Volumetric Method To Estimate Volume In Place and Reserves

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