Understanding CH4 adsorbed on shaly matrix displaced through injecting binary SO2 and CO2: Insights into oxy-coal combustion flue gas sequestration with enhanced shale gas recovery

To assess the feasibility of oxy-coal combustion flue gas sequestration with enhanced shale gas recovery (oxy-coal combustion flue gas-ESGR), the process of adsorbed CH4 displaced by injecting binary SO2 and CO2 as well as single CO2, i.e., control group, was addressed in this study. Results indicated that the co-injection of SO2 and CO2 leads to more CH4 desorption and resultant increased CH4 recovery by 54.88-117.35 % when compared with the injection of pure CO2. The reasons accounting for that include higher boiling temperature difference between SO2 and CH4, alterations in nitrogen-/sulfur-containing functional groups due to SO2 exposure, enhanced CO2 adsorption capability due to SO2 addition, and decreased clay minerals because of SO2 uptake. Thereinto, the promoted CO2 adsorption is ascribed to the less molecular dynamics diameter of SO2 than that of CO2, and strong attraction of SO2-interacted functional groups to CO2. The opposite changing trends of CH4 adsorption capacity and CO2 adsorption capacity raise the adsorption selectivity of CO2/CH4 by 33.90-535.89 % for all the shaly matrices, thereby highlighting the competitiveness of oxy-coal combustion flue gas-ESGR. Ultimately, both physisorption and chemisorption contribute to SO2 abatement as for co-injection of SO2 and CO2. The latter contributing 13.65-88.99 % to the overall SO2 adsorbability is an irreversible process, benefiting stable storage of SO2 in target reservoirs. In summary, the aforementioned results well address that injection of binary SO2 and CO2 instead of single CO2 into gas-bearing shaly reservoirs can not only profoundly enhance CH4 recovery and realize CO2 sequestration, but also achieve SO2 geologic sequestration.