椭圆度和偏心及微裂纹缺陷套管剩余强度分析

Residual strength analysis of ovality and eccentricity and microcrack defects

套管用于支撑油气井井壁,形成完整井筒,部分套管存在椭圆度、内外壁偏心及微裂纹等制造缺陷,这些缺陷会降低套管强度。因此,有必要分析含制造缺陷套管的剩余强度。依据美国石油协会(American Petroleum Institute,API)套管抗外挤强度分析公式,考虑椭圆度和偏心引起的径厚比差异,据此修正径厚比,得到缺陷套管剩余强度分析公式。以P110套管为例,尺寸为127 mm×7.52 mm(外径×壁厚),对套管剩余强度进行理论分析和数值分析,对比分析椭圆度变形量和圆心偏离程度在1.0%~3.0%范围变化时套管的剩余强度。分析表明,椭圆度和偏心度为3.0%时套管剩余强度分别降低了6.81%和7.02%,与数值分析结果误差分别为1.91%和2.04%,剩余强度计算公式的精度满足工程要求。尺寸相同的微裂纹位于管壁内部、内壁和外壁时,强度分别降低了8.51%、3.55%、4.69%,微裂纹位于管壁内部时强度降低最多,套管强度随微裂纹增加呈近似线性降低趋势。依据研究,建立了不同椭圆度和偏心度套管剩余强度速查图表,方便设计和作业人员使用。研究结果可为含制造缺陷的127 mm×7.52 mm P110套管的剩余强度分析提供理论依据,可为弹性力学中含缺陷的筒体问题提供新的解决方案。

Casing pipes are used to support the wellbore of oil and gas wells so as to form complete wellbores.In some cases,these casing pipes suffer from manufacturing defects such as ovality,inner and outer wall eccentricity and microcracks,which lead to strength reduction.Therefore,it is necessary to analyze the residual strength of the sleeve with various manufacturing defects.According to the analysis formula of the anti-extrusion strength of casing pipe from the American Petroleum Institute(API),the diameter-thickness ratio was corrected accordingly taking into consideration of the difference in the diameter-thickness ratio caused by ovality and eccentricity,and the residual strength analysis formula of the defective casing was obtained.Taking the 127 mm×7.52 mm P110 casing pipe as an example,the residual strength of the casing pipe was analyzed theoretically and numerically,and the residual strength of the casing pipes was compared and analyzed when the ellipticism deformation and the deviation degree of the center of the circle changed in the range of 1%-3%.The analysis shows that when the elliptonality and eccentricity are 3%,the residual strength of the casing is reduced by 6.81%and 7.02%,respectively,the error is 1.91%and 2.04%,respectively,and the accuracy of the residual strength calculation formula meets the engineering requirements.When the microcracks of the same size are located in the inside,inner and outer walls of the pipe wall,the strength decreases by 8.51%,3.55%and 4.69%,respectively.The strength of the microcracks decreases the most when the microcracks are located in the pipe wall,and the strength of the casing pipes decreases approximately linear with the increase of microcracks.According to the research,a quick reference chart of the residual strength of the casing pipes with different ovality and eccentricity was established,which was conductive to design and operation.The research results herein can provide not only a theoretical basis for the residual strength analysis of 127 mm×7.52 mm P110 casing pipe with manufacturing defects,but also a new solution for the problem of cylinders with defects in elastic mechanics.