Effects of Pressure and Temperature on Condensation of Distillate from Natural Gas

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Deep drilling has led to the development of numerous pools which yieldproducts consisting predominantly of natural gas accompanied by high-gravitywater-white or slightly straw-colored liquid. Yields of this"condensate" or "distillate," as it is commonly called, vary fromtraces to over a hundred barrels per million cubic feet of gas. The fact thatthis liquid exists in the reservoir as an integral part of the gas phase and isformed by condensation attending simultaneous reduction of pressure andtemperature after its entry into the well was apparently first recognized about1932.1 The nature of the physical phenomena involved in this condensation wasclarified for the petroleum industry in 1933 by a report by Sage, Schaafsma, and Lacey of the results of an experimental investigation of the behavior ofmixtures of methane and propane. Search of the literature has revealed that the condensation of liquid fromgaseous mixtures by isothermal pressure reduction is not a new discovery.Kuenen was led by theoretical considerations to expect such a phenomenon, andin 1892 reported the results of experimental verification of his deductions.This was followed by a considerable amount of work on the phase relations ofmixtures by Kuenen and by others, mostly in European universities. Since 1930investigators in this country have reported experimental data on the properties and behavior of a number of hydrocarbon systems. It is now generally known that in the presence of natural gas the solubility inthe gas phase of the heavier and normally liquid hydrocarbons increasesrapidly with increasing pressure above 500 to 1000 lb. per sq. in. Thus, at pressuresof 2500 lb. per sq. in. or higher a gas may contain appreciable quantities ofhydrocarbons with normal boiling points as high as 600?F., which in turn may becondensed from the gas phase by the reduction of their solubility thereinattending pressure reduction. The combined effects of pressure and temperatureon condensation and vaporization of mixtures are most readily visualized by thestudy of phase diagrams. Since the construction, application, andinterpretation of such diagrams have been described fully in readily availableliterature, this paper will not review the theoretical aspects of thissubject. While the behavior of gas-distillate systems is now clearly understood, atleast in a qualitative way, most of the published work deals with the resultsof laboratory studies of binary systems or of synthetic mixtures ofhydrocarbons, and although several systematic investigations of the effects ofseparator pressure and temperature on the yields of liquid product fromgas-distillate wells have been made it appears that in all cases thelimitations have precluded the construction of even approximately completephase diagrams for naturally occurring systems. T.P. 1269