欠平衡钻井多相流动理论与计算分析

把欠平衡钻井循环流动分为钻柱内向下流动和环空向上流动，按气、液两相流动问题研究该混合体系流动过程，建立新的理论模型。采用时间有限差分离散，空间有限体积离散的方法求问题的数值解。选取中国欠平衡钻井的常见井眼尺寸和常用的注入气、液速度范围，进行数值计算，分析得出，流动过程的流动型式主要为泡状流和段塞流，环空接近地面的井段有时为搅动流。举例分析在环空和钻柱内的稳定流动过程中，随井深增加，压力、含气率、多相流体密度等的变化规律；在环空流动过程中，随井深增加，斜井段与相应垂直井段的压力值不同。分析了在关小出口节流阀开度的不稳定流动过程中，环空流动压力值沿井深增加的变化过程；用实际井欠平衡钻进的随钻环空压力测量数据与本文理论模型及求解方法计算所得结果比较表明：计算结果能很好地反映钻进过程中环空压力的变化规律。图6表2参12

A dynamic modeling of underbalanced drilling multiphase-flow and numerical calculation

Circulation flows in underbalanced drilling include downward flow in drill pipe and upward flow in annulus. The circulation flow system is studied according to the gas-liquid two-phase flow theory and a new dynamic model is established. The dynamic model is numerically solved by the finite difference method scattering time and the finite volume method scattering space. The often-encountered hole sizes in oilfields and the varied combination injection ratios of gas and liquid were inputted to the model, and the numerical calculation show that the dominant flow patterns are bubble flow and slug flow. Churn flow only exists in the annulus near to the surface. The steady-state flows in drill pipe and in annulus are analyzed and the variations of pressure, gas void fraction, and multiphase-flow density along the well depth are simulated. In the annulus flow, the pressures in deviated hole and the corresponding vertical hole vary with depth. The change of the annulus pressure in the non-steady multiphase-flow in closing choke valves is simulated. The calculation results of the model are validated by actually measured data, showing that the model can predict annulus pressure variation in drilling.