiments at various temperatures. A modified assessment method of CO2 geologic storage capacity was established, the curves of CO2 storage capacity in coals at depth profile in the Zhengzhuang Block were analyzed, CO2 adsorption mechanisms at different depths are presented, and the theoretical and effective CO2 storage capacity of No. 3 coal in Zhengzhuang Block, Qinshui Basin was estimated. The results show that: The supercritical Dubinin-Radushkevich (D-R) model for CO2 adsorption data of CZ and SH coals has a high fitting accuracy. Adsorption capacities of the moisture-equilibrated coals at different temperatures ranged from 1. 58 to 2. 47 mmol/g and from 1. 51 to 2. 38 mmol/g, respectively. The curves of the modified calculated CO2 storage capacity per 1 g coal (CO2-SCC) combined adsorbed and free CO2 of CZ and SH coals presented maximum values (2. 71 mmol/g and 2. 80 mmol/g) near 250m in the Zhengzhuang Block. At the depth of 2000 m, the CO2-SCCs of CZ and SH coals still maintained 76% and 67% of their own maximum values, suggesting that the deep coal also has a promising CO2 geologic storage capacity. In depth profiles, the CO2 adsorption behavior can be subdivided into three states: a) surface coverage adsorption at low density, b) micropore filling adsorption at high density and c) surface coverage adsorption at high density. The theoretical CO2 storage capacity of No. 3 coal seam in Zhengzhuang Block is 428. 7 Mt, and they in gas and gas-like supercritical zone, and liquidlike supercritical zone are 277. 91 Mt and 150. 79 Mt respectively. The effective CO2 storage capacity in study area is 340. 59 Mt and they in above areas are 250. 12 Mt and 90. 47 Mt, respectively. It is suggested that CO2-ECBM projects should be preferentially implemented in areas with high permeability and superior gas preservation condition. © 2023 China University of Mining and Technology. All rights reserved.