When oil is transported to the surface, we learned that it shrinks. It shrinks because gas evolves out of the oil as the pressure decreases. The gas that evolves from oil as it is transported from reservoir to surface conditions is called solution gas. The figure below illustrates the concept of solution gas for an undersaturated reservoir (i.e. reservoir pressure above the bubble point).
It is important to realize that solution gas does not include free gas. If the reservoir was saturated (reservoir pressure below the bubble point), a free gas phase would exist. We do not include free gas in the determination of solution gas because under reservoir conditions, the free gas is not dissolved in the oil. To account for solution gas, petroleum engineers have defined a solution gas ratio (
). The solution gas ratio is defined as the number of standard cubic feet of gas which will dissolve in one stock tank barrel of oil when both are taken down to reservoir pressure. It has units of scf/STB. In the lab, the typical shape of solution gas oil ratio is illustrated below:
The figure above shows how the solution gas oil ratio changes as a function of pressure at constant reservoir temperature. The process can be described in steps:
- 1-2: As the reservoir pressure is decreased from initial reservoir pressure (
) to bubble point pressure (
), the dissolved gas oil ratio is constant. This is because, above the bubble point there is no free gas in the reservoir. So the amount of gas that comes out at the surface will be dissolved gas only and the solution gas oil ratio will remain constant. - 2-3: As the reservoir pressure falls below bubble point pressure, free gas will continuously evolve in the reservoir. This leaves less gas dissolved in the oil, therefore the solution gas oil ratio steadily declines below the bubble point pressure.
So why is solution gas oil ratio important?? From the figure above it is clear that there is a transition period where we see a change in solution gas oil ratio. It occurs at the bubble point pressure. We can take advantage of the solution gas oil ratio behavior to identify the bubble point pressure of producing wells that started out producing above the bubble point pressure. Further, the solution gas oil ratio is used in material balance calculations to help identify drive mechanisms.