Design and Calculation of Process Parameters in Bullheading and Pressurized Mud Cap Drilling

This study aims to provide the design and calculation method of key parameters in bullheading/PMCD operations. An improved gas migration velocity model in closed well conditions was developed based on the combination of the equation of motion (bubble flow) and Taylor bubble correlation (slug flow). A detailed derivation of the model can be found in our companion work (Liu et al. 2023). Experiments of Taylor bubble countercurrent behavior in an eccentric 6 in. × 4 in. annulus were conducted. Fluid rheology, annulus inclination, and internal pipe rotational speed were varied to provide a clearer understanding of Taylor bubble physics under non-Newtonian countercurrent flow and its implications for effectively managing upward gas migration that can occur in a wellbore during drilling operations in fractured or vugular rock formations. Computational Fluid Dynamics (CFD) simulations were performed to estimate the velocity of Taylor bubble in vertical annuli with downward fluid flow and a new closure relation for distribution parameter, C0, was proposed. The drift flux model embedded in the new gas migration velocity model was applied to simulate the dynamics of bullheading/PMCD. Good agreement between the model and published data was obtained. The effect of different bullheading rates on surface pressure and gas fraction in PMCD operation was examined.