1. A system for creating and calibrating production and injection well models for a reservoir, the system comprising: a well performance modeling computer having a processor and memory in communication with the processor to store software therein; at least one database stored in memory accessible to the well performance modeling computer; and well performance modeling program product stored in the memory of the well performance modeling computer to create and calibrate production and injection well models for a reservoir, the program product including instructions that when executed by the well performance modeling computer cause the computer to perform the operations of: performing a vertical flow correlation validation of a flow correlation used to model a pressure drop inside a well bore of a well to be modeled to calibrate the flow correlation so that flowing bottom-hole pressure predicted using the flow correlation at gauge depth matches a corresponding field measured value to thereby develop a model of the well, and performing a total system calibration on the well model including decreasing a well productivity index value when a model-predicted liquid rate for the well is greater than a field measured liquid rate for the well and when the well has a valid productivity index (PI) test associated therewith having a performed date later than any well work-over date for the well.