The Use of Flexible Gridding for Improved Reservoir Modeling

Abstract

ABSTRACT This paper describes some model improvements developed to allow new possibilities in reservoirs representation with simulators using a finite difference scheme. It includes: possibility of flow between any grid block and from one to many blocks;a multiple time step calculation which allows to solve flow equations with several time step lengths for different group of grid blocks. These facilities provide a flexible and performant way of handling irregular and complex grid patterns with different nested levels of refinements. They are particularly useful to accurately simulate: –unconfined pilots or areas with smaller wells spacing;–faulted reservoirs;–vanishing or discordant geological layers;–reservoirs connected to a general aquifer. The new approach has been tested on a black oil simulator, implicit in pressure, implicit in saturation. The solutions used for some important aspects of model design are outlined. These are the Input/Output data organisation, the inner model grid pattern representation, the matrix solver, the multiple time steps selector and the associated equation solver. Two examples illustrate the use of the modified black oil simulator, and results are presented. The first example concerns the simulation of a repeated five spot injection. This test is made to check the local mesh refinements efficiency. The second example is a full field reservoir study for which using the modified model is suitable. The performance of a produced oil reservoir connected to an aquifer is simulated. In both cases, results obtained with a composite grid and the modified model are compared to those obtained with a coarse grid and then a fine grid used on a conventional model. The following points are outlined: –the use of local grid refinements give results very closed to those obtained with the fine gridded conventional model;–on the computers used, the running time of the modified model simulations is much lower than the time needed for the conventional model. Moreover, it is shown that the modified model results remain unchanged when the multiple time step selector is used. A different time step length is then automatically calculated for the oil reservoir and the aquifer, thus allowing a noticeable extra gain of processing running time.