基于应变控制的热采井套管试验及安全可靠性研究

With the complex force state, unclear failure mechanism and behindhand design idea of casing in the heavy oil thermal recovery well, the security of casing string has become one of the fatal factors in the heavy oil thermal recovery of China. The main objective of the program is to build up effective models and methods of casing design and reliability evaluation. A new strain design idea is proposed, namely "stress control + uniform deformation". The biaxial strain cyclic tests of casing material under high temperture and multi-physics numerical simulations are carried out to obtain the stresses and deformations of casing in the processes of heavy oil thermal recovery. Based on the elastic-plastic total theory, a new stress- strain equation is given to depict the actual deformation of casing material under injection and production conditions. Moreover, corresponding strain design parameters are also afforded based on the parameters of a thermal recover well in Xinjiang oilfield. On above basis, the security and reliability design overviews and ways are introduced, and the reliability evaluation models of casing designed by strain control method are established. Furthermore, the effects of the changes of casing physical and mechanical properties, geological parameters and boundary conditions, etc, on the reliability of casing are studied systemically. At the same time, the design parameters can be deduced by the given security level accordingly. Eventually, systemic and scientific design methods and reliability evaluation theories are established which can be serviced as the referenced method and theory of the design and reliability evaluation of casing in heavy oil thermal recovery well.

由于受到受力情况复杂、失效机理不明确、设计思路滞后等因素的影响，热采井套管柱安全成为困扰我国稠油热采的主要因素之一。本课题以稠油热采井套管柱设计和安全可靠性评价为主要研究内容，提出一种"强抗+均匀变形"的基于应变控制的设计思路，从高温条件下管材服役行为双轴应变循环实验和多物理场仿真入手，利用弹塑性全量理论建立适用于高温条件下的管材塑性应力-应变模型，并结合新疆油田热采井工况给出基于应变设计的热采井套管柱设计控制参数。在此基础上，通过引入安全可靠性设计概论和方法，建立基于应变设计的热采井套管柱安全可靠性评价模型，综合考虑热采过程中套管物理力学性能参数、地质参数和边界条件等因素的时域性变化规律和耦合作用机理的影响，对热采井套管柱安全可靠性进行评价并能够根据可靠度对设计参数进行反演控制。最终形成适用于热采井套管柱应变设计和安全可靠性的评价方法，为采井套管柱设计和安全可靠性评价提供基础理论依据。

本课题以稠油热采井套管柱设计和安全可靠性评价为主要研究内容，从高温条件下管材服役行为实验和多物理场仿真入手，建立高温条件下的管材塑性应力-应变模型，并通过套管材料应变与结构应变对比给出应变安全系数最小取值，形成热采井套管柱应变设计方法。在此基础上，考虑热采过程中套管力学性能参数的变异性，引入安全可靠性设计概论和方法，建立热采井套管柱安全可靠性评价模型。最终形成适用于热采井套管柱应变设计和安全可靠性的评价方法。研究结果表明：(1)高温350℃下，热采套管存在应力松弛现象，施加的预拉载荷力明显降低，其中HS-N80Q的应力松弛极限最低；(2)HS-N80Q、TP90H、HSTG80H三种套管蠕变速率大小次序为：HSTG80H< TP90H<BG80H；(3) 新疆油田KS-4井套管柱系统在开发初期可靠度为100%，而在运行20年后，整体可靠度降为76.20%。(4)形成的套管柱应变设计方法，已纳入中华人民共和国石油天然气行业标准《SY/T 6952.1-2014 基于应变设计的热采井套管柱第1部分：设计方法》。另外，该方法已在新疆油田现场8口井获得成功应用。