reservoir · recovery
Voidage Replacement Ratio (VRR)
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Inputs
STB/d
RB/STB
STB/d
RB/STB
STB/d
Description
Computes the voidage replacement ratio — reservoir-condition volume injected divided by reservoir-condition volume produced — the standard surveillance metric for whether waterflood injection is keeping pace with the reservoir voidage created by production, and therefore whether reservoir pressure is being maintained.
Variables
| Symbol | Unit | Description |
|---|---|---|
| VRR | — | Voidage Replacement Ratio |
| Winj | STB/d | Surface (stock-tank) water injection rate for the pattern or field being evaluated. |
| Bw | RB/STB | Water formation volume factor at reservoir conditions, from the Water FVF calculator or measured PVT data (copy the value across — not read automatically). |
| Np | STB/d | Surface (stock-tank) oil production rate for the pattern or field being evaluated. |
| Bo | RB/STB | Oil formation volume factor at reservoir conditions, from the Black Oil PVT calculator or measured PVT data (copy the value across — not read automatically). |
| Wp | STB/d | Surface (stock-tank) produced-water rate for the pattern or field being evaluated. |
Assumptions
- Bo and Bw are representative of current reservoir conditions for the pattern or field being evaluated
- Free gas production (if any) is a minor contributor to voidage relative to oil and water — this calculator omits a separate gas voidage term
- Injection and production rates are measured or allocated consistently over the same time period
Limitations
- Omits a free-gas voidage term — for reservoirs producing significant free gas (below the bubble point), a full three-phase voidage balance is needed for an accurate VRR
- A single-period VRR does not reveal spatial imbalance within a pattern or field — localized over- or under-injection can occur even when the field-average VRR looks acceptable
- Does not itself compute Bo or Bw — those must come from the Black Oil PVT / Water FVF calculators (or measured PVT data), copied in manually
Use Cases
- → Waterflood pressure-maintenance surveillance: Track VRR over time to confirm injection is keeping pace with production and reservoir pressure is being maintained near target.
- → Injection allocation review: Compare pattern-level or well-level VRR to identify under-injected patterns at risk of pressure decline and premature decline in offset producers.
- → Waterflood design sanity check: Confirm a proposed injection rate is sufficient to achieve a target VRR given expected production rates before committing to a facility design.
Related Calculations
Region Notes
Permian Basin
Produced water disposal capacity, not injection capacity, is often the binding constraint on VRR management in unconventional development — surface facility limits should be checked alongside the reservoir-engineering target.
Gulf of Mexico
Aquifer-supported reservoirs may already receive substantial natural water influx, so a VRR target based on injection alone can understate total voidage replacement — check for aquifer support before concluding pressure is under-maintained.
North Sea
Brent Group waterfloods typically target VRR of 1.0-1.05 to slightly overbalance voidage and support pressure through late field life.
Global
A VRR near 1.0 confirms volumetric balance but does not by itself confirm good areal conformance — combine with injection profile and interference data to check whether replacement volume is reaching the right part of the reservoir.
References
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