1. n. [Production Logging]
With reference to multiphase flow in pipes, the fraction of a particular fluid present in an interval of pipe. In multiphase flow, each fluid moves at a different speed due to different gravitational forces and other factors, with the heavier phase moving slower, or being more held up, than the lighter phase. The holdup of a particular fluid is not the same as the proportion of the total flow rate due to that fluid, also known as its cut. To determine in-situ flow rates, it is necessary to measure the holdup and velocity of each fluid. Holdup is usually given the symbol y, with the suffixes g, o or w for gas, oil or water. The sum of the holdups of the fluids present is unity. The holdup ratio is the ratio of the holdups of two fluids, and is sometimes used as a parameter to express the phenomenon.