Abstract:Low well wall stability and low permeability of the coal seam are key factors limiting the extraction of coalbed methane. Supercritical carbon dioxide jet drilling technology has its unique advantages in coalbed methane extraction. Due to its high density,low viscosity and strong diffusivity,it not only enhances well wall stability and recovery rates,but also enables underground storage of carbon dioxide. However,the highenergy consumption of the system limits the application of supercritical carbon dioxide jet drilling technology. On this basis,the selfexcited oscillation pulsed supercritical carbon dioxide jet technology reduces the system energy consumption and promotes the application of carbon utilization and storage technology. In order to enrich and improve the theory of selfexcited oscillation pulsed supercritical carbon dioxide jet,the effect of cavity diameter ratio on the performance of selfexcited oscillation pulsed supercritical carbon dioxide jet was investigated. Based on the flow field structure,various numerical simulations were used to select a range of cavity diameter ratios suitable for the selfexcited oscillation pulsed supercritical carbon dioxide jets and to analyze the effect of cavity diameter ratio on the pulse performance of the jets. The effect of the cavity diameter ratio on the pulse pressure of the self excited oscillation pulsed supercritical carbon dioxide jet was investigated. The self excited oscillation pulsed supercritical carbon dioxide jet breaking experiments were carried out using the selfexcited oscillation pulsed supercritical carbon dioxide jet rock breakage experimental system under different cavity diameter ratios to compare and verify the pulse performance of the jet under different cavity diameter ratios. The results show that the cavity diameter ratio is not an isolated factor,but is related to the upstream nozzle diameter to influence the pulse performance of the jet and the coal breakage effect. The higher the matching ratio,the higher the pulse pressure of the selfexcited oscillation pulsed supercritical carbon dioxide jet and the better the coal breakage effect. When the upstream nozzle diameters are 2.6 mm and 2.4 mm,and the cavity diameter ratios are 3.5 and 3.0 respectively,the pulse performance of the selfexcited oscillation pulse supercritical carbon dioxide jet is the best and the coal breakage efficiency is the highest.