reservoir · recovery
Oil & Solution Gas Expansion (Eo)
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Inputs
RB/STB
RB/STB
scf/STB
scf/STB
ft³/scf
Description
Computes the oil-and-solution-gas expansion term of the general material balance equation — the reservoir-volume expansion per stock-tank barrel from oil shrinkage/expansion plus the reservoir-volume contribution of solution gas liberated as pressure depletes below the bubble point. A drive-mechanism-energy quantity, distinct from the raw Bo/Rs PVT correlations it draws on.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Eo | RB/STB | Oil & Solution Gas Expansion |
| Bo | RB/STB | Oil formation volume factor at current reservoir pressure, from the Black Oil PVT calculator or measured PVT data. |
| Boi | RB/STB | Oil formation volume factor at initial (discovery) reservoir pressure. |
| Rsi | scf/STB | Solution GOR at initial reservoir pressure, from the Black Oil PVT calculator or measured PVT data. |
| Rs | scf/STB | Solution GOR at current reservoir pressure. Below the bubble point, Rs < Rsi as gas is liberated from solution; above the bubble point Rs = Rsi. |
| Bg | ft³/scf | Gas formation volume factor at current reservoir pressure, needed to convert liberated solution gas into reservoir-volume terms. |
Assumptions
- Bo, Boi, Rs, Rsi, and Bg are all evaluated using consistent PVT methodology at the same reservoir temperature
- Above the bubble point, Rs = Rsi is expected (no free gas liberated) — the formula still applies but the liberated-gas term should evaluate to zero
- A single, representative pressure is used for the 'current' condition — no attempt to average across a depleting pressure history
Limitations
- Does not itself compute Bo, Boi, Rs, Rsi, or Bg — those must come from the Black Oil PVT / Gas FVF calculators or measured PVT data, copied in manually
- For a reservoir with an original gas cap, Eo alone does not capture total drive energy — combine with the Gas Cap Expansion (Eg) calculator for the full general material balance picture
- Does not account for water or rock compressibility contributions to expansion (Ceppi/pore-volume compressibility terms) — those are separate additive terms in a full material balance
Use Cases
- → General material balance input: Compute the Eo term needed for a Havlena-Odeh straight-line material balance analysis, alongside Eg for a gas-cap-drive reservoir or alone for a solution-gas-drive reservoir.
- → Drive mechanism energy contribution: Quantify how much reservoir voidage is being offset by oil-and-liberated-gas expansion versus gas cap expansion or aquifer influx.
- → Below-bubble-point depletion tracking: Track how the liberated-solution-gas contribution to Eo grows as reservoir pressure depletes further below the bubble point.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Spraberry unconventional reservoirs are commonly depleted well below the bubble point, so the liberated-solution-gas term is often the dominant contributor to Eo late in life.
Gulf of Mexico
Undersaturated Miocene/Pliocene reservoirs early in depletion have Rs=Rsi, so Eo reduces to the pure liquid-expansion term — matching the simpler Material Balance (Undersaturated Oil) calculator over that regime.
North Sea
Brent Group solution-gas-drive fields track Eo trends closely against GOR history to validate material balance N estimates as pressure depletes below the bubble point.
Global
Always confirm Rs/Rsi and Bo/Boi were derived from the same PVT correlation or lab report — mixing methodologies between the initial and current values biases the apparent expansion.
References
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