In field-scale reservoir simulation the demands to the grid-in-use may change significantly during a simulation run. Introduction of new wells (vertical and horizontal ones) at a later stage of the simulation run, phase breakthrough or coning may require a different grid than initially set up.

While in the past the reservoir simulation grid was considered static, this paper will describe a novel approach of a dynamically used grid. It is only applied locally and just for a specific time period. The geometry within such a local subdomain (window) may be regular or irregular and the grid construction is based upon the principle of perpendicular bisectors (PEBI or VORONOI grid).

Windows may be incorporated into the standard Cartesian simulation grid at any time and at any location. No non-neighborhood connections are used. The windows are linked to the system of equations based on their natural sequence and they are "switched on and off" as required. A multiple timestepping technique is applied that allows the use of different timestep lengths inside and outside the windows.

Examples will demonstrate the potential of this new concept. A local radial grid may be used for well test simulation, or arbitrary direction horizontal wells may be incorporated after history matching. The whole simulation is conducted using the familiar Cartesian grid with the irregular grids included only during a short period.

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