Zhao Chaojie, Jin Yanxin, Wang Xue
Sinopec Research Institute of Safety Engineering Co., Ltd., Qingdao, 266000, China.
State Key Laboratory of Safety and Control for Chemicals, SINOPEC Research Institute of Safety Engineering Co., Ltd., Qingdao, 266000, China.
Sci Rep. 2024 Jun 7;14(1):13164. doi: 10.1038/s41598-024-63923-x.
Fracturing horizontal well casing deformation has become very prominent, particularly in tectonic stress-concentrated shale gas fields, limiting the efficient development progress of shale gas. The main failure mode of casing shearing deformation had been attributed to fault slip caused by multi-fracturing. The current research did not provide a clear picture of the dynamic evolution relationship between hydraulic fracturing, fault slip, and casing deformation. In this paper, the dynamic model of fault slip induced by formation pressure change is established, incorporating the effects of stress drop, physical change of friction, and casing and cement-sheath resistance loads. The discontinuous displacement approach and explicit/implicit coupling iteration methods are used to reveal the relationships between the effective normal stress, shear stress, friction coefficient, and sliding velocity during the fault slip process. Furthermore, the microscopic process of casing deformation sheared by fault slip is investigated using static equilibrium theory, and a characterization method for determining the amount casing deformation caused by real-scale fault slip is proposed. The results show that three stages exist in the process of casing deformation sheared by fault slip, including trigger activation stage, accelerated slip stage, and deceleration slip stage. Fault slip is clearly influenced by fault strike. To reduce the amount of fault slip, the fault direction with the maximum in-situ stress should be avoided as much as possible. Serious casing deformation still occurs for large-scale activated faults even though the optimization measure of wellbore structure has been well taken. To fundamentally reduce the possibility of casing shear deformation, it is necessary to prevent fault slip through optimizing the design of hydraulic fracturing. This study lays the theoretical groundwork for the casing deformation control method in shale gas wells.
压裂水平井套管变形问题十分突出，特别是在构造应力集中的页岩气田，制约了页岩气的高效开发进程。套管剪切变形的主要失效模式被认为是多次压裂导致的断层滑动所致。目前的研究尚未清晰呈现水力压裂、断层滑动与套管变形之间的动态演化关系。本文建立了地层压力变化诱发断层滑动的动力学模型，考虑了应力降、摩擦物理变化以及套管和水泥环阻力载荷的影响。采用间断位移法和显式/隐式耦合迭代方法，揭示了断层滑动过程中有效正应力、剪应力、摩擦系数和滑动速度之间的关系。此外，利用静力平衡理论研究了断层滑动剪切套管变形的微观过程，提出了一种确定实际规模断层滑动引起套管变形量的表征方法。结果表明，断层滑动剪切套管变形过程存在触发激活阶段、加速滑动阶段和减速滑动阶段三个阶段。断层滑动明显受断层走向影响。为减少断层滑动量，应尽量避开最大地应力方向的断层。即使采取了优化井身结构的措施，大规模激活断层仍会导致严重的套管变形。要从根本上降低套管剪切变形的可能性，有必要通过优化水力压裂设计来防止断层滑动。该研究为页岩气井套管变形控制方法奠定了理论基础。
