Evaluation of CO2 storage in fractured coal seam and the effect of coal fines

Gas channeling in fractures during CO2 injection into the deep coal seam seriously reduces the CO2 storage efficiency after the development of coalbed methane. The generation and migration of coal fines causes blockages in the fractures in the stage of drainage and gas production, reducing the gas channeling effect of injected CO2 caused by the heterogeneity of the coal seam. To explore the impact of coal fines within coal seam fractures on the efficacy of CO2 storage, experiments on the production stage and CO2 injection for storage were conducted on coal combinations containing propped fractures, fractures, and matrix. The CO2 storage characteristics of coal at the constraint of coal fines, as well as the influence of multiple rounds of intermittent CO2 injection and different injection parameters on the CO2 storage effect, were analyzed. The research results show that blockage by coal fines increases the resistance to fluid flow in the fractures by 71.2%. The CO2 storage capacity and storage potential of coal with coal fines are 6.5 cm3/g and 8.8% higher than those of coal without coal fines, while the CO2 storage capacity of fractured coal under the influence of coal fines has the largest increase of 9.4 cm3/g. The CO2 storage of coal containing coal fines is significantly higher (6.6%) than that of the coal without coal fines. The CO2 storage effect of the coal with coal fines is improved with the increase in injection rate, whereas the CO2 storage effect of the coal without coal fines decreases significantly (by 7.8%). Multiple rounds of intermittent injection increases the CO2 storage volume of coal by 20.4% (with coal fines) and 17.1% (without coal fines). The presence of coal fines in fractures also slows down the downward trend of CO2 storage fraction after multiple rounds of CO2 injection. The blockage in fractures significantly increases the CO2 injection time and difficulty, but can increase the CO2 storage fraction by 4.7%–17.1%, and the storage volume by 1.9%–14%, increasing the feasibility of CO2 storage in fractured coal seams that have previously been exploited for methane production. The multiple rounds of intermittent CO2 injection and shut-in periods has shown potential for greater CO2 storage and injection efficiency.