Gas Migration Model for Non-Newtonian Fluids Under Shut-In Well Conditions

Abstract

Addressing gas migration in a static mud column during the shut-in period is a major concern in Pressurized Mud Cap Drilling (PMCD). Significant discrepancies have been found between the field data and existing correlations for gas migration velocity, since the latter are based on either small-scale experiments or overly simplified assumptions, resulting in overly conservative estimations. To meet the Light Annular Mud (LAM) requirement for managing gas migration and to monitor the transient pressure experienced throughout the PMCD operation, an improved gas migration velocity model was developed by combining the equation of motion (bubble flow) and Taylor-bubble correlation (slug flow). In the bubble flow model, the effects of non-Newtonian fluid properties and drill pipe rotation are considered through a modified drag coefficient (CD) that incorporates the bubble Reynolds number (Reb) and dimensionless shear rate (Sr). The effect of bubble swarm is taken into account through a void fraction (αg) term. The slug flow model is based on a Taylor bubble correlation in terms of Eötvös number (Eo) and inverse viscosity number (Nf). For the first time, the dependence of Taylor bubble velocity on drill pipe rotation has been shown and correlated as a function of Sr. Predictions of the gas migration velocities in PMCD operations are made and successfully compared with the existing models and test-well experimental data. The drift flux model embedded in the new gas migration velocity model was applied to simulate the gas migration in a test well. Good agreement between the model and measured pressure results in the full-scale test-well experiments can be obtained. Its companion work (Liu et al., 2023) provides the design and calculation method of key parameters in bullheading/PMCD operations.