Wet gas is a particularly important concept in the field of flow measurement, as the varying densities of the constituent material present a significant problem.
This exists in either a gaseous or liquid phase or in solution with crude oil in porous rock formations.
The number of hydrocarbons present in the liquid phase of the wet gas extracted depends on the reservoir temperature and pressure conditions, which change over time as the gas and liquid are removed.
Changes in the liquid and gas content also occur when a wet gas is transported from a reservoir at high temperature and pressure to the surface where it experiences a lower temperature and pressure.
It is important to be able to measure these wet gas flows accurately to quantify production from individual wells and to maximise the use of equipment and resources which will assist with the reduction of costs.
Hold up is the cross sectional area occupied by the liquid in the pipe carrying the wet gas flow.
To account for both the flow rates and densities of the liquid and gas phases it is common practice to define the wetness or liquid loading of the gas using the Lockhart–Martinelli parameter, referred to as χ (Greek letter chi), which is a dimensionless number.
[6] The behavior of the gases and liquid in a flowing pipe will exhibit various flow characteristics depending on the gas pressure, gas velocity and liquid content, as well as orientation of the piping (horizontal, sloping or vertical).
Despite the complexity of gas and liquid interaction, attempts have been made to categorize this behavior.
These slugs may also be in the form of a surge wave that exists upon a thick film of liquid on the bottom of the pipe.
Dispersed flow assume a pipe is completely filled with liquid with a small amount of entrained gas.
The pressure and temperature at the test separator should be the same as at the wet gas flow meter or the gas and liquid flow rates must be corrected for the conditions at the meter as phases could be different from those measured.
Advanced signal processing is used where a liquid phase has an effect on the measurement signal such as pressure fluctuations in a DP flow meter or shift in the speed of sound in ultrasonic flow meter.
Complex analysis and modelling of these signals can determine the liquid and gas flows.