Rock Typing and Flow Units

One Trend Is Not Enough

Earlier you saw permeability predicted from porosity with a single trend. Real reservoirs contain several rock types, each with its own pore geometry and therefore its own poro-perm relationship. Forcing one trend through all of them blurs the good rock into the bad. Rock typing sorts the data into flow units that each behave consistently.

The Flow Zone Indicator

The standard tool is the flow zone indicator. From the reservoir quality index $\text{RQI} = 0.0314\sqrt{k/\phi}$ and the normalized porosity $\phi_z = \phi/(1-\phi)$, the indicator is $\text{FZI} = \text{RQI}/\phi_z$. Rocks with the same FZI share a pore geometry and fall on one curve in the porosity-permeability plane, so iso-FZI lines partition the crossplot into rock types. The widget colors a core dataset by FZI band.

Why It Matters

Once the rock types are defined, each gets its own poro-perm transform, its own capillary-pressure curve, and often its own variogram. Permeability modeled per rock type is far more faithful than a single global fit, and the rock types become the framework that ties the petrophysics, the facies model, and the flow behavior together. Rock typing is where static modeling and dynamic behavior first meet.