Dual-layer pipe dual-gradient drilling technology is an emerging technology for solving the problem of the narrow safety density window in deepwater drilling. The unstable spacer fluid interface in this technology directly affects the dual-gradient pressure system in the annulus, causing changes in the drilling mud performance and affecting the control of bottom hole pressure and rock removal with drilling mud. Therefore, the key to the stable operation of dual-layer pipe dual-gradient drilling technology is to maintain the stability of the spacer fluid interface. Based on this, a seawater-spacer fluid-drilling mud annular flow model was established in this study, with a bottom hole pressure control step of 0.2 MPa, and the spacer fluid height after a single control was used as the evaluation index to study the influence of annular flow velocity, the spacer fluid properties, and the drill string rotation speed on the stability of the spacer fluid interface. The results show that in the determined conditions of the seawater and drilling mud system, the annular fluid flow rate and the physical parameters of the spacer fluid are the main factors affecting the stability of the spacer fluid interface. When the annular fluid flow rate increased within the range of 0.04~0.2 m/s, the liquidity index of the spacer fluid increased between 0.5 and 0.9, the consistency coefficient increased in the range of 0.6 to 1.4 $Pa\cdot s^n$, and the stability of the spacer fluid interface decreased. However, the stability of the spacer fluid interface increased with the increase in its density in the range of 1100~1500 kg/m³. The results obtained in this study can provide a reference for selecting the operating parameters to ensure the stable operation of dual-gradient pressure systems.

Although the world is rich in deepwater oil and gas resources, deepwater drilling faces the challenge of a narrow safety density window. Dual-gradient drilling is an effective approach to address this narrow safety density window, and subsea pump-lift dual-gradient drilling is currently one of the most widely used methods. To improve handling capacity and effectiveness under well kick conditions, we established a simplified hydrodynamic model for subsea pump-lift dual-gradient drilling that accounts for the subsea pump’s head-flow performance curve. Based on this hydrodynamic model, we proposed automatic control methods based on throttling regulation and variable-speed regulation and conducted simulation analyses. The results indicate that both control methods can safely, efficiently, and automatically handle well kicks; throttle control has shorter response times and higher precision, while variable-speed control regulates bottom hole pressure by adjusting the subsea pump’s rotational speed and offers better energy-saving performance. These findings provide new insights into handling well kicks in subsea pump-lift dual-gradient drilling.