What the grid does
A reservoir simulator solves conservation of mass — and sometimes energy — over a set of control volumes, which are the grid cells. Between any two neighbors it computes a transmissibility, a single number combining geometry and permeability that governs how fast fluid moves from one cell to the other. The grid is not just bookkeeping: its shape sets the flow field the simulator can resolve.
Structured vs unstructured
Two broad families dominate:
| Structured (corner-point) | Unstructured (PEBI/Voronoi) | |
|---|---|---|
| Topology | logical i, j, k index | general cell adjacency |
| Geometry | distorted hexahedra on pillars | Voronoi polyhedra |
| Faults | stair-stepped, NNCs | faces land on the fault |
| Strength | simple, fast, well supported | conforms to complex structure |
Corner-point grids are the incumbent and excel on gently structured fields. PEBI grids win when faults, pinch-outs and wells demand geometry a Cartesian index cannot follow.
Upscaling
Geologic models are built fine — often millions of geocellular cells — then upscaled to a coarser simulation grid so runs finish in reasonable time. Upscaling averages porosity and permeability and computes effective transmissibilities. How well it preserves flow behavior depends on how faithfully the coarse grid still honors faults, layering and high-permeability streaks.
Grid quality and flow accuracy
Grid choices show up directly in results:
- Grid-orientation effect: structured grids can bias displacement fronts along their axes.
- Numerical dispersion: coarse or poorly aligned cells smear sharp fronts and plumes.
- Orthogonality: non-orthogonal cells undermine two-point flux accuracy.
How AutoMesh-Geo helps
AutoMesh-Geo produces conforming Voronoi (PEBI) simulation grids straight from a structural model, so the geometry that drives flow accuracy — faults, horizons, near-well refinement — is honored by construction. See how it fits oil & gas and subsurface work.