Reservoir Simulation Grids: Opportunities and Problems

Summary Grid selection is one of the most difficult and time-consuming tasks in the simulation of geologically complex reservoirs. A grid is imposed on a reservoir so that we can solve the nonlinear flow equations that predict the reservoir's response to changes at wells or other boundaries. Therefore, I first discuss the role of grid geometry in the evaluation of each term in the flow equations. Then I describe several conventional and some new gridding techniques, as well as their advantages and disadvantages. Next I make recommendations based on my own experience and discussions with colleagues. My assessment is that grids should be as close to orthogonal as possible. Also, local grid refinement should be used with care; its inappropriate use can be counterproductive. Considerable progress has been made recently on the use of flexible grids, but efficient use of unstructured, locally orthogonal grids requires further research. Techniques that automatically align the grid with major reservoir features (e.g., faults) are being developed and are expected to simplify grid generation. Introduction A reservoir simulator predicts reservoir performance by solving flow equations on a discrete grid chosen by the simulation engineer to represent the reservoir. The grid normally is selected with one or more of the following considerations in mind.Reservoir geology and size and the data available for reservoir description.Type of fluid displacement or depletion process to be modeled.Past and anticipated field development (location and type of wells).Numerical accuracy desired.Available software options.Objectives of the simulation study.Competence of the simulation engineer or team.Available computer resources, time constraints, or project budget. In the early days of reservoir simulation, the last consideration often determined the number of gridblocks, and the available software limited the choice of grid type, usually to block-centered Cartesian or cylindrical grid. Research in reservoir simulation and hardware developments, especially over the past 10 years, has greatly extended the gridding options available.