Abstract:

When drilling marine carbonate reservoirs, it is difficult to identify the early gas cut in reservoirs with fractures and pores due to the complex flow resulted from the substitution effect of wellbore fluid and formation fluid. Based on the models of gas cut in the drilling processes, this study discussed the mechanisms, occurrence conditions and interconnections of different gas cut models. New thoughts and methods were proposed for qualitative and quantitative analyses of S-containing gas cut in a direct invasive model. The upward migration and expansion patterns of gas along the wellbore were further investigated. During the upward migration process, the closer to the wellhead, the severed of gas expansion. Accordingly, the variations of the bottom hole pressure during the gas cut process were simulated using a multi-phase flow model that took into account the effects of gas-oil phase changes in the wellbore. It was shown that during the early gas cut, the bottom hole pressure fluctuated in the deep slim holes, with small increases followed by decreases. Based on the annular-pressure-while-drilling system that provides real-time down-hole pressure information, a new method was proposed for identification and early warning of gas cut through comparison with the normal trend line. The application of such method will provide effective protection for safe drilling of complex carbonate reservoirs.

Key words: carbonate rock, hydrogen sulfide, gas cut, early identification, pressure measurement while drilling

摘要：

针对海相碳酸盐岩油气藏钻井过程中钻遇裂缝孔洞发育储层时，井筒流体与地层流体置换效应形成的复杂流动造成的气侵识别困难，从钻井过程中气体侵入方式研究入手，探讨了各种气侵方式的侵入机理、发生工况及其相互联系，提出了直接侵入方式下含硫天然气侵从定性研究到定量化研究的思路和方法，研究了气体沿井筒的运移、膨胀规律，即气体在上升过程中越接近井口气体膨胀越剧烈。在此基础上，通过对气侵发生时的井筒环空气液两相流动井底压力变化特征进行模拟，揭示了气侵初期在深井小井眼段井底压力会出现小幅上升后下降的变化规律，提出了以井下实时环空压力随钻测量仪器监测井底压力变化对比正常趋势线来识别和预警气侵的方法。利用该方法，可以在气侵第一时间实现预警，为碳酸盐岩复杂油气井安全钻井提供了有效保障。

关键词: 碳酸盐岩, 硫化氢, 气侵, 早期识别, 压力随钻测量