Hydrocarbon Mixtures

Characteristics of Oil and Gas

Hydrocarbon Mixtures

In a mixture of two components, the system is no longer so simple in its behavior as the one component, or pure, substance. Instead of a single line representing the pressure-temperature relationship, there is a broad region in which two phases (liquid and gas) coexist. The next figure is a diagrammatic correlation, or phase diagram, of the phase behavior of a 50:50 mixture of two hydrocarbons such as propane and heptane. Superimposed on the correlation are vapor-pressure curves of the two components in their pure state.

Vapor pressure curves for two pure components and phase diagram for a 50:50 mixture of the same components

The two-phase region of the phase diagram is bounded on one side by a bubble-point line and on the other by a dew-point line, with the two lines joining at the critical point. A bubble point occurs where gas begins to leave solution in oil with decreasing pressure, while a dew point is reached when liquid begins to condense from gas with either decreasing or increasing pressure. Of importance is the fact that, for a given temperature, the pressure at which a mixture of two components is condensed to total liquid is lower than the pressure at which the lighter component in the mixture would condense if it were not in the mixture.

The pressure at which the mixture of two components is vaporized to total gas is higher than the pressure at which the heavy component would vaporize if it were not in the mixture. This is caused by the attractive forces between molecules of like and unlike sizes. Attraction of the heavy for light molecules pulls the light material into the liquid at a lower imposed pressure than would the light molecules among themselves in the absence of the heavy molecules. Attraction of the light for heavy molecules pulls the heavy material into a gas at a higher imposed pressure than would the heavy molecules among themselves in the absence of light molecules.

At the critical point, properties of both gas and liquid mixtures are identical. It is significant that the definition of critical point, as applied to a single-component system, no longer applies because both liquid and gas phases exist at temperatures and pressures above the critical point, although the degree may be slight in a system of only two components. As the system becomes more complex with a greater number of different components, the pressure and temperature ranges in which two phases exist increase greatly.

Phase diagram of low shrinkage oil

An idealized pressure-temperature phase diagram of a common, naturally occurring petroleum is shown in figure above. If the oil existed at its bubble point, or if the original reservoir pressure was the saturation pressure of the mixture, the petroleum would be existing at A. If the pressure of this same oil should be at A, the oil would be undersaturated. Separator and stock tank temperatures and pressures are indicated. Quantities of liquid and gas represented by location of the stock tank point on the diagram, however, do not indicate what would occur in the stock tank because the original mixture changes at the separator in the oil and gas production process.

Vertical or constant temperature line A-B, represents the path the material takes into the two-phase region as pressure is dropped at reservoir temperature and represents quantities of liquid and gas existing in equilibrium at temperature and pressure conditions represented by the given location of the point in the phase loop. This is physically represented in the reservoir by gas coming out of solution, the amount of which is governed by the amount of drop in pressure.

keywords: petroleum, critical point, properties, pressure.