Optimization of Heavy Mud Cap in Narrow Density Window Precise Managed Pressure Drilling

When managed pressure drilling was conducted in the deep carbonates in western China, a simple heavy mud cap mode was generally used, but it resulted in larger fluctuation of bottom hole pressure (BHP) and even precipitated the risk of downhole failures. In this paper, therefore, the borehole pressure control method was improved and a heavy mud cap design was optimized after analysis was conducted on the real-time variation of heavy mud distribution at each stage of heavy mud injection. Nozzle killing heavy mud and heavy mud cap (two drilling fluids with different densities) were optimized and combined. A pressure control design was carried out in five stages with the returning of nozzle killing heavy mud to the well head as a pressure control node, and pressure control ranged from zero to 5 MPa, and based on that, real-time wellhead pressure control strategies were adjusted in order to ensure the fluctuation range of the wellbore pressure to less than ±0.35 MPa. When the method was applied to 13 wells in central Tarim, heavy mud consumption during the trip was reduced by 21.5% and injection time was shortened by 17.7%. It was shown that the new method could keep the BHP steady more efficiently and achieve better effects of precise wellhore pressure profile control and well control risk reduction when it was applied to the injection of a heavy mud cap in precise managed pressure drilling regime.