Undersaturated Oil Formation Volume Factor (B_o,us)

The oil formation volume factor (Bo) is defined as the ratio of the volume of oil (plus the gas in solution) at the prevailing reservoir temperature and pressure to the volume of oil at standard conditions.

With increasing pressures above the bubble-point pressure, the oil formation volume factor decreases due to the compression of the oil. To account for the effects of oil compression on Bo, the oil formation volume factor at the bubble-point pressure is first calculated, then it is adjusted to account for the effect if increasing the pressure above the bubble-point pressure.

Reference: Reservoir Engineering Handbook, Tarek Ahmed