钻具旋转时同心窄间隙环空流动的数值模拟

Numerical simulation of flow in the concentric narrow annulus during drilling tool rotation

摘要:

钻井过程中，当循环钻井液在井眼环空中作轴向流动时，由于钻具旋转而导致周向运动的出现，整个环空中因此形成螺旋流动，从而可能会对环空（特别是窄间隙）摩擦压降存在潜在的影响。对此，本文借助数值模拟软件，对同心窄间隙环空中钻具旋转下的层流流动进行直接数值模拟计算并分析。其中，钻具理想化为钻杆或圆柱体；钻井液则选用3种不同流变参数的宾汉流体。当前数值模拟计算结果表明，在不同钻具/杆转速下，PLR（压损比）几乎保持不变；这就表明，钻具/杆旋转过程中，并不会引起环空中摩擦压降的显著变化；然而，对比一些经验公式后发现，除了Ooms（1999）的公式能与当前计算吻合外，其余公式均不能反应该现象。进一步地，通过观察环空压力沿径向的变化过程时，结果表明，在当前180 rad/min范围内，压力沿径向几乎不变；即使当转速增大到600 rad/min，这种径向变化也十分有限，占比大约是2×10^-5量级。因此，在当前工程应用范围（钻具转速在180 rad/min以内）内，钻具旋转既不会导致轴向摩擦压降增大，也不会引起压力沿着径向显著变化。本文研究结果为实际钻井过程中对钻具旋转引起的摩擦压降的估算提供了一些参考，有助于优化钻井工艺和设备设计，提高钻进效率和安全性。

关键词:钻具旋转;窄间隙;环空流动;宾汉流体;摩擦压降;数值模拟

Abstract:

During drilling， when circulating drilling fluid flows axially in the wellbore ring， the circumferential motion occurs due to the rotation of drilling tools， and spiral flow is formed in the whole annulus. This may have a potential impact on the friction pressure drop in the annulus， especially the narrow gap. In this paper， the laminar flow under the rotation of concentric narrow gap ring drilling tools is calculated and analyzed by numerical simulation software. Among them， the drilling tool is ideal as drill pipe or cylinder， and the drilling fluid is composed of three kinds of Bingham fluid with different rheological parameters. The drilling fluid is the current numerical simulation results show that the PLR （pressure-loss ratio） is almost constant at different drilling tool/rod speeds； this indicates that the friction pressure drop in the annulus does not change significantly during the drilling tool/rod rotation； However， a comparison of some empirical formulas reveals that， with the exception of Ooms （1999） formula， which matches the current calculations， none of the other formulas can reflect the phenomenon. Moreover， by observing the radial variation of annulus pressure， the results show that the pressure is almost constant along the radial direction in the current 180rad/min range， and even when the rotational speed increases to 600 rad/min， the radial variation is very limited， accounting for about 2×10^-5 order of magnitude. therefore， in the current engineering application range （drilling tool speed within 180rad/min）， drilling tool rotation will neither cause an increase in axial friction pressure drop nor cause significant changes in pressure along the radial direction. The results of this paper provide some references for the estimation of friction pressure drop caused by rotating drilling tools in the actual drilling process， which can help to optimize the drilling process and equipment design， and improve the drilling efficiency and safety.

Key words:drilling tool rotation;narrow gap;annular flow;Bingham fluid;friction pressure drop;numerical simulation