reservoir · volumetrics
Remaining Reserves
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Inputs
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STB / scf
STB / scf
Description
Computes remaining (net-of-production) reserves as ultimate recoverable reserves minus cumulative production to date — the routine update every reserve report needs as an asset produces. A purely arithmetic calculator: it does not itself estimate URR or forecast the timing of remaining production.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Remaining | STB / scf | Remaining Reserves |
| Phase | — | Numeric code for the hydrocarbon phase: 1 = oil (URR/Np/remaining in STB), 2 = gas (URR/Np/remaining in scf). Rounded to the nearest integer. Added so this calculator's phase-dependent STB/scf fields can be correctly converted between Imperial and Metric display, the same convention volumetric_reserves already uses. |
| URR | STB / scf | Current best estimate of ultimate recoverable reserves (from the Volumetric Reserves calculator, decline-curve EUR, or another source), in the same phase and units basis as Np. |
| Np | STB / scf | Cumulative oil or gas production to date, measured or reported as of a common reference date. |
Assumptions
- URR and Np are reported in the same phase (oil or gas) and volume basis (standard/stock-tank conditions)
- Np is a reliable, up-to-date cumulative production figure as of a known reference date
- URR reflects the current best estimate, understood to be revised as more production and reservoir data become available — not treated as a fixed original number
Limitations
- Purely arithmetic — the result is only as reliable as the URR and Np inputs, neither of which this calculator itself derives or verifies
- Does not forecast the rate or timing at which the remaining reserves will be produced — pair with a decline curve calculator for that
- A negative or near-zero result signals the URR estimate needs revision; it is not evidence of a genuine negative or exhausted reserve
Use Cases
- → Routine reserve report update: Recompute remaining bookable reserves at each reporting period as cumulative production (Np) advances and URR is periodically revised.
- → Divestiture or acquisition evaluation: Quickly estimate the remaining reserves being bought or sold, given the seller's reported URR and up-to-date production data.
- → URR consistency check: Compare remaining reserves against an independent decline-curve EUR projection to sanity-check whether the booked URR still looks reasonable.
Related Calculations
Region Notes
Permian Basin
Unconventional wells typically produce a large share of URR in the first 2-3 years — expect remaining reserves to fall quickly early in life even with a well-calibrated URR.
Gulf of Mexico
Deepwater fields often revise URR upward mid-life as waterflood or infill drilling improves recovery beyond the original primary-depletion estimate — remaining reserves can increase even as Np grows.
North Sea
Mature North Sea fields frequently see multiple URR revisions over field life as new wells, 4D seismic monitoring, and improved recovery projects refine the estimate.
Global
Always date-stamp both the Np figure and the URR estimate used — remaining reserves computed from mismatched reference dates can materially misstate the current position.
References
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