Oil in place

Oil in place
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "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.

Calculating STOOIP

Accurate calculation of the value of STOOIP 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]

$N = \frac{6.289808\ V_b\ \phi\ \left(1 - S_w\right)}{B_{oi}}\ \$ [m³

where

$N\$ = STOIIP (barrels)
$V_b\$ = Bulk (rock) volume (acre-feet or cubic meters)
$\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 must be used to convert cubic meters to stock tank barrels. A 1 cubic meter 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. The ratio of stock tank volume to oil volume under reservoir conditions is called the formation volume factor (FVF). 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.

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