深水钻井表层导管喷射钻进机理研究

Deepwater subsea generally has the characteristics of low temperature and loose soil. Conventional surface drilling methods have the risk of low cementing quality and the conductor hard to find the wellhead position in deepwater. Surface conductor jetting technology can effectively adapt to the environment characteristic of deepwater seabed, it can install the conductor as well as drilling process, solving the problem of the conductor can not find the well hole after surface drilling operation in deepwater, what is more, jetting technology can complete two layer of borehole opearations with one trip of drilling string, using of this technology can greatly improve the efficency. Research the drilling mechanism of surface jetting is the premise and foundation grasping and applying this technology. Nowadays, analysis of the enginnering practice is emphasised at home and abroad, but theoretical research of jet drilling mechanism is nearly blank.This project will base on the instance of deepwater drilling engineering in the foreign cooperation block of South China Sea, through theoretical analysis and numerical calculation, combined with a large number of indoor and on-site simulation experiments to research the interaction mechanism of water jet and seabed soil in the process of jet drilling under different seabed soil conditon, explore the variation method of stress and displacement field relation with time for soil around the conductor, and the calculation method for conductor bearing, analysis the influence of weight on bit, pump discharge paramenters to penetration rate during jetting process, research the control method of optimal hydraulic parameters for conductor jeting technology, to grasp the core theory and drilling mechanism for deepwater surface drilling, providing the scientific basis and technical guidance for our deepwater drilling engineering design.

由于深水海底普遍具有低温、土质疏松等特点，深水表层采用常规的钻井方法存在固井质量低、导管找井口困难等钻井风险。表层导管喷射钻进方法能有效应对深水海底环境特点，边钻进边下导管，解决了常规钻孔后导管无法找到井口的难题，且一趟管柱能完成两开井眼钻进，作业时效高。研究表层喷射钻进机理是掌握和应用该技术的基础，目前国内外研究偏重于工程实践分析，对喷射钻进机理的理论研究几乎处于空白状态。本课题将根据南海国外合作区块深水钻井工程实例，通过理论分析和数值计算，结合大量室内和现场模拟实验，研究不同海底土质条件下喷射钻进过程中的水射流与海底土体相互作用机理，摸索水力喷射中导管周围土体应力和位移场随时间变化规律以及导管承载力计算模型，分析喷射过程中钻压、排量等参数对下入速度的影响规律，研究导管喷射钻进的最优水力学参数控制理论，掌握深水表层导管喷射钻进机理和核心理论，为我国深水钻井工程设计提供科学依据和技术指导。

表层导管是深水钻井安装的第一层管柱，是重要的持力结构和循环通道。安装表层导管是深水钻井施工的关键环节。深水海底具有土质疏松、温度低等特点，采用常规的钻井方法安装导管存在井壁坍塌、固井质量差等钻井风险。喷射安装表层导管方法是针对深水海底环境特点，边钻进边下导管，能有效解决常规钻井方法钻孔缩径难题，同时减少了表层导管固井工序，实现一趟管柱完成两开井眼钻进，作业时效高。基于淹没水射流理论，分析了喷射安装表层导管过程水射流特征，结合海底土体特征，建立了喷射水射流破土模型；针对喷射施工过程水力扩孔直径小于导管直径和水力扩孔直径大于导管直径两种情况，分别建立了基于小孔扩张理论的表层导管稳定分析模型和基于小孔收缩理论的表层导管稳定分析模型。基于岩土工程理论，分析了表层导管与海底土相互作用特征，建立了表层导管与海底土相互作用挠曲微分方程；基于桩基理论，确定了表层导管极限竖向承载力计算方法及软黏土、砂性土极限侧向抗力计算方法；提出了喷射排量、钻头伸出量与表层导管承载力降低系数之间的函数关系，建立了考虑喷射排量、钻头尺寸和时间效应影响的表层导管实时承载力计算模型。表层导管承载力随着喷射排量、钻头伸出量的增加呈指数关系降低，当喷射排量值超过1.2倍最小破土排量时，表层导管承载力降低幅度达到最大，钻头喷嘴完全伸出表层导管外部后，钻头伸出量变化对表层导管承载力影响减小，表层导管下入速度随喷射排量、钻头伸出量增大呈幂函数增大；表层导管喷射安装到位后，表层导管承载力随静置时间呈对数关系恢复。根据理论模型和模拟实验结果，综合考虑“喷射水力破土能力、表层导管承载力”双因素约束，建立了的喷射排量、钻头伸出量设计方法和表层导管安装深度与静置时间窗口设计方法。该设计方法在南海自营深水井得到成功应用，有效保证了表层导管稳定，提高表层导管喷射下入效率。