Abstract: In order to provide a practical bottom hole pressure prediction model to the engineering design of underbalanced drilling (UBD), six pressure prediction models which are established according to homogenous flow approach (Guo Boyun model),empirical correlation approach (BeggsBrill Model, Orkiszewski Model, Han HongshenCheng Jialang Model) and mechanistic approach (HasanKabir model and modified Ansari model) are used to predict pressure drop in UBD. The six algorithms are implemented into a FORTRAN 90 computer program. Gasliquid twophase flow in annuli in UBD is simulated by the six models and bottom hole pressure is predicted in gasifiedliquid UBD. The prediction results are validated with field data and fullscale experimental data. The predictions precision of the models is presented.The comparative analysis shows that modified Ansari model better pressure prediction in underbalanced drilling and it satisfies the accuracy demand of underbalanced drilling. The method is recommended in the design of underbalanced drilling to maintain full underbalanced status and economic benefit target of underbalanced drilling is obtained.

Key words: underbalanced drilling, multiphaseflow model, evaluation, analysis, accuracy comparison