11.6 Faulted Grids

Discretized representations of faults are a part of the minimum grid export specification. Two fault representations are recommended; for faults with pillars, you must use BOTH approaches:

• A grid connection set representation (recommendation, use only one grid connection set for each fault) is a collection of cell-face-pairs that details local fault topology required for flow simulation or for fault property characterization (transmissibility, throw, fault smear, etc.). Because of the detail it provides, this is the preferred method. Historically this information has often been lost on grid export, and has led to some of the largest discrepancies in reservoir performance prediction between different reservoir simulation applications.
• A subrepresentation of the grid with an ordered list of “pillars” as the indexable element. To define a single path for the fault representation, order the pillars from one (lateral) extremity of the fault to the other. This approach provides a qualitative description of the location of pillar-based faults, and will be natural to many geologic modeling applications. However, it cannot be used for XYZ stair-step faults, and it lacks the information on the local discretized topology of the fault, which is present in the grid connection set representation.

Use of cell faces is NOT a recommended practice. Just as with the use of a pillar subrepresentation, this approach is lacking the detailed local fault topology. However, if required to transfer historical data sets that lack the information required for a grid connection set representation, this practice can be reproduced using a subrepresentation based on the “faces per cell” for a column-layer grid or the “faces” of an unstructured grid.