天然气井下节流临界状态的判别方法

临界状态参数，尤其是临界压力比，这是天然气井下节流嘴设计计算以及判断节流嘴内是否达到临界流动，能否实现气井稳产的关键数据。以往对天然气井下节流临界流动状态的判别往往基于经验公式。由于井下节流，节流嘴上游压力的不易测得，使得临界流动状态判别困难。以空气动力学的基本原理对井下节流过程进行了理论分析，分析指出实际临界压力比要小于理论临界压力比，理论临界压比只是临界流状态的必要而非充分条件。进而提出了临界状态判别的新方法，即用临界流动的原始定义（即马赫数）来判定。若节流嘴出口处的马赫数大于1，则说明流动已处于超临界状态，在节流嘴的最小截面处即喉部可以达到临界流动；若截面2处的马赫数小于1，则节流嘴中流动处于亚临界状态。该方法物可有效的判别井下节流的流动状态，在此基础上，可进一步完成节流嘴入口处、井底的气体流动预测研究。

DISCRIMINANCE OF THE FLOW STATE IN DOWNHOLE CHOKE OF NATURAL GAS WELLS

Flow state parameters, in particular, critical pressure ratio, are essential in design and calculation of gas well downhole chokes, determining flow state in the choke and in guarantee of steady production of a gas well. In the past, the empirical equations were always used to determine the flow state. However, it is so hard to measure the upstream pressure of downhole choke that the empirical equations are useless. This paper analyzes downhole flow choke process based on the aerodynamic theory. It showed that the value of the actual critical pressure ratio is less than that of ideal critical pressure ratio. So, critical pressure ratio is not a sufficient condition but a necessary condition. A new method comparing the value of the Mach (Mach number at the outlet of the choke) is recommended to determine the flow state. If the Mach value >1, it indicates a super critical flow state, that is, critical flow is achieved at the choke throat (Min. cross section) while Mach value at Cross Section 2 <1 indicates that flow in the choke is in sub-critical state.