Pinchouts and Zero-Thickness Cells

Layers That Run Out

Geological layers do not extend across the whole field; a sand commonly thins updip until it pinches out to zero thickness against the unit above. A grid has to represent this, because the cells in the layer must somehow handle the place where the layer no longer exists. The widget shows a sand wedge thinning to a pinchout; move the pinchout and toggle how the grid copes.

Collapse, Do Not Sliver

The corner-point answer is elegant: where the layer thins to nothing, the cell's top and base corners meet, so the cell simply collapses to zero thickness and is marked inactive. It holds no volume, takes no part in the flow, and causes no trouble. The naive alternative, forcing every cell to keep some minimum thickness, leaves a fringe of tiny sliver cells where the layer should be gone.

Why Slivers Are Dangerous

A sliver cell has a minuscule pore volume, and a simulator advancing in stable time-steps must take a step small enough for the smallest cell. One sliver can therefore throttle the entire run, or destabilize it. This is why honoring pinchouts with zero-thickness cells, rather than papering over them, is part of building a grid that will actually run. With the geometry under control, the next concern is how the grid interacts with the flow itself.