热采井高温对套管强度的影响

INFLUENCE OF HIGH TEMPERATURE ON CASING STRENGTH IN THERMAL WELL

注蒸汽热采技术已广泛应用于提高重质油的采收率。在注蒸汽期间,井下高温使管柱产生附加热应力,从而可能使其发生塑性屈服、变形、断裂或屈曲。热采井中套管的失效几率非常高,研究套管柱在较高压力的高温蒸汽以及外围的地应力共同作用下的应力分布规律,对于评估套管的服役状况,提高套管设计的安全性,从而减少套管的失效几率具有重要意义。文章根据套管-水泥环-地层体系受力情况建立了平面力学分析模型,根据位移的连续条件求解了管柱与水泥环间的接触应力,探讨了注气条件下井筒温度的分布情况,分析了热采井中管柱所受三轴热应力的分布。算例分析结果表明,三轴热应力值远大于管柱实际的屈服强度,从而使管柱发生屈服破坏,为减小热应力对套管寿命的影响,应合理选择注气参数并合理设计管柱。

Steam injection is an effective method to enhance oil recovery of heavy oil. Extra thermal stress caused by high temperature during steam injection will result in plastic yield,deformation,fracture or buckling of casing string. The probability of casing failure is very high in thermal well. It is very useful to evaluate the service status of casing and improve the designed safety margin of casing to reduce the risk of casing failure by investigating stress distribution which was induced by steam pressure,terrestrial stress simultaneously. The plane mechanical analysis model of casing-cement-formation system was established and the contact force between casing and cement was discussed based on the displacement continuity conditions in this paper. Temperature and stress distribution were also presented. Sample analysis indicated that casing tri-axial thermal stress was much larger than actual yield stress of casing which could increase the probability of casing failure. Reasonable parameters of steam injection should be determined in order to reduce the influence of thermal stress on casing string.