Undersaturated Black Oil Reservoir

An undersaturated black oil reservoir is a type of oil reservoir where the pressure within the reservoir is higher than the bubble point pressure of the oil.  The bubble point is the pressure at which gas begins to come out of solution and form bubbles within the oil.  This type of reservoir is contrasted with a saturated reservoir, where the pressure is at or below the bubble point, and gas is already present as a separate phase within the reservoir.

Key Points About Undersaturated Black Oil Reservoirs

No Free Gas  -  There is no free gas present in the reservoir.  The gas remains dissolved in the oil, which means the oil is in a single-phase (liquid) state.
Pressure Decline  -  As oil is produced from the reservoir, the pressure in the reservoir will gradually decline.  Once the pressure drops to the bubble point, gas will begin to come out of the oil, marking the transition from an undersaturated to a saturated condition.
Production Characteristics  -  These reservoirs typically have higher initial pressures and oil production rates.  However, as production continues and pressure drops below the bubble point, gas begins to evolve from the oil, which can lead to changes in the production characteristics, such as decreased oil production rates and increased gas production.
Reservoir Management  -  Managing an undersaturated black oil reservoir involves maintaining the pressure above the bubble point as long as possible to avoid premature gas breakout, which can complicate production and reduce the recovery efficiency of the oil.