Total Formation Volume Factor

At pressures greater than bubble point pressure, the total (two-phase) formation volume factor increases gently as the reservoir pressure decreases. That behavior occurs because the oil phase, holding the maximum amount of gas in solution, expands gently due to the pressure decrease. When the pressure decreases below the bubble point pressure, the total (two-phase) formation volume factor increases rapidly.​ That’s because more gas is liberating from the liquid phase, making the whole system expanded rapidly. Actually, almost all the black oil properties respond differently to the decrease in reservoir pressure.