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
or
where
- = STOOIP (barrels)
- = Bulk (rock) volume (acre-feet or cubic metres)
- = Fluid-filled porosity of the rock (fraction)
- = Water saturation - water-filled portion of this porosity (fraction)
- = Formation volume factor (dimensionless factor for the change in volume between reservoir and standard conditions at surface)
Gas saturation 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.
Read more about this topic: Oil In Place
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