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 a well performance modeling program stored in the memory of the well performance modeling computer to create and calibrate production and injection well models for a reservoir, the program 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 multi-phase flow correlation used to model a pressure drop inside a well bore of a well to be modeled to include calibrating the multi-phase flow correlation so that flowing bottom-hole pressure predicted using the flow correlation at gauge depth matches a corresponding field measured flowing bottom hole pressure value to thereby develop a well model of the well, comparing a performed date of a valid productivity index (PI) test for the well to a latest work-over date for the well, and performing a total system calibration on the well model including: decreasing a well productivity index value for the well model responsive to a model-predicted liquid rate for the well is greater than a field measured liquid rate for the well and responsive to the valid productivity index (Pt) test having a performed date being later than any well work-over date for the well to thereby adjust the model-predicted liquid rate, so that the model-predicted liquid rate is within a preselected value of the field measured liquid rate, and modifying flow correlation parameters for the well model to increase the model-predicted liquid rate responsive to the model-predicted liquid rate being less than the field measured liquid rate and responsive to the valid productivity index test associated therewith having the performed date being later than any well work-over date for the well to thereby adjust the model-predicted liquid rate, so that the model-predicted liquid rate is within the preselected value of the field measured liquid rate, performed without significantly adjusting the well productivity index value.