变梯度控压钻井井控过程中井口回压变化规律

为了准确掌握变梯度控压钻井井控过程中井口回压的变化规律，采用考虑密度突变的井筒气液两相变质量流动模型，分析了基于井底恒压的变梯度控压钻井井控过程中井口回压的变化，探讨了不同因素变化对井口回压的影响，并开展了最大井口回压影响因素敏感性分析。研究发现：变梯度控压钻井在控制井底压力恒定时井口回压的调节受气体膨胀、分离器位置处液相密度突变、套管鞋及海底泥线处环空变径等多个因素综合影响，其变化规律更加复杂；其他条件不变时，分离器与钻头间距越小、轻/重质钻井液密度差越大、地层压力越大、循环排量越小，循环排气过程中的井口回压越大；对于最大井口回压而言，地层压力、轻/重质钻井液密度差、分离器与钻头间距、循环排量的比变异系数值依次减小，敏感性程度也随之降低。研究结果可为变梯度控压钻井井控过程中的井口回压准确预测和井筒压力精细控制提供理论支撑。

Variation law of wellhead back pressure under well control during variable gradient managed pressure drilling

To accurately understand the variation law of wellhead back pressure under well control during variable gradient managed pressure drilling (MPD), based on a wellbore gas-liquid two-phase variable mass flow model considering the sudden change in density, this paper analyzes the variation of wellhead back pressure under well control during variable gradient MPD based on bottom hole constant pressure, explores the influence of the changes of different factors on the wellhead back pressure, and performs the sensitivity analysis of the factors affecting the maximum wellhead back pressure. It is found that the regulation of wellhead back pressure in variable gradient MPD is comprehensively affected by many factors, such as gas expansion, sudden change of liquid phase density at the separator, annulus diameter change at the casing shoe and seabed mud line, and its variation law is becoming more complicated. Under the scenario where other conditions remain unchanged, the smaller the spacing between the separator and the drill bit, the greater the density difference of light and heavy drilling fluids, the greater the formation pressure, and the smaller the pumping rates, the greater the wellhead back pressure in the circulating process. For the maximum wellhead back pressure, the specific variation coefficient values of formation pressure, density difference of light and heavy drilling fluids, spacing between separator as well as bit and circulating displacement decrease successively, and the corresponding sensitivity also decreases. The research results can provide theoretical support for the accurate prediction of wellhead back pressure and the fine control of wellbore pressure during the variable gradient MPD.