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.



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;



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 example



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.



Abhijit Y. Dandekar, 2013. Petroleum Reservoir Rock and Fluid Properties, Second Edition. 2 Edition. CRC Press.

L.P. Dake, 1983. Fundamentals of Reservoir Engineering, Volume 8 (Developments in Petroleum Science). New impression Edition. Elsevier Science.

Tarek Ahmed PhD PE, 2011. Advanced Reservoir Management and Engineering, Second Edition. 2 Edition. Gulf Professional Publishing.

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