欠平衡气侵与重力置换气侵特征及判定方法

摘要

During well drilling in fracture-vuggy carbonate oil and gas reservoirs, gas may enter into wellbore in the forms of underbalanced pressure gas kick and gravity displacement gas kick. In order to analyze the gas kick velocity and identify the two different gas kick types, a wellbore gas kick model was proposed based on gas-liquid two phase flow to calculate downhole pressure and the incremental amount of drilling fluid in mud pit. It is possible to determine the types of gas kicks by observing the volume change in the mud pit via changing wellhead back pressure. The results show that, when the wellhead back pressure is increased, the downhole pressure may change and result in a transfer from underbalanced to overbalanced, in which only a slight volume increase in mud pit can be observed. On the other hand, the volume in mud pit may maintain its steady increase trend if a gravity displacement gas kick occurs downhole. This can be used to identify the gas kick types, and the method is verified in the field trials, which is useful for the managed pressure drilling to control gas kick quickly and safely.%在裂缝性碳酸盐岩地层中，地层气体以欠平衡或重力置换方式进入井筒形成气侵。对重力置换气侵和欠平衡气侵类型及进气量进行分析；基于气液两相流理论建立井筒气侵模型，改变边界条件计算精细控压钻井气侵期间井底压力和泥浆池增量的变化；通过改变井口压力观察泥浆池总量或出口流量的变化，分析井底气侵方式。结果表明：井口增加回压后，井底压力由欠平衡状态转变为过平衡状态，泥浆池增量略微增加，而重力置换气侵泥浆池增量保持原趋势增加，由此可以判断出井底的气侵方式；该判定方法在现场试验中得到验证，对精细控压钻井安全快速控制气侵有指导意义。