The volume occupied by a given amount of hydrocarbon varies depending on the pressure and temperature. A volume factor is a ratio of the volume at one set of conditions to a set of standard conditions. Typical volume factors include the formation volume factor, two-phase formation volume factor and gas volume factor. We’ll review the single-phase and two-phase volume factors for a specific reservoir fluid.

The fluid as it exists in a undersaturated oil reservoir is shown in A. At this point, 1.31 barrels is the formation volume factor Bo. Point B could represent a point within the wellbore; the pressure has decreased, but the temperature remains the same resulting in an increase in volume. As the fluid continues up the wellbore the pressure declines further. For the purposes of this illustration we are going to assume it stays at the same temperature. At some pressure between B(2500 psia) and C (1200 psia) the fluid passes the bubble point, and gas begins to break out of solution. This results in a much larger increase in the overall volume occupied and a decrease in the liquid volume. As the fluid reaches atmospheric pressure (D), it expands to its maximum extent and the last traces of gas break out of solution. Finally, at point E, the fluid cools to ambient temperature. The volume of both the liquid and the gas decrease slightly. This same progression is shown in the graph below, starting from the right and moving to the left.

A saturated reservoir, meaning a reservoir in which both liquid and gas are present at initial conditions, would start at C rather than at A.

The gas volume factor (Bg) is calculated as follows (for standard conditions of psc is 14.7 psia and Tsc is 60°F):

where T is temperature, p is pressure and z is the gas compressibility factor.

The two-phase formation volume factor (Bt) is calculated:

where Rsoi and Rso are the initial solution gas-oil ratio and solution gas-oil ratio respectively. The solution gas oil ratio mirror the formation volume factor.

Terry, Ronald E., J. Brandon. Rogers, and B. C. Craft. *Applied Petroleum Reservoir Engineering*. Third ed. Massachusetts: Prentice Hall, 2014. Print.