Well Modelling is a technique used by drilling, subsurface, and well test engineers to help achieve an optimum well design in terms of perforations, tubing size, and fluid and underbalance design, as well as to provide some of the key data inputs for the design of surface facilities. The objective is to model both the inflow performance of reservoir fluid into the wellbore and the outflow performance of reservoir fluid through the tubing. The inflow performance relationship (IPR) plots the drop in reservoir pressure with the production rate to produce a characteristic curve for a given set of conditions, that is, reservoir permeability, thickness, pressure drop, wellbore radius, fluid viscosity, and skin. The outflow, or tubing performance, plots pressure loss in the tubing against increasing flow rate for a given set of conditions, including fluid weight, friction losses, and wellhead pressure. The plotted results produce a characteristic outflow performance curve. Plotting these two curves together provides a pressure and corresponding flow rate for a given position along the tubing or at the wellhead.