窄安全密度窗口地层钻井起下钻井底压力瞬态波动规律

LAWS OF BOTTOMHOLE TRANSIENT PRESSURE FLUCTUATION DURING TRIPPING IN NARROW SAFETY DENSITY WINDOW FORMATION

窄安全密度窗口地层压力敏感,钻井起下钻作业引起井底压力波动,易诱发溢流、井漏等井下复杂。文章以一维瞬态流动模型,考虑井筒液-固两相介质,建立了起下钻井底压力瞬态波动理论模型,并通过数值模拟研究了影响钻井起下钻井底压力瞬态波动的主要因素。研究表明,起下钻速度、井深、钻井液密度、起下钻深度是影响井底压力波动的主要因素。起下钻速度、钻井液密度越大,井深越深,起下钻深度越深,起下钻作业引起的井底压力波动越剧烈,压力波峰值滞后越严重。起下钻作业时,尤其在起钻的早期和下钻后期,应尽量降低起下钻速度,以降低井底压力波动,保障井底压力处在安全密度窗口之内。

In gas and oil reservoir with narrow safety density window,where the reservoir pore pressure is close to fracturing pressure,the complex circumstances such as blowout and loss circulation are easy to be induced.A mathematical model for predicting swab and surge pressure was proposed based on the one-dimensional transient flow model considering the wellbore medium as liquid-solid two phase.Numerical simulation was conducted to elevate the factors influencing swab and surge pressure.The results showed that the velocity of round trip,drilling fluid density,well depth and the drill bit depth when tripping were the major influencing factors of bottomhole pressure fluctuation.With increase of the velocity of round trip,drilling fluid density,well depth and drill bit depth,the swab and surge pressure increased gradually,and the delay of the peak pressure was greater.In tripping operation,especially in the early time of pulling out the hole and later time of run back into the hole,controlling the velocity of round trip in wellbore with narrow safety density window was beneficial to the drilling safety.