reservoir · recovery
Gas Cap Expansion (Eg)
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Inputs
RB/STB
ft³/scf
ft³/scf
Description
Computes the gas-cap expansion term of the general material balance equation — the incremental reservoir volume, expressed per stock-tank barrel of oil-equivalent, contributed by an expanding gas cap as reservoir pressure depletes. A drive-mechanism-energy quantity, distinct from a raw Bg PVT correlation. Also distinct from Drilling's Gas Kick Expansion despite the shared name: that calc tracks a kick gas bubble's near-isothermal Boyle's Law expansion over the minutes to hours it migrates up a wellbore during a well control event, a completely different physical process on a completely different timescale from this calc's production-life, PVT-driven gas-cap depletion term.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Eg | RB/STB | Gas Cap Expansion |
| Boi | RB/STB | Oil formation volume factor at initial (discovery) reservoir pressure — the reference volume basis for expressing gas-cap expansion in oil-equivalent reservoir barrels. |
| Bgi | ft³/scf | Gas formation volume factor at initial reservoir pressure, from the Gas FVF calculator or measured PVT data. |
| Bg | ft³/scf | Gas formation volume factor at current reservoir pressure. Bg increases as pressure depletes, so Bg > Bgi is expected for a gas cap that has expanded. |
Assumptions
- Bg and Bgi are both evaluated using consistent PVT methodology (same z-factor correlation and gas composition assumption)
- The gas cap is in pressure communication with the oil column, so Bg/Bgi reflects the same average reservoir pressure depletion affecting both
- No significant gas cap gas has been produced separately from the oil column between the initial and current conditions being compared
Limitations
- Does not itself compute Bg or Bgi — those must come from the Gas FVF calculator or measured PVT data, copied in manually
- Assumes a single, well-defined gas cap in pressure communication with the oil zone — not applicable to reservoirs without an original free gas cap (use the Oil & Solution Gas Expansion calculator alone for those)
- The gas-cap-to-oil-volume ratio (m) needed to combine Eg into a full material balance equation is not computed by this calculator — it must be supplied separately
- Not applicable to a migrating kick gas bubble during a well control event — that is a different physical process (near-isothermal Boyle's Law expansion over minutes to hours as confining pressure drops) with its own calculator, Drilling's Gas Kick Expansion, despite the coincidental name overlap
Use Cases
- → General material balance input: Compute the Eg term needed for a Havlena-Odeh straight-line material balance analysis of a gas-cap-drive reservoir.
- → Drive mechanism energy contribution: Quantify how much reservoir voidage is being offset by gas-cap expansion versus other drive mechanisms (oil/solution-gas expansion, aquifer influx) as pressure depletes.
- → Gas cap size (m) diagnostic support: Compare Eg trends over time as part of estimating or validating an assumed gas-cap-to-oil-volume ratio in a full material balance study.
Related Calculations
Region Notes
Gulf of Mexico
Many Miocene/Pliocene reservoirs have a well-defined original gas cap; Eg is routinely tracked alongside pressure and GOR trends to confirm the assumed gas-cap size (m) remains consistent.
North Sea
Brent Group gas-cap-drive fields often manage gas cap expansion deliberately (blowdown or gas injection) rather than letting it expand freely — Eg trends help quantify the energy being added or withheld.
Permian Basin
Unconventional Wolfcamp/Bone Spring reservoirs rarely have a conventional free gas cap in the material balance sense — this calculator is more applicable to conventional, structurally-trapped reservoirs.
Global
Always confirm Bg and Bgi were computed with consistent gas composition and z-factor methodology — a correlation change between the initial and current PVT reports will bias the apparent expansion.
References
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