Reservoir Classification is determined by the reservoir fluid’s phase behavior. They are classified into four reservoir types: single phase gas, gas condensate, undersaturated oil and saturated oil reservoirs. The differences in their phase behavior is illustrated in the following diagram.
We’ll start with a single phase gas reservoir. Point A represents the virgin reservoir. As The reservoir is produced, the fluid inside the reservoir remains at the same temperature, but decreases in pressure and follows the dashed line toward A1. This reservoir never enters the two-phase envelope and as a result, the reservoir is entirely gas throughout its entire life. The produced fluid, on the other hand decreases both in temperature and pressure towards A2. It does enter the two phase envelope and some liquids will be produced.
The retrograde gas condensate reservoirs also start out as gas but at point B. As the fluid is produced the fluid remaining in the reservoir drops into the two phase envelope (B1) and liquids start to be produced in the reservoir. The amount of liquids continue to increase until it reaches B2 and then vaporization of that retrograde liquid in the reservoir begins to occur until it reaches abandonment pressure (B3).
The third, undersaturated reservoirs (also called dissolved gas reservoirs) differ from the first in that the fluid exists as a liquid in the reservoir at initial conditions. As pressure declines (C1), the first bubbles of ‘dissolved’ gas begin to appear (at which point the reservoir is said to be saturated). As pressure continues to drop, the liquid volume in the reservoir decreases and more gas is produced. As oil is the primary product, pressure maintenance strategies in this reservoir type is crucial.
Finally we come to our saturated oil reservoirs. At initial conditions, this reservoir already has both liquid and gas present. These phases have separated overtime due to density differences resulting in a ‘gas cap’ over the reservoir. Typically, the reservoir is produced in the oil zone, allowing the expansion of the gas cap to assist in maintaining a high reservoir pressure.
Terry, Ronald E., J. Brandon. Rogers, and B. C. Craft. Applied Petroleum Reservoir Engineering. Third ed. Massachusetts: Prentice Hall, 2014. Print.