深水油气井筒环空注氮控压机理

Mechanism of pressure management by injecting nitrogen in casing annulus of deepwater wells

通过深水井筒环空圈闭压力影响因素分析,结合环空圈闭介质温度、压力特性室内模拟实验,提出环空注氮气控制圈闭压力的方法。受深水井身结构及水下井口系统限制,技术套管和生产套管固井后存在较长的自由段套管环空,由水基、合成基或油基钻井液充填,油气测试、生产过程中,套管环空圈闭液体受井筒流体影响温度显著升高,圈闭液体受热膨胀出现圈闭压力。实验表明：圈闭流体热膨胀系数和等温压缩系数是环空圈闭压力关键影响因素,圈闭压力对圈闭介质类型（液、气）敏感度差异极大;向套管圈闭环空注入5%~20%体积分数的氮气,可以有效控制圈闭压力。现场实践表明：环空注入氮气的圈闭压力控制方法操作方便、可靠性高,能保证深水油气测试、生产过程井筒安全。

The influence factors of annular pressure buildup are analyzed, and the temperature and pressure characteristics of annulus trap medium are simulated for deepwater wells. The method of pressure management by injecting nitrogen in easing annulus is proposed based the experiment result. Limited by the well structure and subsea system, long casing annulus exists between technical casing and production casing and it is filled with water-based, synthetic based or oil-based drilling fluid. In the process of oil/gas test and production, the temperature of the trapped fluid rises significantly under the influence of the well-bore fluid and the trap pressure buildup appears because of liquid heat expansion. Experiments show that isothermal compressibility coefficient and thermal expansion coefficient are the key influencing factors for annulus pressure buildup. Trap pressure is very sensitive to the type of trapped medium （liquid, gas）. Injecting 5%-20% volume fraction of nitrogen into the annulus can effectively control the annulus pressure build-up, and avoid casing collapse. Field practice shows that the method, convenient and highly reliable, can ensure the borehole safety during testing and production of deepwater oil and gas.