Transmissibility

The Flow Between Two Cells

The flux between neighboring cells is the transmissibility of their shared face times the pressure difference, $Q = T,\Delta P$. The transmissibility is the face area over the cell spacing, times an averaged permeability.

Why the Harmonic Mean

The two cells sit in series along the flow path, so the face permeability is the harmonic mean $k_f = \dfrac{2,k_1 k_2}{k_1 + k_2}$, which is pulled toward the smaller value rather than the arithmetic mean.

The Low-Permeability Throttle

One tight cell therefore throttles the flux through the pair, the way the narrowest section of a pipe sets the flow. The simulator stores one transmissibility per face and uses it to move fluid across the grid.