ResInsight/ApplicationLibCode/ReservoirDataModel/Eclipse_Grid_Format.md

Eclipse Grid Format: Pillars, COORD and ZCORN

Overview

Eclipse uses a corner point geometry representation for reservoir grids. This format consists of two main arrays:

• COORD: Defines the pillar coordinates
• ZCORN: Defines the depth values at pillar intersections

Grid Structure and Pillars

What are Pillars?

Pillars are vertical lines that define the structural framework of the grid. Each pillar connects corresponding points from the top to the bottom of the reservoir.

Grid Layout (2x3 example):
   
   Y ^
     |
     +----> X

Pillars (shown as |):
     |     |     |
     |     |     |  <- (nx+1) = 3 pillars in X direction
     |     |     |
   j=3 --- --- ---
     |     |     |
     |     |     |  
     |     |     |
   j=2 --- --- ---
     |     |     |
     |     |     |
     |     |     |
   j=1 --- --- ---
     |     |     |
     |     |     |
     |     |     |
   j=0 --- --- ---
    i=0   i=1   i=2

Total pillars: (nx+1) × (ny+1) = 3 × 4 = 12 pillars

COORD Array

Structure

• Size: (nx+1) × (ny+1) × 6 values
• Each pillar has 6 values: [x1, y1, z1, x2, y2, z2]
  • (x1,y1,z1): Top point of pillar
  • (x2,y2,z2): Bottom point of pillar

Organization

For pillar at position (i,j):
  pillarIndex = j × (nx+1) + i
  coordIndex = pillarIndex × 6
  
  COORD[coordIndex + 0] = x_top
  COORD[coordIndex + 1] = y_top  
  COORD[coordIndex + 2] = z_top
  COORD[coordIndex + 3] = x_bottom
  COORD[coordIndex + 4] = y_bottom
  COORD[coordIndex + 5] = z_bottom

Example (2x3 grid)

Pillar (0,0): x1=456064, y1=5.936e6, z1=1709.18, x2=456064, y2=5.936e6, z2=1715.78
Pillar (1,0): x1=456194, y1=5.936e6, z1=1707.43, x2=456194, y2=5.936e6, z2=1714.08
...

ZCORN Array

Structure

• Size: nx × ny × nz × 8 values
• Contains Z-coordinates for each corner of every cell
• 8 corners per cell: 4 top corners + 4 bottom corners

Cell Corner Indexing

Each cell has 8 corners with this indexing convention:

Top Face (NEG_K):        Bottom Face (POS_K):
    3 ---- 2                 7 ---- 6
    |      |                 |      |  
    |      |                 |      |
    0 ---- 1                 4 ---- 5

Corner mapping:
0: (-I,-J,top)    4: (-I,-J,bottom)
1: (+I,-J,top)    5: (+I,-J,bottom)  
2: (+I,+J,top)    6: (+I,+J,bottom)
3: (-I,+J,top)    7: (-I,+J,bottom)

ZCORN Organization

The ZCORN array is organized by layers and interfaces:

For each K layer:
  Top interface (NEG_K face):
    For each J row:
      Face 1: corners (0,3) for all cells in row → [z0, z3, z0, z3, ...]
      Face 2: corners (1,2) for all cells in row → [z1, z2, z1, z2, ...]
      
  Bottom interface (POS_K face):  
    For each J row:
      Face 1: corners (0,1) for all cells in row → [z4, z5, z4, z5, ...]
      Face 2: corners (3,2) for all cells in row → [z7, z6, z7, z6, ...]

Pillar Intersection Sharing

Adjacent cells share pillar intersections, creating the characteristic duplication pattern:

2x3 Grid Cell Layout:
+---+---+ j=2
| 4 | 5 |
+---+---+ j=1  
| 2 | 3 |
+---+---+ j=0
| 0 | 1 |
+---+---+
i=0 i=1 i=2

Cell 0 corners (0,1,3,2) share pillars with:
- Corner 1 shared with Cell 1 corner 0
- Corner 2 shared with Cell 2 corner 0  
- Corner 3 shared with Cell 2 corner 1, Cell 4 corner 0, Cell 1 corner 3

Example ZCORN Pattern (2x3x4 grid)

For layer k=0, the pattern shows shared intersections:

Layer 0 Top Interface:
J=0, Face 1: 1709.18 1707.43 1707.43 1715.72  <- Cell(0,0) and Cell(1,0)
J=0, Face 2: 1706.67 1705.04 1705.04 1709.57  <- shared pillar values
J=1, Face 1: 1706.67 1705.04 1705.04 1709.57  <- duplication from sharing
J=1, Face 2: 1701.68 1697.96 1697.96 1703.33
...

Key Insights

1. Pillar Continuity: Pillars extend vertically through all layers, maintaining structural continuity
2. Shared Intersections: Adjacent cells share pillar intersection points, creating value duplication in ZCORN
3. Interface Organization: ZCORN is organized by layer interfaces (top/bottom), not individual cell corners
4. Face-Based Grouping: Within each interface, values are grouped by faces rather than individual corners

Implementation Notes

The conversion from RigCell to Eclipse format requires:

1. Extracting pillar coordinates for COORD array
2. Organizing cell face corners according to Eclipse ZCORN specification
3. Ensuring proper pillar intersection sharing between adjacent cells
4. Following the specific layer-interface-face organization pattern