Permeability and the Poro-Perm Relationship

How Easily Rock Flows

Where porosity is how much fluid a rock holds, permeability $k$ is how easily that fluid flows through it, measured in millidarcies (mD). It is arguably the most important and most uncertain reservoir property, because it spans many orders of magnitude and cannot be read by any standard log. We measure it on core and in well tests, then must predict it everywhere else.

The Poro-Perm Transform

Permeability correlates with porosity, so we calibrate a poro-perm transform on core measurements and apply it to the porosity model. On a crossplot of porosity against log permeability the data fall along a trend, usually fitted as $\log_{10} k = a + b,\phi$. The widget shows such a crossplot. Notice that sand and shale form separate trends: at the same porosity, clean sand is far more permeable.

Connectivity, Not Just Volume

Why do two rocks with the same porosity differ so much in permeability? Because permeability depends on how the pores connect, on the size of the throats between pores, not just on the total pore volume. A rock full of large, well-connected pores flows easily; a rock with the same porosity but tiny, poorly connected pores (a shale) barely flows. That is also why each rock type needs its own poro-perm transform, a theme we return to in rock typing.