reservoir · inflow performance
Water-Oil Ratio (WOR)
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Inputs
STB/d
STB/d
Description
Computes water-oil ratio — produced water rate divided by produced oil rate — a production-ratio metric closely related to water cut but reported directly as a ratio rather than a fraction, and commonly used in its cumulative form (Wp/Np) for long-term production-decline diagnostics.
Variables
| Symbol | Unit | Description |
|---|---|---|
| WOR | fraction | Water-Oil Ratio |
| qw | STB/d | Produced water rate, measured at the same conditions/time as the oil rate. |
| qo | STB/d | Produced oil rate, measured at the same conditions/time as the water rate. |
Assumptions
- qw and qo are measured (or well-test-derived) rates at the same point in time and the same measurement conditions
- Both rates are surface (stock-tank/standard) rates, not reservoir-condition rates
- This calculator's WOR is the instantaneous rate form — not the cumulative (Wp/Np) form sometimes used interchangeably in decline diagnostics
Limitations
- Unbounded above as oil rate approaches zero — a small oil rate produces a very large or undefined WOR, which can be numerically unstable to plot or trend near well shut-in
- A single-point measurement — does not itself reveal the underlying water-source mechanism, the same limitation as water cut
- Uses instantaneous rates, not the cumulative (Wp/Np) form used in classic semi-log WOR decline-diagnostic plots — compute that form separately if needed
Use Cases
- → Production surveillance: Track WOR over time as an alternative to water cut, particularly useful on a log scale for wells spanning a wide range of water fractions.
- → Water-handling facility and disposal design: Size water-handling and disposal capacity directly from the produced water-to-oil ratio.
- → Economic limit / workover screening: Flag wells whose WOR has crossed an operator-defined threshold as candidates for a water-shutoff workover or economic-limit review.
Related Calculations
Region Notes
Permian Basin
Unconventional wells often show a high early-life WOR from flowback fluids that declines and stabilizes at a lower baseline once fracturing fluid recovery is complete.
Gulf of Mexico
Aquifer-supported reservoirs commonly show WOR rising sharply, sometimes by an order of magnitude within a year or two, once the water contact reaches a well's perforations.
North Sea
Waterflooded fields track WOR against cumulative injection to manage sweep efficiency; many Brent Group producers show a classic semi-log-linear late-time WOR trend.
Global
A sharp, sudden jump in WOR (rather than a gradual rise) more often indicates a mechanical issue (casing/tubing leak) than a genuine reservoir water-breakthrough event.
References
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