Discretization and the Grid Block

One Value per Cell

A simulator cannot carry the continuous pressure field; it stores a single value per grid cell and treats the cell as uniform. The smooth solution is replaced by a staircase of cell values.

The Discretization Error

The gap between the staircase and the true curve is the discretization error. Refining the grid shrinks the cells and the error, but the cell count and the run time climb with it.

Where to Refine

Because the cost rises fast in three dimensions, the grid is refined only where it pays: near wells and across steep gradients, where coarse cells would smear the physics that matters.