# Oil Material Balance

From wiki.pengtools.com

## Brief

The general form of the **Oil Material Balance** equation was first published by **Schilthuis** in **1941**^{[1]}.

## Math & Physics

Equating all underground withdrawals to the sum of the volume changes^{[1]}:

For use in the code to find Pres:

Pres = Pi - (Np * Bo + Np * (Rp - Rs) * Bg + (Wp * Bw - Wi * Bw - Ggi * Bginj - We * Bw) - (N * (Bo - Boi) + N * (Rsi - Rs) * Bg + m * N * Boi * (Bg / Bgi - 1))) * (1 - Swc) / (N * (1 + m) * Boi * (cw * Swc + cf))

For use in the code to find Np:

Np = (N * (Bo - Boi) + N * (Rsi - Rs) * Bg + m * N * Boi * (Bg / Bgi - 1) + N * (1 + m) * Boi * (Pi - Pres) * (cw * Swc + cf) / (1 - Swc) - (Wp * Bw - Wi * Bw - Gging * Bgi - We * Bw)) / (Bo + (Rp - Rs) * Bg)

## Discussion

... most powerful tool for investigating reservoirs and understanding their performance ...— L.P. Dake^{[2]}

... the safest technique in the business since it's minimum assumption route through the subject of reservoir engineering ...— L.P. Dake^{[2]}

## See also

Gas Material Balance

Gas Flowing Material Balance

Oil Flowing Material Balance

## Nomenclature

- = gas formation volume factor at Pres, bbl/scf
- = initial gas formation volume factor, bbl/scf
- = injection gas formation volume factor at Pres, bbl/scf
- = oil formation volume factor at Pres, bbl/stb
- = initial oil formation volume factor, bbl/stb
- = water formation volume factor at Pres, bbl/stb
- = formation compressibility at initial pressure and temperature, 1/psia
- = water compressibility at Pres, 1/psia
- = gas injection volume, scf
- = gas cumulative production volume, scf
- = initial gas cap hydrocarbon pore volume, bbl
- = initial oil hydrocarbon pore volume, bbl
- , initial gas cap oil leg ratio, dimensionless
- = stock tank oil initially in place, stb
- = oil cumulative production volume, stb
- = initial reservoir pressure, psia
- = average reservoir pressure, psia
- , cumulative GOR, scf/stb
- = solution oil-gas ratio, scf/bbl
- = initial solution oil-gas ratio, scf/bbl
- = initial gas saturation, fraction
- = initial oil saturation, fraction
- = connate water saturation, fraction
- = water influx volume, stb
- = water injection volume, stb
- = water production volume, stb

## References

- ↑
^{1.0}^{1.1}Dake, L.P. (1978).*Fundamentals of Reservoir Engineering*. Amsterdam, Hetherlands: Elsevier Science. ISBN 0-444-41830-X. - ↑
^{2.0}^{2.1}Dake, L.P. (1994).*The Practice of Reservoir Engineering*. Amsterdam, Hetherlands: Elsevier Science. ISBN 0-444-88538-2.