This paper describes an analytical approach to investigate the nature of short overbalanced conditions and time effects during underbalanced drilling (UBD) in a naturally fractured reservoir. This study uses an analytical model which is developed for kinetic invasion of mud into the fractures. The model is based on fluid flow between two parallel plates, which is further extended to model the fluid flow in a fractured formation. The effect of short overbalanced pressure and the time effect during UBD as well as the aspects of well productivity and flow efficiency are explained. This model is an Excel-based program and provides a fast and convenient tool for analysis and evaluation of drilling conditions (mud properties, time, and pressure of drilling) in a fractured formation. The model can also predict the impact of the fracture and mud properties on the depth of invasion in the fractured formations.