Oil in place

Oil in place is the total hydrocarbon content of an oil reservoir and is often abbreviated STOOIP, which stands for Stock Tank Original Oil In Place, or STOIIP for Stock Tank Oil Initially In Place, referring to the oil in place before the commencement of production. In this case, stock tank refers to the storage vessel (often purely notional) containing the oil after production.

Oil in place must not be confused with oil reserves, that are the technically and economically recoverable portion of oil volume in the reservoir. Current recovery factors for oil fields around the world typically range between 10 and 60 percent; some are over 80 percent. The wide variance is due largely to the diversity of fluid and reservoir characteristics for different deposits.^[1]

Calculating STOIIP

Accurate calculation of the value of STOIIP requires knowledge of:

• volume of rock containing oil (Bulk Rock Volume, in the USA this is usually in acre-feet)
• percentage porosity of the rock in the reservoir
• percentage water content of that porosity
• amount of shrinkage that the oil undergoes when brought to the Earth's surface

and is achieved using the formula

$N = \frac{7758\ V_b\ \phi\ \left(1 - S_w \right)}{B_{oi}}\ \$ [stb]

or

$N = \frac{\ V_b\ \phi\ \left(1 - S_w\right)}{B_{oi}}\ \$ [m³]

where

• $N\$ = STOIIP (barrels)
• $V_b\$ = Bulk (rock) volume (acre-feet or cubic metres)
• $\phi\$ = Fluid-filled porosity of the rock (fraction)
• $S_w\$ = Water saturation - water-filled portion of this porosity (fraction)
• $B_{oi}\$ = Formation volume factor (dimensionless factor for the change in volume between reservoir and standard conditions at surface)

Gas saturation S_g is traditionally omitted from this equation.

The constant value 7758 converts acre-feet to stock tank barrels. An acre of reservoir 1 foot thick would contain 7758 barrels of oil in the limiting case of 100% porosity, zero water saturation and no oil shrinkage. If the metric system is being used, a conversion factor of 6.289808 can be used to convert cubic metres to stock tank barrels. A 1 cubic metre container would hold 6.289808 barrels of oil.

Formation volume factor

When oil is produced, the high reservoir temperature and pressure decreases to surface conditions and gas bubbles out of the oil. As the gas bubbles out of the oil, the volume of the oil decreases. Stabilized oil under surface conditions (either 60 F and 14.7 psi or 15 C and 101.325 kPa) is called stock tank oil. Oil reserves are calculated in terms of stock tank oil volumes rather than reservoir oil volumes. Oil formation volume factor ( Bo ) can be defined as ratio of Volume at reservoir condition to Volume at the surface condition (at 60F and 14.7psi). It usually varies from 1.0 to 1.7. A formation volume factor of 1.4 is characteristic of high-shrinkage oil and 1.2 of low-shrinkage oil.

References

1. ^ "Defining the Limits of Oil Production". International Energy Outlook 2008. U.S. Department of Energy. June 2008. http://www.eia.doe.gov/oiaf/ieo/oilproduction.html. Retrieved 2008-11-22.