1. The gas tubing or VFP (vertical flow performance) rate equation defines the relationship between surface gas rate and pressure drawdown (bottomhole flowing pressure minus tubinghead flowing pressure). This relationship is also called “tubing performance” or tubing “backpressure curve”.
Tubing diameter is the most important variable defining a well’s tubing performance under steady (turbulent-dominated) flow conditions.
Tubing performance deteriorates dramatically when the gas tubing velocity drops below a certain minimum value - a value where 100% of the liquids is no longer carried as droplets or a film to the surface. Instead, some of the liquid falls downward causing significant liquid-dominated hydrostatic pressure drop.
Very high gas flow rates may cause tubing failure due to errosion.
Optimal tubing design should try to minimize pressure losses, liquid loading, errosional velocity, tubing cost, and hole- size (drilling & completion) requirements.
The basic form of the gas tubing equation qg=C(pw2-pt2)0.5 is often applicable even for very rich gas condensates, though it may be difficult to predict a priori the tubing constant C with standard (‘dry gas’) equations.