Porosity

Part 2, Chapter 2: Petrophysics for Modeling

Learning objectives

Define porosity and distinguish total from effective porosity
Describe how core and the density and neutron logs measure porosity
Relate porosity to bulk density through mass balance
Explain why effective porosity is what matters for flow

The Room in the Rock

Porosity, written $\phi$ , is the fraction of the rock that is pore space rather than solid grain. It is the room available to hold oil, gas, and water, so together with the rock volume it sets how much fluid a reservoir can contain. A clean sandstone might be 25 to 30 percent pore space; a tight rock only a few percent. The widget draws a grain pack at the porosity you choose: raise it and the grains pull apart, opening more pore volume.

Total and Effective Porosity

Not all pore space is useful. Total porosity counts every void, including pores sealed off from their neighbors and clay-bound water that never moves. Effective porosity counts only the connected pores that can actually store and give up fluid, and it is the number that feeds volumetrics and flow. The difference matters most in shaly rocks, where clay traps water the reservoir can never produce.

Measuring Porosity

A core measures porosity directly in the lab, but cores are sparse. The workhorse is the density log, which reads bulk density $\rho_b$ and inverts a simple mass balance, $\phi = \frac{\rho_{ma} - \rho_b}{\rho_{ma} - \rho_f}$ , where $\rho_{ma}$ is the matrix (grain) density and $\rho_f$ the fluid density. The neutron log gives an independent porosity, and the two together flag gas (which pushes them apart) and help correct for shale. These log porosities, tied to core, are the porosity data the 3D model must honor.