非均匀椭圆载荷下套管抗外挤能力分析

Analysis on Collapse Resistance of Casing under Non-uniform Elliptic Load

摘要:

目前大多数油井套管的损坏很大程度上是由非均匀载荷引起的，外载荷的非均匀性等因素大大降低了套管的承载能力，为此，揭示这些因素对套管抗挤能力的影响具有重要的实际意义。根据非均匀椭圆载荷作用下套管的受力特点，利用材料力学理论建立了理想套管的环向应力计算模型，分析了套管的应力分布规律，结果表明：套管承载的危险点为圆周方向与最大水平地应力成90°夹角的内壁上，与有限元数值模拟结果有着很好的一致性。并以危险点应力状态为依据，得到了套管在不同的非均匀载荷下的等效外挤压力计算的新公式。计算结果表明，椭圆载荷中的非均匀部分对套管等效外挤压力影响极大；外载荷非均匀度越大，径厚比越大，套管承受外挤载荷能力越差，越容易发生挤毁失效。

关键词:椭圆载荷;环向应力;危险点;等效外挤压力;抗外挤能力

Abstract:

Most of the damage of oil well casing is caused by non-uniform load, the non-uniform of external load and some other factors greatly reduce the bearing capacity of the case. There is important practical significance to reveal the influence of these factors on the collapse resistance of casing. According to the characteristics of hoop stress on casing under non-uniform elliptic load condition, a calculation model of hoop stress for ideal casing is established by the theory of material mechanics. Analysis is made on the regularity of stress distribution, the results show that the dangerous point is on the inner wall of casing by 90 angle between circumferential direction and the maximum horizontal stress, which has good consistency with the finite element numerical simulation result. Based on the stress state at the dangerous point, the new formula is proposed to calculate the equivalent external pressure on casing in different non-uniform loading. The calculation shows that non-uniform of elliptic load has great influence on casing equivalent external pressure; the more the non-uniform and the larger diameter-thickness ratio are, the poorer the bearing capacity of external load is and more easily collapse failure of casing happen.

Key words:elliptic load; hoop stress; endangered point; equivalent external pressure; collapse resistance