Volumetric is a method to estimate fluid in reservoir based on volume of pore space in a rock and water saturation.
Volume of Oil Initially In Place (OIIP)
To estimate oil initially volume in place, the following formula is a volumetric calculation for oil.
Where;
STOIIP = stock tank oil in place, stb
A= area, acre
h = reservoir thickness, ft
ɸ = rock porosity, %
Swc =connate water saturation, %
Boi = oil formation volume factor, rb/stb
Note: the stock tank condition is a standard surface condition of oil and gas at 60F and 14.7 psia.
Volume of Gas Initially In Place (GIIP)
The formula to determine gas in place is listed below;
Where;
G = gas oil in place at standard condition, scf
A= area, acre
h = reservoir thickness, ft
ɸ = rock porosity, %
Swc =connate water saturation, %
Bgi = gas formation volume factor, rcf/scf
Note: This is the same formula as the oil in place but only constant is different because of volume of gas is reported in cu-ft.
Example Calculations
Oil reservoir
Area = 10,000 acre
Thickness (H) = 100 ft
Average porosity (ɸ) = 20%
Connate Water Saturation (Swc) = 25%
Oil formation volume factor (Bo) = 1.29 rb/stb
STOIIP = 902.1 MM STB
Reserves
In reservoir engineering, volume of hydrocarbon in a reservoir called volume in place (oil and/or gas). Volume of hydrocarbon that can be commercially recovered is called “Reserves”. Reserves shall satisfy four criteria which are discovered, recoverable, commercial and remaining based on the development method. Each company may rate the reserves differently based on several criteria.
If you are interested in how to classify reserves, this is a recommended document provided by Social of Petroleum Engineering (SPE) – Petroleum Resources Management System, Society of Petroleum Engineers et al, 2007.
http://www.spe.org/industry/docs/Petroleum_Resources_Management_System_2007.pdf
References
