Definition - What does Annular Flow mean?
Annular flow is a type of multi-phase flow in which the lighter fluid generally gas flows in the center of the pipe while the heavier fluid either oil or water flows as thin film over the walls of the pipe. This kind of flow is also called a channel flow.
Petropedia explains Annular Flow
During production phase, wells may produce either gas or oil/water alone where the flow regime is considered as a single phase flow. In some cases wells may produce both the gas and oil/water where the flow involves two or more phases indicating the flow is multi-phase flow.
Annular flow regime exists when gas produced from the formation expands and occupies the entire space of the tubing allowing the heavier fluid (oil/water) to flow on the walls of the tubing. For this phenomenon to occur the velocity of the gas should be at critical velocity where the gas can be able to lift the heavier fluid. As the gas velocity increases above the critical velocity the gas-liquid interface will disappear and the heavier fluid will entrain in the form of mist.
Various factors like gas pressure, gas velocity, temperature, solubility of liquid in gas, type of liquid content and orientation of a well (horizontal or vertical) will influence the occurring of annular flow in the tubing. This kind of phenomenon can be commonly seen in gas injection wells where dry gas is pumped in annulus to lift the heavier fluid.