Multiphase flow

In fluid mechanics, multiphase flow is simultaneous flow of (a) materials with different states or phases (i.e. gas, liquid or solid), or (b) materials with different chemical properties but in the same state or phase (i.e. liquid-liquid systems such as oil droplets in water).

Each of the phases is considered to have a separately defined volume fraction (the sum of which is unity) and velocity field. Conservation equations for the flow of each species (perhaps with terms for interchange between the phases) can then be written down straightforwardly.

The momentum equation for each phase is less straightforward. It can be shown that a common pressure field can be defined and that each phase is subject to the gradient of this field, weighted by its volume fraction. Transfer of momentum between the phases is sometimes less straightforward to determine, and, in addition, a very light phase in bubble form has a virtual mass associated with its acceleration. (The virtual mass of a single bubble is about half its displaced mass).

These terms, often called constitutive relations, are often strongly dependent on flow regime.

In nature and environment, rain, snow, fog, avalanches, mud slides, sediment transport, debris flows are all examples of multiphase flow where the behaviour of the phases are studied in different fields of natural science.

In oil and gas industries, multiphase flow often implies to simultaneous flow of oil, water and gas. The term is also applicable to signifies the properties of a flow in some field where there is a chemical injection or various types of inhibitors. Volumetric flow rates, mass flow rates (per phases and total) and also phase fractions are the important parameters to be accounted for when designing and operating multiphase-including systems. For example, multiphase flow can occur during the different stages of oil and gas recovery. Stage 1: Single-phase flow (oil) or multiphase flow (gas production related decrease of the average pressure of the reservoir). Stage 2: Waterflooding: Injection of water to increase oil recovery Stage 3: EOR (Enhanced Oil Recovery): Use of chemicals (surfactant) or bacteria to improve the recovery of the residual oil.

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