A New Global Upscaling Technique for 3D Unstructured Grids

New procedures for unstructured coarse-model generation are presented and applied. The underlying fine-grid model is considered to be unstructured, and the coarse-model cells are defined as groupings of fine-grid cells. The key flow quantity that must be computed for the coarse model is the upscaled transmissibility for each cell-to-cell connection. We introduce a global upscaling procedure for this computation. The method first requires several (minimum of three) global single-phase flow solutions. Appropriately defined linear combinations of these solutions are used to compute each upscaled transmissibility. This approach circumvents some of the limitations of existing (local and global) upscaling procedures. It also enables transmissibility to be quickly computed for a number of different coarse grids without performing any additional pressure solutions. Results are presented for an idealized two-phase flow problem. The fine grid contains nearly 200,000 cells, and coarse models of varying resolution are considered. Accurate results for total injector-producer flow rate are observed for all grid-resolution levels for the three different well configurations considered. Oil rate as a function of time is shown to improve in accuracy with increasing resolution, and is quite accurate for a model of about 10,000 cells.