摘要
为明确稠油火驱井底高温环境对套管损伤的影响,基于传热学和材料力学原理,构建了套管水泥环地层三维有限元模型,系统分析了井底套管在500~650℃燃烧条件下的温度场及等效应力分布特征。对火驱井使用的套管材料进行了高温拉伸试验,研究了材料在高温环境中的力学性能,确定了高温下的许用应力范围,并利用扫描电镜观察了不同温度下的断口形貌,分析了温度变化对套管材料失效机理的影响。结果表明:当温度超过500℃时,套管材料的屈服强度与抗拉强度显著下降,拉伸断口呈现典型的韧性断裂特征,伴随严重的高温氧化现象。采用Von Mises屈服准则作为套管失效判据,对比数值模拟获得的等效应力与高温拉伸试验确定的许用应力,识别了存在变形失效风险的套管段。储油岩层套管段在高温作用下因热膨胀受顶部与底部约束,导致轴向受压,发生明显的轴向压缩变形;而普通岩层套管段受地层应力影响,发生了明显的颈缩现象。套管的等效应力分布与温度场呈正相关,井底温度越高,套管的失效风险区间越大,但失效位置始终未超越点火井段。套管损伤的主要机制是热应力引起的轴向压缩变形,特别是在储油岩层段,套管更易因受限膨胀产生局部变形而发生失效。该研究为火驱井套管设计与风险评估提供了重要参考依据。
[Objective]To thoroughly investigate the impact of high temperatures at the bottom of in-situ combustion wells on casing integrity,a finite element model(FEM)was developed to simulate the casing-cement sheath-formation system.The model was constructed based on the principles of heat transfer and material mechanics,enabling an in-depth analysis of the temperature distribution and equivalent stress distribution under combustion temperatures ranging from 500℃to 650℃.This approach aimed to elucidate the thermal and mechanical behaviors of casing materials under extreme high bottomhole temperature conditions,thereby providing a reliable basis for casing risk assessment and design optimization.[Methods]High-temperature tensile tests were performed on casing materials specifically designed for in-situ combustion wells.The tests assessed the mechanical properties of these materials,including yield strength and tensile strength,under elevated temperatures typically encountered in in-situ combustion operations.Allowable stress for different temperature ranges was determined using the safety factor method,which is widely used in engineering field to ensure the reliability of the casing structure.Additionally,a microscopic analysis of tensile fracture surfaces at various temperatures was conducted using scanning electron microscopy to investigate the temperature-induced casing failure mechanisms.[Results]The results reveal significant changes in the fracture morphology of casing materials,highlighting a progressive degradation in the material properties as temperature increases.The Von Mises yield criterion is used as the failure assessment standard,allowing for a detailed comparison between equivalent stresses obtained from numerical simulations and experimental allowable stresses.This comparison identifies casing segments at a high risk of deformation and failure.Numerical simulations demonstrate that the casing within oil-bearing formations experiences substantial thermal expansion.Meanwhile,due to axial constraints at the top and bottom of a wellbore,the casing undergoes significant axial compression,resulting in compressive deformation.Furthermore,in ordinary rock formations,the casing expands freely but is subjected to formation stresses,leading to pronounced necking deformation.Experimental results show that once the temperature exceeds 500℃,the yield strength and tensile strength of casing materials decrease dramatically.The tensile fracture surfaces exhibit typical ductile fracture characteristics,accompanied by severe oxidation,confirming the adverse effects of high-temperature exposure on the mechanical performance and structural integrity of the casing.This mechanical performance degradation poses a considerable risk of failure,especially in high-temperature zones near the ignition segment.[Conclusions]This study further reveals that the equivalent stress distribution within the casing is positively correlated with the temperature field.High bottomhole temperatures significantly expand the risk zones for casing damage,though the failure locations remain confined to the casing segment near the igniter.The primary failure mechanism is identified as compressive deformation caused by thermal stress,with oil-bearing formation segments being particularly vulnerable to failure due to restricted casing expansion caused by surrounding constraints.These findings provide valuable insights into the failure mechanisms of the casing under high-temperature conditions and offer practical guidance for improving the casing design and reliability in in-situ combustion for heavy oil.By integrating numerical simulations with experimental tests,a more comprehensive understanding of the casing behavior under extreme thermal and mechanical conditions achieves.This approach not only enhances risk assessment strategies but also supports the development of more robust casing solutions capable of withstanding harsh environments encountered in in-situ combustion wells.Ultimately,this research contributes to the sustainable development and improves safety of heavy-oil extraction projects,reducing operational risks and ensuring long-term profitability.
作者
刘添琪
武胜男
张来斌
LIU Tianqi;WU Shengnan;ZHANG Laibin(Hainan Institute of China University of Petroleum(Beijing),Sanya 572000,China;College of Safety and Ocean Engineering,China University of Petroleum(Beijing),Beijing 102249,China;Key Laboratory of Oil and Gas Production Safety and Emergency Technology,Ministry of Emergency Management,Beijing 102249,China;Key Laboratory of Oil and Gas Production Equipment Quality Inspection and Health Diagnosis,State Administration for Market Regulation,Beijing 102249,China)
出处
《清华大学学报(自然科学版)》
北大核心
2025年第6期1112-1119,共8页
Journal of Tsinghua University(Science and Technology)
基金
中海石油(中国)有限公司北京研究中心科研项目(CCL2022RCPS0523RSN)。
关键词
稠油热采
热应力
失效机理
套管损伤
in-situ combustion
thermal stress
failure mechanism
casing damage
