Synergistic damage effects and mechanism of CO₂-alkaline-water on physicochemical properties of bituminous coal

During gas displacement, CO₂ enrichment is the key challenge restricting the field application of CO₂ enhanced coalbed methane recovery (CO₂-ECBM). For conventional coal seam water injection, it is difficult for injected water to enter the micropore, resulting in an unsatisfactory water injection effect. The CO₂-alkaline-water two phase displacing gas and wetting coal (the CADW) method is proposed in recent years, which can significantly promote the coalbed methane recovery efficiency, effectively eliminate the CO₂ outburst risk, and improve the coalbed water injection effect. However, both the carbonic acid, formed by the residual CO₂ dissolution, and alkaline-water will cause obvious dissolution effects on coal. To grasp the synergistic damage effect of CO₂-alkaline-water, the damage characteristics of mineral composition, pore structure and mechanical strength of bituminous coal were studied using various techniques, including the X-ray diffraction, water quality analysis, SEM, low-temperature CO₂ adsorption, uniaxial compression experiment and acoustic emission characterization test. The results showed that: ① Compared with conventional coal seam water injection, after using the CADW, coal was subjected to double erosion of carbonic acid, formed by residual CO₂ dissolution, alkaline-water and stronger water dissolution, the dissolution degree of coal minerals was obviously improved. The dissolution of minerals in bituminous coal increased evidently, the contents of minerals such as kaolinite and muscovite decreased distinctly. After direct water injection, \mathrmHCO_3^- was the main ion component in the leaching solution of LS bituminous coal, accounting for 63.89% of the total ions. However, after the CADW, the \mathrmHCO_3^- disappeared, while the contents of Na⁺, \mathrmCO_3^2- , K⁺, \mathrmNH_4^+ and Cl⁻ ions increased sharply, and the content of \mathrmCO_3^2- accounted for 57.57% of the total ions. ② Compared with direct water injection, after adopting the CADW, the pore structure of bituminous coal was developed obviously, the number and size of pores were increased clearly. Some pores connected and formed obvious grooved fracture structure. Single water invasion had little influence on the micropore structure of bituminous coal, while the CO₂-alkaline-water had a strong coupling dissolution effect on the micropores of bituminous coal, which was subjected to strong “pore-enlargement” and “pore-expansion” effects. The specific surface area and total pore volume of micropores in LS coal increased by 1.77% and 3.63% respectively. ③ Compared with conventional water injection, the mechanical strength of bituminous coal was reduced significantly after using the CADW. The elastic modulus, uniaxial compressive strength and cumulative energy of LS coal were reduced by 57.35%, 45.99% and 22.83%, respectively. The research results can provide important references in breaking through the technical bottleneck of the CO₂-ECBM and preventing coal and gas outburst risk.