PetroPulse Insights

  Understanding the Key Characteristics of Oil and Gas Some hydrocarbon mixtures exist naturally above their critical temperature as gas condensates. When pressure is decreased on these mixtures, instead of expanding (if a gas) or vaporizing (if a liquid) as might be expected, they tend to condense. Conversely, when pressure is increased, they vaporize instead of condensing. The process is illustrated by temperature condition Tr3 in Fig. 41, and the gas condensate curve in the next figure. This process is caused by forces acting on molecules of unlike sizes and depends upon a balance of these forces, as illustrated in the figure. Phase diagram of retrograde condensate gas Equilibrium retrograde behavior of condensate gas Normal vaporization and condensation, on the other hand, depend more upon balance between molecular forces of like-size component molecules. As pressure drops (at constant temperature) below dew-point pressure (A), the attraction between light and heavy component m...

  Characteristics of Oil and Gas Equilibrium Separation Low Shrinkage Oils A given volume of oil existing at its bubble point at reservoir temperature is considered to be saturated with gas at the given temperature and pressure conditions. Thus, the term "saturation pressure" is synonymous with bubble-point pressure at a given temperature. A decrease in pressure will cause the original sample to change into two phases as shown in the next figure. The physical change is evidenced as gas being liberated from the liquid. It is a common misbelief that a certain amount of gas is dissolved in oil and that a pressure drop results in the gas coming out of solution. Actually, the first gas liberated is composed principally of the lightest components (methane, ethane and propane) because these components possess the highest molecular energy and the lowest molecular attraction for other molecules. Equilibrium vaporization of low shrinkage oil. Vaporization of the lighter components is u...

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 ...

Characteristics of Oil and Gas Changes in  Petroleum  Phases   If pressure is increased, molecules are forced closer together so that gas will be compressed or changed to a liquid. However, as pressure is decreased, the reverse occurs; gas expands and liquid tends to vaporize to gas. The molecules in the latter case are thrown apart by their own kinetic energy and molecular repulsion. These phase changes caused by changes in pressure are termed normal or regular behavior. If the molecules are smaller, as in the case of methane and ethane, there is less attraction between molecules and greater tendency for them to be thrown apart by their kinetic energy into gas; whereas, if molecules are larger, as in hexane and heptane, they tend to be attracted together into the liquid rather than thrown into the gaseous state by their kinetic energies. As the temperature of the compounds increases, kinetic energy increases. The tendency then is for all molecules in the liquid stat...