RPI® uses a mathematically rigorous extension of the classic pressure transient theory to model changing rates and pressures through time, as wellbore and reservoir flow conditions change. The program emphasizes graphical presentation of the observed data, so the user can recognize the pattern signatures that the well data contains. By using a “weight rate” approach to account for all produced fluids contributing to the energy consumed by producing the well, it is possible to very effectively approximate multi-phase behavior. The program is designed to allow evaluation of the individual transient events, that often exist in a well’s production history. The program allows very high time-resolution datasets to be evaluated, for example from the second-by-second data available in an electronic gas measurement system.