Fluid is characterized by its:
- phase envelope
- initial gas-oil producing ratio
- oil gravity (API gravity)
- oil formation volume factor
- heavy components composition based on laboratory analysis (mole fraction, reported as “heptanes-plus” fraction, C7+)
Pure Component Phase Diagram
A pure (one) component phase diagram is a pressure-temperature cartesian plot that has lines separating the solid, liquid, and gas phases.
The line separating the liquid and gas regions is called the vapor pressure curve. The end of the vapor pressure curve is called the critical point.
The line separating the solid and liquid regions is called the melting point curve. At the triple point, all three phases can coexist in equilibrium.
The cartesian coordinate of the critical point is the critical temperature at the critical pressure. Critical temperature is the highest temperature at which two phases can exist. Beyond this temperature, there’s nothing you can do in terms of increasing pressure to transition from a liquid to a gas. Similarly, critical pressure is the highest pressure at which two phases can exist. In the region above both the critical temperature and critical pressure, a component doesn’t look like either a liquid or gas.
For the single phase pure component above, given a PVT cell at the pressure and temperature indicated by point 1, we begin in a liquid state. As we isothermally decompress the cell, we transition along the line between points 1 and 2. At point 2, we hit the vapor pressure curve and incipient gas formation begins. As we continue to withdraw fluid, more and more gas breaks out until the point that no more liquid exists. From the moment that gas first appears until the last liquid disappears, pressure and temperature are fixed at point 2. Once all liquid is gone, pressure can continue to drop in the gas region toward point 3.
Mixed Component Phase Diagram
For a mixed component, the vapor pressure line is expanded into a phase envelope, with a bubble point line separating the liquid phase from the mixed phase, and a dew point line separating the mixed phase from the gas phase. The lines within the envelope represent different volume percentages of liquid and gas.
As we reduce pressure from an all liquid region, we’ll begin to evolve gas once we hit the bubble point line. As we continue to reduce pressure, we get more and more gas, and less liquid, until we hit the dew point line. Below that point, we produce only gas.
We still have a critical point, but it no longer has the same definition as in the pure component diagram. Notice in this diagram that there are temperatures and pressures higher than the critical point at which 2 phases can exist. The critical point is now defined as the pressure and temperature at which the properties (composition, density, viscosity, etc.) of the liquid and gas converge. The cricondenbar is the highest pressure at which any two phases can exist, while the cricondentherm is the highest temperature at which any two phases can exist.
See also Mixtures of Methane and Ethane.