In high school we all learn about the single phase diagram for pure substances. Unfortunately, petroleum engineers don’t normally use it. In petroleum engineering we are dealing with multiple-component mixtures. In these cases, the phase diagram is different. Instead of a single line separating the vapor and liquid phases, there is a two phase region where gas and liquid can coexist. This region is called the phase envelope. Without further ado a phase diagram for a multiple component mixture is shown below:
The phase diagram divides the fluids into three distinct regions. The liquid region is to the left of the critical point and above the bubble point line. The gas region is to the right of the critical point and above the dew point line. The two-phase region (i.e. saturation envelope) is enclosed by the bubble point and dew point lines. The important elements of the phase diagram are defined below:
- Bubble Point Line: The point at which the first bubble of gas comes out of the liquid
- Dew Point Line: The point at which the first drop of liquid drops out of the gas
- Critical Point: Represents the condition where the properties of liquid and gas are identical
- Cricondentherm: The highest temperature on the saturation envelope
- Cricondenbar: The highest pressure on the saturation envelope
- Two Phase Region: Region where gas and liquid coexist
- Retrograde Region: Region where liquid condensate forms by lowering pressure or increasing temperature
- Quality Lines: The lines showing liquid %’s which intersect the critical point
Proper analysis of many petroleum engineering problems requires basic knowledge of the phase behavior diagram. Understanding the components of the diagram above will allow you to proceed intelligently with more complex problems and understand the different kind of reservoirs.