This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 95759, "Optimal Coarsening of 3D Reservoir Models for Flow Simulation," by M.J. King, SPE, K.S. Burn, P. Wang, V. Muralidharan, SPE, and F. Alvarado, BP America Inc., and X. Ma, SPE, and A. Datta-Gupta, SPE, Texas A&M U., prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 912 October.
A new constrained optimization approach to the coarsening of 3D reservoir models for flow simulation has been developed. The optimization preserves a statistical measure of the heterogeneity of a fine-scale model. Constraints arise from the reservoir fluids, well locations, pay/nonpay juxtaposition, and large-scale reservoir structure and stratigraphy. The approach has been validated for several oil and gas projects in that flow simulation through the coarsened model was shown to provide an excellent approximation to high-resolution calculations performed in the original model.
The development of coarsened reservoir-simulation models from high-resolution geologic models remains an active field of research. This study had success with the use of coarsening algorithms to determine a reservoir-simulation grid obtained by grouping fine-scale geologic-model cells into effective simulation cells. Results differ from previous studies in that a statistical analysis of the static properties of the model appeared to identify the best grid for dynamic reservoir simulation. Coarsening beyond the degree indicated by the analysis discarded too much of the underlying heterogeneity. It would overly homogenize the properties on the simulation grid. Finer models would retain at least as much reservoir heterogeneity, but they are more costly.
This analysis uses a statistical technique for layer grouping and a constrained approach for areal gridding. The resulting composite corner-point grid (CCPG) has many of the advantages of unstructured perpendicular-bisection grids in that it can follow major features of the geologic description. Compared with perpendicular-bisection grids, CCPGs have exact alignment of the simulation cells with the geologic model, which will minimize property-scaleup errors, and they may be used without the development of new simulation pre- and post-processing applications. This approach also moves closer to having an Earth model shared between the reservoir engineer and the reservoir geologist: The 3D geologic model will provide the grid on which the high-resolution initialization of the simulation model will be calculated.
The full-length paper details the new statistical approach applied to layer coarsening followed by unstructured vertical coarsening and volumetric coarsening.
Static methods are fast. Unfortunately, they do not predict the appropriate number of layers for a dynamic model. The dynamic model must be tested by simulation and the appropriate number of layers determined by trial and error.