Microstructure characteristics of bituminous coal under the synergistic effect of VES, acids and oxidants: Implications for CO2 injection capacity

Injecting CO2 into deep coal seams is very promising as it can achieve large-scale safe carbon sequestration. However, a major challenge currently is low injection efficiency, prompting exploration of a fracturing fluid that can modify coal to increase the efficiency of CO2 injection. In this study, three different CO2-injection-enhancement fracturing fluids with varying molar ratios of acetic acid and hydrogen peroxide were prepared (IEFFs-A(1:0.5), IEFFs-B(1:1), IEFFs-C(1:1.5)), combined with quaternary ammonium Gemini surfactants and potassium chloride. The effects of IEFFs on bituminous coal surface morphology, molecular structure, pore structure, and wettability were investigated by atomic force microscope, mercury intrusion porosimetry, fourier-transform infrared spectroscopy, and contact angle tests. Results showed that IEFFs can significantly improve CO2 injection efficiency by regulating coal microstructure and wettability compared with deionized water. Specifically, IEFFs modification simplified pore structure and improved pore connectivity, resulting in a 45%–86 % increase in seepage pores volume and a 2–4 times increase in permeability. Meanwhile, the increase in oxygen-containing functional groups (13%–30 %) and the decrease in aromatic structures improved wettability by 25%–30 %, reducing capillary pressure by 12%–15.5 %, which lowered the resistance to flow. IEFFs-B was preferred to show the best modification effect, enhancing the efficiency of CO2 injection into coal seams.