The Phase Behavior of a Natural Hydrocarbon System

Abstract

The phase behavior of a naturally occurring hydrocarbon system whosecritical temperature is near the reservoir temperature has beendescribed. The same volume per cent liquid was observed for the first time at threedifferent pressures for isotherms immediately below the critical temperature.The shapes of the isothermal equilibrium constant curves necessary to predictthis phenomena are discussed and illustrated. Introduction The phase behavior of a number of hydrocarbon systems has been reported;however, very little data are available on natural occurring hydrocarbonsystems whose critical temperature is near the reservoir temperature. Thisappeared to be the case for the fluid studied here; therefore thepressure-volume-temperature data for this system were determined forpresentation. Method The PVT data were obtained with equipment similar to that described byWeinaug and Katz, consisting principally of a two-section, double-windowedgauge mounted in a constant temperature air bath. The procedure followed wasessentially that used by Katz and Kurata except that a cathetometer, readingdirect to 0.01 cm, was used to determine the height of each extreme phaselimit. The system studied was formed by recombining vapor and liquid samples from thefirst stage separator of the well. Portions of these samples were displacedwith mercury into the cell which had previously been filled with mercury. Apredetermined amount of the liquid sample was introduced, while an excess ofvapor was admitted. The resulting mixture was brought to equilibrium at thecondition of the separator during sampling. By adding mercury to maintain thepressure, enough vapor was displaced to give a gas/oil ratio within the cellequal to that produced by the separator at the time of sampling. The remainingmaterial was then considered to be equivalent to the reservoir fluid. Ananalysis of material in the cell, computed from analysis of vapor and liquidsamples and the separator gas/oil ratio, is given in Table I. T.P. 3097

Fundamentals of Reservoir Fluids

“…(3) the effects of variation of separator pressure on the amounts of gas liberated and on the shrinkage of produced oil in the stock tank which results from such separation process; ( 4 ) differential gas liberation data; ( 5) the density or specific volume of reservoir fluid; (6) the relation of reservoir oil viscosity at reservoir temperature and at pressures ranging from reservoir pressure to atmospheric pressure with the viscosity of stock-tank oil; (7) a hydrocarbon analysis of the mixture as it existed originally in the reservoir; and (8) ASTM distillation analyses of residual oil. Complete and fairly accurate data on these various properties can be obtained with comparative ease and at moderate cost through a laboratory examination of a sample of reservoir oil.…”