Undersaturated volatile oil reservoirs are single-phase oil systems that reside in reservoirs with an original temperature relatively close to the critical point (relative compared to low shrinkage oils). As with the undersaturated black oils, the term undersaturated implies that the original reservoir temperature and pressure lies in the single-phase liquid region of the phase diagram. Such a system is shown in Figure 2.14.
We are now in a position to discuss the terms “low shrinkage oil” and “high shrinkage oil” as they pertain to the oil and gas industry. The blue bars on Figure 2.14 (A) represent equal pressure drops below the bubble-point pressure (these bars are of equal length). We can see from this figure that due to the convergent nature of the quality lines (dashed lines) near the critical point, equal pressure drops result in different volume percentages of the liquid phase in the two-phase region. The pressure drop represented by the blue bar further from the critical point results in a two-phase system with 90 percent of the volume occupied by the liquid phase; while the blue bar closer to the critical point results in a two-phase system with 80 percent of the volume occupied by the liquid phase. Thus, the volume of the liquid phase shrinks more in a system that is closer to the critical point for similar pressure drops. Since the liquid phase occupies less volume for a volatile, high shrinkage oil for comparable pressure drops, at abandonment the secondary gas cap will occupy a larger volume.
The definition of a “volatile fluid” is a fluid that easily evaporates or vaporizes. This definition gives rise to the name “volatile oils.” These systems develop larger gas phase volumes for comparable pressure drops when entering the two-phase region of the phase envelope.