A dynamic model, based on the drift-flux formulation, is presented for treating transient phenomena in UBD operations. A set of mechanistic steady state procedures for dealing with the definition of flow patterns, pressure drops, gas volumetric fractions, and in-situ velocities completes the model. The iteration between those mechanistic laws and the conservation equations is discussed. The numerical solution, which presents an easy implementation, uses a composite explicit scheme that properly combines the second order MacCormack and the first order Lax-Friedrichs methods, reducing false oscillations and numerical diffusion. In addition, details about the boundary treatment are also presented.

Model predictions are validated through comparison to full-scale experimental data in two distinct transient situations. First, a typical unloading scenario in an underbalanced operation is studied. Further, the model simulates the injection of a high velocity single pulse and a sequence of two pulses of a gas-liquid mixture. Measured variables as pressure and returning liquid and gas rates at surface are compared to the predicted ones. The comparisons address model limitations and improve the understanding of the physics involved.

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