The measurement of natural gas liquids (NGL) is similar in many respects to that of other hydrocarbon liquids but is markedly different in other aspects. The main difference in NGL measurement is the need to properly address the effects of solution mixing. Measuring NGL by mass measurement techniques will properly address solution mixing effects because the mass measurement process is not sensitive to the effect that pressure, temperature and solution mixing have on the fluid measured.
Another difference is the effect of higher vapor pressures on the measurement of natural gas liquids. In static measurement methods, the liquid equivalent of the vapor space must be determined. In dynamic measurement methods, the equilibrium vapor pressure, or that pressure at which a liquid and its vapor phase is in equilibrium at a given operating temperature, must be deducted from the operating pressure when determining the compressibility effects on measured volumes.
This paper will describe the basic concepts used to measure and report the quantities of NGL streams.