Meitan xuebao (Jun 2024)
Study on the impact frequency modulation of self-excited oscillating pulsed SC-CO2 jet
Abstract
The SC-CO2 jet drilling and completion technology can ensure the stability of coalbed methane wells, improve drilling speed and rock breaking efficiency, and has a wide application prospect. However, the high energy consumption and complex system of coal breaking limit its engineering application. The impact frequency of self-excited oscillation pulse SC-CO2 jet is the same as the natural frequency of coal and rock mass or satisfies the phase relationship, which can give full play to the resonance effect and achieve a better coal breaking effect. The nozzle structure is the key to affect the jet impact frequency. The existing frequency modulation method of self-resonating pulsed water jet is not suitable for the SC-CO2 jet. In order to realize the modulation of the impact frequency of the SC-CO2 jet, a large eddy simulation was used to study the influence law of different nozzle structures on the impact frequency of self-excited oscillation pulse SC-CO2 jet, and the influence degree of different nozzle structure parameters on jet impact frequency was obtained by weight analysis. The influence rule of nozzle structure on the jet impact frequency and the reliability of jet impact frequency modulation method are verified by jet impact frequency measurement experiment and coal breaking experiment. The results show that the diameter and length of the oscillating cavity, and the angle of the collision wall are the key factors affecting the jet impact frequency. The jet impact frequency can be substantially modulated by adjusting the structure of the oscillating cavity, so as to achieve the resonance effect. By adjusting the oscillating cavity structure, the jet impact frequency forms resonance with the inherent frequency of coal, then the coal breaking effect is significantly improved, and the smaller the resonance multiplier, the better the coal breaking effect of the jet, which can effectively improve the coal breaking efficiency. When the natural frequency of coal is 25 Hz, the outlet diameter of the upstream and downstream nozzle is set as 2.5 mm and 3.0 mm, and the diameter, length and angle of the oscillating chamber are adjusted to 10 mm, 3 mm and 120° respectively. The jet impact frequency reaches 25051.83 Hz, which is 1002.0 times of the natural frequency of coal and forms a resonance effect with the natural frequency of coal, effectively improves the coal breaking efficiency.
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