2.5.1 Introduction

The steady-state simultaneous flow of petroleum liquids and gases in wells and surface pipes is a common occurrence in the petroleum industry. Oil wells normally produce a mixture of fluids and gases to the surface while phase conditions usually change along the flow path. At higher pressures, especially at the well bottom, flow may be single-phase. But going higher up in the well, the continuous decrease of pressure causes dissolved gas to gradually evolve from the flowing liquid, resulting in multiphase flow. Even gas wells can produce condensed liquids and/or formation water in addition to gas. Both oil and gas wells may be vertical, may be inclined, or may contain longer horizontal sections. Surface flowlines as well as long distance pipelines are laid over hilly terrain and are used to transfer oil, gas, and water simultaneously.

Multiphase flow, however, is not restricted to the previous cases. Most artificial lifting methods involve a multiphase mixture present in well tubing, like gas lifting when the injection of high-pressure gas into the well at a specific depth makes the flow in the well a multiphase one. Other lifting methods, e.g. the use of rod or centrifugal pumps, also are similar. It is easy to understand then that the proper description of multiphase flow phenomena is of prime importance for petroleum engineers working in the production of oil and gas wells. It is the pressure drop or the pressure traverse curve, which the main parameters of fluid flow...