reservoir · recovery
Mobility Ratio
click formula to derive ↑
Inputs
fraction
fraction
cp
cp
Description
Computes the mobility ratio — the ratio of the displacing fluid's mobility (water) to the displaced fluid's mobility (oil) — the single most important parameter governing waterflood displacement efficiency and areal sweep.
Variables
| Symbol | Unit | Description |
|---|---|---|
| M | — | Mobility Ratio |
| krw | fraction | Relative permeability to water at the displacing-phase end-point saturation (e.g. at residual oil saturation), from core relative permeability (SCAL) data. |
| kro | fraction | Relative permeability to oil at the displaced-phase end-point saturation (e.g. at initial/connate water saturation), from core relative permeability (SCAL) data. |
| μw | cp | Viscosity of the displacing water phase at reservoir conditions. |
| μo | cp | Viscosity of the displaced oil phase at reservoir conditions. |
Assumptions
- krw and kro are representative end-point values from core SCAL data (or a calibrated correlation), not a full relative permeability curve
- Viscosities are evaluated at representative reservoir conditions for each phase
- A single, homogeneous mobility ratio is assumed — reservoir heterogeneity and viscous fingering effects are not modeled
Limitations
- Uses fixed end-point relative permeabilities, not a full kr(Sw) curve — this platform has no relative-permeability-curve infrastructure yet, so M is evaluated at a single representative saturation point rather than swept across the full displacement
- Does not itself predict sweep efficiency or recovery — it is an input to those calculations (e.g. Fractional Flow, or empirical sweep-efficiency correlations), not a replacement for them
- An unfavorable M does not by itself preclude a successful flood — pattern geometry, heterogeneity, and injection strategy also strongly affect outcomes
Use Cases
- → Waterflood screening: Quickly assess whether a candidate reservoir's oil/water mobility contrast is favorable or unfavorable before committing to detailed flood design.
- → EOR/polymer flood justification: Quantify how unfavorable the native mobility ratio is to build the case for a mobility-control method such as polymer flooding.
- → Fractional flow input: Provide the same end-point kr/viscosity inputs used by the Fractional Flow calculator, for a consistent displacement-efficiency picture.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Spraberry unconventional benches rarely see conventional waterflood (matrix permeability too low) — mobility ratio here is more relevant to fracture-fluid cleanup than to pattern flood design.
Gulf of Mexico
Miocene/Pliocene sandstone waterfloods with light oil (low μo) often show near-favorable M, contributing to the strong recovery factors typical of the region.
North Sea
Brent Group waterfloods with moderate oil viscosity typically show mildly unfavorable M, managed through infill drilling and pattern optimization rather than mobility-control chemicals.
Global
Heavy oil waterfloods (μo > 100 cp) show strongly unfavorable M and often require polymer flooding or thermal methods rather than conventional waterflooding to achieve acceptable sweep.
References
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