Abstract:

During long-time fracturing，the high speed proppant-carrying fluid flowing will cause serious erosion for downhole tools and casings. Therefore，it is important to understand the flow field distribution inside the tool and the casing erosion pattern during fracturing，propose improvement measures，optimize fracturing parameters，and protect wellbore integrity. A finite element mechanical model of casing erosion at the decoupled sandblaster during fracturing was developed，and a systematic analysis and study of erosion in an ultra-deep well was conducted. The results show that without sheath protection，the erosion rate of the casing at the outlet reaches 1.46×10^-3 kg/（m²·s），with a casing thickness reduction of 1.35 mm，which decreases the casing strength，and the pipe string with erosion defect is more prone to stress concentration. After the sheath is added，the wall thickness reduction of the casing caused by erosion is significantly lower than that of the working condition without sheath protection. The finite element model developed allows a complete and quantitative study and analysis on all the points near the outlet of the sandblaster. The safety window of wall thickness thinning of the eroded sheath under different injection rate of fracturing fluid and proppant ratio was predicted. The safety window can quantitatively provide guidance for the operators to ensure effective protection of the casings and prevent casing erosion by controlling the injection rate and proppant ratio. The study is of great significance to parameter optimization and wellbore protection during fracturing.

Key words:

fracturing, decoupled sandblaster, erosion, finite element, sheath wall thickness, safety window

摘要：

长时间压裂过程中，高速流动的携砂液会对井下工具与套管造成严重冲蚀。应掌握压裂过程中工具内部流场分布及套管冲蚀规律，有针对性地提出改进措施，优化压裂参数，保护井筒完整性。建立了压裂脱接喷砂器处套管冲蚀的有限元力学模型，针对某超深井的冲蚀进行了系统分析和研究。结果表明，无护套保护时出口处套管的冲蚀速度达到1.46×10^-3 kg/（m²·s），套管壁厚减薄1.35 mm，套管强度下降，带有冲蚀缺陷的管柱更容易发生应力集中；加护套后冲蚀引起的套管壁厚减薄明显低于无护套保护的工况。建立的有限元模型可以对喷砂器出口附近的所有位置进行完整定量的研究与分析，并预测了不同压裂液排量与砂比下冲蚀护套壁厚减薄安全窗口，可以定量指导现场通过控制排量与砂比的方法保证护套有效，防止套管冲蚀。此研究对压裂过程中的参数优化及井筒保护具有重要意义。

关键词:

压裂, 脱接喷砂器, 冲蚀, 有限元, 护套壁厚, 安全窗口