This paper presents an advanced dynamic model and computer simulator for underbalanced drilling. The model is formulated based on the theory of multiphase transient flow referring to the drilling mud, water, oil, gas and solid particles. All the important factors affecting underbalanced drilling have been taken into account comprehensively in the model, including IPR relation, physical properties and mass transfer behavior of fluids, flow regime and phase migration features, geometry and deviation of wellbore as well as the different operating modes which may be carried out in practical underbalanced drilling.

A specific numerical approach is adopted for solution of the theoretical models and on the basis, an advanced computer simulator is developed for undertaking simulation, study and planning of underbalanced drilling operations. The computer simulator differs from existing simulators because it is fully dynamical and interactive with users. Researchers and users can change different operating modes according to their will at any time during simulating process just as they are really performing an underbalanced drilling operation. The simulator can track all the complicated and complex conditions in underbalanced drilling process, and all the related parameters and profiles of parameter distribution along wellbore at any time can be shown dynamically.

Through a number of simulating computations, tests and verifications, it is shown that the model and simulator are valid and in accordance with actual drilling practice under different and complicated conditions. It is of great importance for consummating underbalanced drilling theory and on the spot application. A typical simulation result and case study are also presented in the paper.

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