Abstract: Based on the theory of multiphase transient flow, a dynamic model with respect to gas/liquid two- phase flow in borehole was developed, it was also calculated and analyzed the effect of gas injection rate on borehole pressure in aerated drilling in this paper. The bottomhole pressure is not only governed by the gas injection rate, but also closely related to some other factors, including the size of wellbore and drill string,physical properties and circulating rate of drilling mud, etc.The results indicate that only increasing the gas injection rate is not certain to result in decreasing the bottomhole pressure, it
depends on the balanced relationships between the hydrostatic pressure and flow resistance pressure in the given borehole gas/liquid two-phase fluid. The model and approach may provide some useful help for the different related parameters, the matched compressors, and the design plans in inderbalanced
drilling.

Key words: underbalanced drilling, multiphase flow, theoretical model, numerical simulation