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
