Formation Volume Factor
Oil swells with dissolved gas at reservoir conditions, so a barrel underground is not a barrel in the tank: the formation volume factor is the exchange rate.
The equation
Bo = reservoir volume / stock tank volume
- Bo is expressed in reservoir barrels per stock tank barrel
- Typical black oils run 1.1 to 1.6
- Bo rises with solution gas and temperature
Worked example
A reservoir producing 1,000 reservoir barrels per day of oil with Bo of 1.25 delivers only 800 stock tank barrels at surface. Ignore the shrinkage and every volumetric and material balance number inflates by a quarter.
When to use it
Take Bo from PVT analysis where samples exist, correlations otherwise. It changes with pressure, peaking at the bubble point, so use the value at the pressure your calculation actually concerns.
Related Calculators and Reference
- Mud Weight and Pressure Gradient Calculator: convert between ppg, SG, and psi per foot.
- Probabilistic STOIIP Calculator: volumes with uncertainty ranges.
- PetroWiki by SPE: the industry reference for petroleum engineering.