Introduction

From a phase-behavior view, we presently have no explanation of why so many reservoirs are discovered at their dew- and bubble-point pressures. Early reservoir engineering pressures. Early reservoir engineering concepts - which still prevail - imply some mysterious origin of "solution gas" in crude oil. It is said that undersaturated crude oils do not contain sufficient gas "in solution". But we really do not know why. Nor do we have an explanation for the observed natural grade from surface tar to methane gas at great depths.

Technically, we recognize that gases are products of liquid evaporation. All compounds products of liquid evaporation. All compounds exhibit critical points at which gas and liquid phases are identical. All gases can be phases are identical. All gases can be liquefied at some pressure and temperature. Frequently, in the case of butane, for example, phase changes occur well within normal working phase changes occur well within normal working pressure and temperature ranges. Yet we ignore pressure and temperature ranges. Yet we ignore phase behavior in explaining deviation from the phase behavior in explaining deviation from the perfect gas laws. perfect gas laws. Suppose that a new phase-behavior relation is observed in the laboratory. The easy way to account for the new or unusual behavior is to name the process and experimentally determine how to handle it. Then, we do not know what causes the unusual behavior, but we do bridge a gap. Unfortunately, if we bridge enough gaps in this manner, it soon becomes difficult to explain subsequent observations.

This is one reason why it is difficult to reconcile our present phase-behavior concept with observed patterns of natural hydrocarbon occurrence. Over the years, names have been given to observations such as the critical state, hydrocarbon series, retrograde behavior, convergence pressure, gas "compressibility", etc. We know that these occur, but we do not really know what causes them. So, we vaguely explain them as "phenomena".

If we are to reconcile our phase-behavior concept with the observed patterns of natural hydrocarbon occurrence, a review of the phase-behavior phenomena is a logical starting phase-behavior phenomena is a logical starting point. Such a review, the subject of this point. Such a review, the subject of this article, is justified on the basis of one simple reinterpretation of a presumption made several years ago.

VARIABLE VOLUME - VARIABLE MIXTURE COMPOSITION

In the laboratory, hydrocarbon mixtures are separated in two ways. The mixture is charged to a PVT cell at representative reservoir pressures and temperatures. Then the system pressure is reduced either by removing small portions of the mixture (the differential process), or by increasing the volume of the process), or by increasing the volume of the container (the flash process). The differential process is said to be a "variable composition" process is said to be a "variable composition" process because material is removed from the process because material is removed from the container.

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