Flow Units: RQI and FZI
Learning objectives
- Define RQI, normalized porosity phiz, and FZI
- Explain that FZI captures the pore geometry porosity misses
- Recognize flow units as unit-slope lines on the RQI-phiz plot
- Use flow units to tighten the permeability-porosity relation
Making Rock Type Rigorous
The last section said to sort by rock type, but how, exactly? Amaefule turned the vague instruction into arithmetic. Define a reservoir quality index, a normalized porosity, and their ratio:
The flow zone indicator rolls everything porosity misses, grain size, sorting, tortuosity, and surface area, into one number for the pore geometry. Rocks that share an FZI are a hydraulic flow unit: they flow alike.
Flow Units Are Unit-Slope Lines
The plot is where it becomes beautiful. Rearrange the definition and , so on log-log axes of RQI against every flow unit is a straight line of unit slope whose height is set by its FZI. The fat poro-perm cloud of the last section splits into a few clean, parallel lines, one per flow unit, and each rock falls unambiguously onto one of them. Drop a point with the sliders and read its FZI straight off.
Why It Matters
Two payoffs follow. First, within a flow unit permeability really is a tight function of porosity, so the poro-perm transform finally works without huge scatter. Second, because FZI is a rock-fabric property, a flow unit identified on core can be recognized in uncored wells from the logs, carrying a calibrated permeability everywhere. Flow units are therefore the rigorous backbone of permeability prediction, and the natural unit for the upscaling that feeds a reservoir model.
References
- Amaefule, J. O. et al. (1993). Enhanced reservoir description: using core and log data to identify hydraulic flow units. SPE 26436.
- Tiab, D. and Donaldson, E. C. (2015). Petrophysics, 4th ed. Gulf Professional Publishing.