Abstract:

Freezing point is an important indicator for evaluating the fluidity of oil products (including crude oil) in the petroleum industry, but its mechanical meaning has not been accurately quantified for a long time. In this paper, the stress and deformation of oil samples under the critical conditions in the gel point measurement experiment are quantitatively analyzed and numerically simulated. The study clarified the movement of this critical condition for the first time: It is a bow-shaped unyielding region rotating around a fixed center above the free surface. And the arc angle of the bow-shaped unyielding region is 137.5°. According to this result, the mechanical meaning of the gel point is, in fact, the temperature at which the yield stress of the crude oil reaches a certain critical value. This value is proportional to the oil density and the diameter of the test tube, so the critical yield stress is 14.14 Pa according to the GB 510—1983 and the critical yield stress is 19.99Pa according to the SY/T 0541-2009. The clarified mechanical meaning of the gel point will help to deepen the understanding of oil fluidity and its measurement and application.

Key words: non-Newtonian fluids, yield stress, petroleum, gel point, numerical simulation

摘要：

凝点是石油工业中评价油品(包括原油)流动性的一个重要指标，但长期以来对其力学涵义缺乏准确的定量分析。通过对测凝实验中油样受力与变形的定量分析与数值模拟，研究发现在判断凝点的临界状态下，油样的运动形式为一弓形未屈服区绕自由液面上方一固定圆心的转动，且该弓形所对应的弧角固定为137.5°。据此，含蜡油凝点的力学涵义事实上是油品的屈服应力刚好达到可产生该特定运动形式的临界温度。该临界屈服应力与油品密度及试管直径呈正比。在国家标准GB510—1983所规范的测量条件下，临界屈服应力为14.14 Pa；在石油行业标准SY/T0541所规范的测量条件下，临界屈服应力为19.99 Pa。对凝点力学涵义的准确认识，将有助于加深对油品流动性测量与应用的全面理解。

关键词: 非牛顿流体, 屈服应力, 石油, 凝点, 数值模拟