1. Introduction

Petroleum Engineering is a broad discipline with several areas of specializations such as petroleum geology, petrophysics, drilling engineering, mud and cementing, reservoir engineering, production (surface & subsurface) engineering, completion, formation evaluation, economics etc. These specialized areas work together as an integrated team to achieve one goal; to recover the hydrocarbon in a safe and cost-effective way. Petroleum engineering is one of the key aspects of Engineering that is concern with the exploration and production of hydrocarbons for consumption by human or to meet the host countries or global energy needs. This chapter presents an understanding of the essential features of petroleum reservoir, the job description of a reservoir engineer, the concept of drainage and imbibition processes, the hydrocarbon phase envelope and all its terminologies, identification of various types of reservoir fluids and their respective phase envelope/diagrams, understanding of the types of fluids in terms of flow regime and reservoir geometry and write the mathematical equations representing the flow regimes. Thus, for a better understanding of the flow regime, several solved example questions are given.