reservoir · material balance
Water Drive Index (WDI)
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Inputs
bbl
STB
RB/STB
bbl
RB/STB
Description
Computes the water drive index — the fraction of total reservoir voidage replaced by natural water influx — one of the three drive indices (alongside depletion and gas-cap/segregation drive) that partition a reservoir's total energy balance and sum to 1.0. Takes cumulative water influx (We) as a known input rather than solving for it, consistent with this platform's aquifer-influx calculators remaining a separate, not-yet-built step.
Variables
| Symbol | Unit | Description |
|---|---|---|
| WDI | fraction | Water Drive Index |
| We | bbl | Cumulative water influx into the reservoir to date, already estimated from an aquifer model (e.g. Fetkovich or van Everdingen-Hurst) or measured — not solved for by this calculator. |
| Np | STB | Total oil produced to date at stock-tank conditions, from production accounting. |
| Bo | RB/STB | Oil formation volume factor at current reservoir pressure — from a PVT report or the black_oil_pvt calculator. |
| Wp | bbl | Total produced water to date at surface conditions, from production accounting. |
| Bw | RB/STB | Water formation volume factor at current reservoir pressure — close to 1.0 for most oilfield brines. |
Assumptions
- We (cumulative water influx) is already known — from a separate aquifer-influx analysis (Fetkovich, van Everdingen-Hurst) or a field-calibrated estimate — this calculator does not derive it
- Np, Wp, Bo, and Bw are all measured or estimated at the same point in the reservoir's production history as We
- No significant gas cap or free-gas term is present in F beyond what Bo already reflects — a full three-drive-index breakdown for a reservoir with a gas cap needs the gas expansion (Eg) and oil expansion (Eo) terms from the Gas Cap Expansion / Oil & Solution Gas Expansion calculators as well
Limitations
- Provides no independent check that DDI+SDI+WDI actually sum to 1 for a given reservoir — computing all three drive indices from consistent inputs and confirming the sum is the standard cross-check, not performed automatically here
- A single-point-in-time WDI can be misleading early in a reservoir's life before aquifer response has stabilized — trend WDI over successive reporting periods rather than trusting one early value
- Sensitive to the accuracy of We, which is itself typically the least certain term in a reservoir material balance
Use Cases
- → Drive mechanism diagnosis: Compute WDI alongside the depletion and segregation drive indices to determine which mechanism dominates a reservoir's production energy, informing recovery-factor and waterflood-timing decisions.
- → Aquifer strength screening: Track WDI over successive material balance updates — a rising WDI trend over time confirms an actively contributing aquifer, supporting the case for pressure maintenance rather than assuming a purely volumetric (depletion) reservoir.
- → Water Drive Index consistency check: Cross-check an aquifer-model-derived We by confirming the resulting WDI, combined with independently-computed DDI and SDI, sums to approximately 1.0 — a large deviation flags an inconsistency in one of the underlying terms.
Related Calculations
Region Notes
Gulf of Mexico
Strong aquifer-supported Miocene/Pliocene sandstone reservoirs commonly show WDI rising toward 0.6-0.9 as the field matures, consistent with the sustained pressure support and rising water cut typically observed.
Permian Basin
Wolfcamp/Spraberry unconventional reservoirs are overwhelmingly depletion-drive (WDI near 0) — a computed WDI significantly above 0 in these plays should prompt a review of the We estimate or local aquifer connectivity assumptions.
North Sea
Brent Group fields with strong aquifer support (or supplemented by water injection) often show WDI trending upward over field life; injection volumes should be excluded from We, which represents natural aquifer influx only.
Global
Always compute DDI, SDI, and WDI together and confirm they sum to approximately 1.0 before relying on any single index — an inconsistent sum usually indicates an error in one of the underlying expansion or influx terms.
References
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