# 11.3.1 Fundamental Grid Classes

Topic Version | 1 | Published | 09/11/2015 | |

For Standard | RESQML v2.0.1 |

The three fundamental grid classes have 3, 2, or 1 grid cell indices.

- IJK Grids have 3 cell indices: NI, NJ, and NK.

- I0 index = I-1, I=1…NI.
- J0 index = J-1, J=1…NJ.
- Layer index = K0 = K-1, K=1…NK.
- Column index = I0 + NI*J0.
- Cell index = I0 + NI*J0 + NI*NJ*K0.

- The corner-point grid of RESQML v1 corresponds to a RESQML v2 IJK grid.

- Unstructured Column-layer Grids have 2 cell indices: #Columns and NK.

- Column index = Column# = 0…#Columns-1.
- Layer index = K0 = K-1, K=1…NK.
- Cell index = Column# + #Columns*K0.

- Unstructured column-layer grids are also known as 2.5D grids and PEBI grids, although there is no RESQML requirement to constrain the grid cell shapes to have perpendicular bisector (PEBI) cells.

- Unstructured Grids have a single cell index: #Cells.

- Cell index = Cell# = 0…#Cells-1.

IJK grids and unstructured column-layer grids are both instances of column-layer grids. As such they each have a layer count, NK.

The (I,J,K) indices for an IJK grid, and the K indices for a column-layer grid have a special status compared to all other indices in the RESQML grid description. These indices may reference information external to a RESQML model, i.e., within a reservoir simulation data deck, which imposes a number of constraints. First, the ordering of the indices of a grid is not arbitrary, but is instead fundamental to the grid. So, if an application flips the order of an index, e.g., to change the parity of a grid, then the resulting grid is a new grid with a new GUID. RESQML may use the Representation Identity to indicate that these two grids are co-located in space, but they should be thought of as two different grids. Second, these indices are constrained by reservoir modeling domain usage to be 1-based. In contrast, all RESQML indices are 0-based. To prevent any confusion, this document and the schema reference the corresponding 0-based indices: I0=I-1, J0=J-1, and K0=K-1.

Notation:

- # prefix and “count” are used inter-changeably in this document, i.e., #Objects = object count.

- # suffix and “index” are used inter-changeably in this document, i.e., Object# = object index.

With the exceptions of NI, NJ, and NK, just noted, elements which provide counts are consistently named “Count” within the RESQML schema. With the exception of the general purpose (GP) grid representation discussed below, the RESQML v2 schema constrains all grid element counts to be positive integers, i.e., degenerate grids are not allowed.