深井大尺寸环空气液两相流动规律数值模拟研究

Numerical simulation of two-phase flow of gas-liquid two-phase flow in large-size annulus of deep well

  • 摘要: 深层和超深层油气井由于井身结构复杂且部分井眼尺寸较大,在钻进过程中容易遇到异常压力,导致安全作业窗口变窄。当发生气侵时,井筒环空内会形成气液两相流。传统的基于常规尺寸流型转化理论的压井施工方法,容易超出窄窗口,导致涌漏交替,从而错过最佳压井时机。为解决这一问题,基于VOF模型开发了一种适用于大尺寸环空气液两相流动的数值模拟方法,并通过文献数据验证了其准确性。在水力当量直径196.8 mm环空内进行的气液两相流动模拟中,识别出泡状流、弹帽流、段塞流和搅拌流4种流型特征,并据此绘制了气液两相流流型图,建立了流型转化判据,揭示了环空尺寸对流型转化的影响规律。研究结果表明,与常规尺寸环空相比,大尺寸环空中的泡状流范围扩大,且在泡状流与段塞流之间存在过渡流型——弹帽流,各流型转化边界均有不同程度的右移。由于常规尺寸环空更易发生气泡聚并形成Taylor泡,压井操作困难,因此,根据常规尺寸环空的流型转化判据为大尺寸环空设计的压井参数往往偏大。相比之下,基于新判据设计的压井参数能够更好地适应窄窗口和大尺寸井眼的压井需求,提高了压井的效率和安全性。

     

    Abstract: Deep and ultra-deep oil and gas wells often encounter abnormal pressures during drilling due to their complex wellbore structures and larger borehole sizes, which results in a narrowed safe operating window. When gas intrusion occurs, a gas-liquid two-phase flow forms in the annulus. Conventional well control methods based on standard flow pattern transition theories are prone to exceeding this narrow window, leading to alternating influx and loss, thereby missing the optimal well control timing. To address this issue, this paper develops a numerical simulation method for gas-liquid two-phase flow in large-diameter annuli using the Volume of Fluid (VOF) model, validated against literature data for accuracy. In simulations conducted within a 196.85 mm annulus, four flow patterns—bubbly flow, cap-bubbly flow, slug flow, and churn flow—were identified. A gas-liquid two-phase flow pattern map was created, and criteria for flow pattern transitions were established, revealing the influence of annular size on these transitions. The results indicate that, compared to conventional annuli, the range of bubbly flow expands in larger annuli, with a transitional flow pattern—cap-bubbly flow—occurring between bubbly and slug flows. The boundaries for flow pattern transitions shift to the right to varying degrees. In conventional annuli, bubble coalescence and the formation of Taylor bubbles are more likely, making well control operations more challenging. Consequently, well control parameters designed for large annuli tend to be oversized. In contrast, the well control parameters designed based on the new criteria better accommodate the needs of well control in narrow windows and large borehole sizes, thereby improving the efficiency and safety of well control operations.

     

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