This paper describes the design of FLEX, an object-oriented, flexible grid, black-oil reservoir simulator. An object-oriented decomposition of a reservoir simulator helps in dealing with the complexity of this problem. This approach is particularly useful because of the difficulties associated with generation and use of flexible grid geometries (like Voronoi, median, boundary adapting grids, etc.).

The entire problem is divided into subsystems like geometry, gridnodes, gridnode connectivity, grid, reservoir fluid flow, and matrix. Each of these subsystems have objects which are closely related. The dependency of these subsystems is established. A detailed analysis of each subsystem leads to identifying the classes, which are a set of objects having similar behavior. Attributes and behavior of the classes are assigned. After establishing relationships between the classes, they are arranged into hierarchies. About one hundred major classes have been identified and designed to achieve the desired behavior from FLEX. The programming language used is C++.

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