Abstract: Natural gas hydrate resources are distributed in deep-sea sediments and permafrost layers under high-pressure and low-temperature conditions. CO₂ fluid is widely applied in various unconventional oil and gas drilling and extraction projects. However, there is a lack of systematic research on the characteristics and modulation methods of liquid CO₂ jet flow in the downhole environments during the drilling and completion processes. The paper establishes a CO₂ fluid phase transition model controlled by cavitation and heat transfer, conducts comparative analyses on the jet velocity and vapor phase distribution characteristics of conical nozzles, convergent-divergent cavitation nozzles, and self-resonating cavitation nozzles under the downhole pressure and temperature conditions of hydrate wells, and investigates the influence of CO₂ fluid temperature on jet mass flow rate, axial velocity, and vapor phase distribution. The results show that the phase transition of liquid CO₂ jets and the formation of Mach disc structures can be inhibited under the low-temperature and high-pressure conditions at the bottom of natural gas hydrate wells, and significant self-excited pulsation phenomena exist in the flow field. The cavitation clouds formed by the three nozzles exhibit strip-shaped, fan-shaped, and spindle-shaped patterns, respectively. Compared to conventional water jets, liquid CO₂ jets can achieve higher jet velocity under lower energy consumption. The CO₂ fluid temperature has limited effects on jet mass flow rate, axial velocity, and vapor phase evolution cycles, but significantly affects the vapor phase distribution range. By contrast, convergent-divergent nozzles have faster jet velocity and stronger cavitation effect. Increasing the fluid temperature can suppress the self-excited pulsation phenomenon of jets and promote cavitation in the external flow field.

Key words: natural gas hydrate, liquid carbon dioxide jet, low temperature and high pressure, Mach disc, gas-liquid phase transition

摘要： 天然气水合物资源分布于高压低温环境的深海沉积物和永久冻土层中,而CO₂则被广泛应用于各类非常规油气资源的钻采工程,但在钻完井过程中井底环境液态CO₂射流特性和调制方法缺乏系统研究。通过建立由空化和传热共同控制的CO₂流体相变模型,对比分析了水合物井底压力温度条件下锥形喷嘴、收缩—扩张型空化喷嘴和自振空化喷嘴的射流速度和气相分布特征,揭示了CO₂流体温度对射流质量流量、轴线速度和气相分布的影响规律。研究结果表明:天然气水合物井底的低温高压条件会抑制液态CO₂射流相变和马赫盘结构的产生,流场存在明显的自激脉冲现象;3种喷嘴的空化云分别呈条状、扇形和梭形;相较于常规水射流,液态CO₂射流消耗较少能量即可获得较高的射流速度;CO₂流体温度对射流质量流量、轴线速度和气相演化周期影响较小,但对气相分布范围影响较大;收缩—扩张型喷嘴的射流速度更快,空化作用更强;提高流体温度会抑制射流的自激脉冲现象,促进外流场空化作用。

关键词: 天然气水合物, 液态二氧化碳射流, 低温高压, 马赫盘, 气液相变