A new technique for evaluating coarse grids based on flow thresholding

Current upgridding techniques usually require the scale-up factor to be known a priori and, in practice, the scale-up factor is selected rather subjectively. The main consideration is that the scale-up factor should be large enough for practical use and not so large that the results are likely to be very inaccurate. A more objective approach to selecting the scale-up factor is desirable. This paper proposes a novel approach to determine a more appropriate scale-up factor for a coarse model. The new approach is based on an analysis of the flows in the fine-scale model. The regions that make a significant contribution to the flow are identified using a thresholding technique. A range of scale-up factors is tested and high flow regions are identified in each case. The percentage overlap (denoted by R-o) of the high flow regions in the coarse and fine grids is then calculated. The value of R-o can be used as a predictor of the accuracy of upscaling and the appropriate coarse grid size can thus be evaluated. When the upscaling is performed using a global method, there is little extra computational cost. This method is suitable for use in multi-phase flow simulations of 3D heterogeneous reservoir models. The technique is demonstrated using a simulation of a waterflood in a highly heterogeneous 2D model.