Studies on the Effects of Cement Sheath on Casing Stress Based on Stratum Arching Effect
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摘要: 【目的】随着对天然气需求的增长以及开采技术的进步，油气钻井深度不断刷新记录。随着钻井深度的增加，地应力急剧增加，因此超深油气井套管损伤问题日益突出。为了提高套管在高地应力条件下的承载力，通常采用大壁厚高强套管来解决这个问题，但收效甚微。【方法】通过套管-水泥环-地层系统的有限元模型，证实在套管系统中成拱效应的存在以及柔性水泥环对套管的保护作用。在此基础上，通过参数分析，研究不同地层、水泥环的材料参数及其厚度对系统内应力传递机制的影响。【结果和结论】结果表明：柔性固井水泥通过允许套管附近地层发生有限变形的形式使地层能够承担地应力，进而减小套管所承担的载荷；刚性固井水泥通过限制套管附近地层发生变形来承担地应力，从而将使套管与其共同承担地应力。在前者的受力体系中，固井水泥的弹性模量越小，套管应力越小，属于主动拱效应；而在后者的受力体系中，固井水泥的弹性模量越大，套管应力越小，属于被动拱效应。然而，主动拱效应场景中的应力减小幅度远远大于被动拱场景。因此，主动拱效应对套管具有更好的保护效果，而且，在硬地层中要比软地层中明显。此外，水泥环泊松比及其厚度对套管应力也有一定的影响，但影响程度不及水泥环弹性模量。鉴于此，认为采用柔性水泥固井的条件应为地层是否具备成拱条件，即地层是否具有抗剪强度，而非地层本身的软硬程度。Abstract: [Objective] With the increasing demand for petroleum and gas and the development for the associated technology, the record is constantly broken in the drilling depth. The in-situ stress increases with the drilling depth, leading to severe casing failure issues in the ultra-deep wells. In order to improve the bearing capacity of casing under high in-situ stress, large-wall thick high-strength casing is traditionally employed but without evident improvement. [Methods] In this paper, the finite element model of casing-cement sheath-formation system is used to prove the presence of arching effect in the casing system and the protective effect of flexible cement sheath on the casing. On this basis, a series of parameter analyses are conducted to study the influence of the elastic moduli of strata and cement sheath and the thickness of cement sheath on the stress transferring mechanism in the system. [Results and Conclusions] The results show that the soft cement can make the formation bear the in-situ stress by allowing the formation near the casing to deform with limited movement, while hard cement resists in-situ stresses by limiting the formation deformation near the casing. In the former scenario with active arching effect, the smaller elastic modulus of the cement sheath leads to the smaller stress in the casing. In the latter scenario with passive arching effect, the larger elastic modulus of cement leads to the smaller stress in the casing. It is worth noting that the stress reduction in the casing in the scenario of active arching is more obvious than that in the scenario of passive arching. Apart from the effects of the elastic modulus of the cement, the Poisson’s ratio and thickness of the cement also has a limited influence on the casing stress, but the extent of their influence is not as significant as that of the elastic modulus of the cement sheath. In view of this, it is suggested that the selection criterion for the cement of in the ultra-deep well cementing should be whether the formation has shear strength to form arching effect rather than the stiffness of the formation.
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Keywords:
- cement sheath /
- elastic modulus /
- casing stress /
- fe model /
- arching effect
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