Effect of pulsation frequencies on the fracture characteristics of CO2 foam fractured coal: An experimental study

The pulsating CO₂ foam fracturing technology represents an efficacious approach to enhance coalbed methane recovery, exhibiting considerable potential for widespread application. However, the influence of pulsation frequency on the acoustic emission response characteristics and fracture propagation mechanism during CO₂ foam fracturing of coal remains elusive, impeding the engineering application. To address the issues above, a self-built pulsating CO₂ foam fracturing experiment system was used to conduct the experiments of CO₂ foam fracturing with different pulsation frequencies (0–20 Hz), with simultaneous acquisition of pressure-time curves and acoustic emission signals throughout the process. The results show that as the pulsation frequency increases, the average breakdown pressure of fractured coal exhibits a cubic polynomial decrease, while the average fracturing duration increases. Concurrently, the cumulative energy of acoustic emission increases gradually, and the number of acoustic emission location points grows exponentially. The macroscopic cracks in coal exhibit a symmetrical “wing-shaped” distribution on both sides of the borehole. The fracture type of fractured coal is dominated by tensile failure and supplemented by shear failure, and the larger the pulsation frequency, the higher the proportion of tensile failure. Notably, before the formation of macroscopic fractures in the coal, the improved b-value of acoustic emission exhibits a significant decrease. The fracture propagation of pulsating CO₂ foam fractured coal results from the synergistic effects of multiple factors, including pulsating fatigue impact, cold shock from the fracturing fluid, mineral dissolution by carbonic acid and surfactants, CO₂ adsorption-induced coal matrix expansion, and CO₂ phase transition. In conclusion, the pulsating CO₂ foam fracturing technology significantly enhances the fracture complexity and pore connectivity of coal through high-frequency pulsation and modification synergy, and effectively enhances the coalbed methane recovery, as well as reduces the consumption of water resources and realizes the CO₂ sequestration, which provides an innovative solution for the economic and efficient development of low-permeability coal beds and the reduction of greenhouse gas emissions. The research results can provide basic theoretical support for the engineering application of pulsating CO₂ foam fracturing, which is of great significance for the prevention and control of coal mine gas disasters and the enhancement of coalbed methane recovery.