An interactive system for modelling of multiphase inflow performance of long, highly deviated and horizontal wells is presented. It's prime areas of use are design of well trajectories and completions, and evaluation of wells inflow performance including calculation of well indices for full reservoir simulation. Further, the system provides a basis for improved integration of the reservoir engineering and the well completion design. The reservoir pressure and multiphase flow distribution is modelled accurately using a finite element method combined with automatic local grid refinement around wells. A comprehensive multiphase wellbore flow model is available for accurate flow modelling in the wellbore. A well inflow model including a generalized formula for calculation of well indicies of wells in arbitrary directions is proposed. Examples are given showing improved inflow performance calculations using the new well indices.