Interactive Generation of Irregular Simulation Grids and Its Practical Applications
Authors: Heinemann Z. E. 
Published: December 2007

In the last five years many new concepts and tools have been developed to increase the flexibility of reservoir simulation. The most important ones are: a) PEBI (or Voronoi) and Median grids with the use of the Control Volume Method (Integral formulation of the fluid flow equations); b) Modeling of non-vertical sealing and non-sealing faults; c) Windowing technique, d) Dual timestepping; e) Vertical and horizontal well models.

All of them were presented and validated in numerous papers. Now it is necessary to make those tools practically applicable and available for everyday work. This paper presents a concept and an integrated solution for the generation of these types of flexible simulation grids.

The basic idea of grid generation is to keep the underlying Cartesian grid wherever appropriate and to use the irregular grid only locally and limited in time. The fundamental Cartesian grid is regular in the productive area and the grid spacing is gradually increased in the aquifer. The aquifer has a different layering than the productive area and can be two dimensional in the simplest case. The faults are modeled by block boundaries which follow exactly the geological traces. These traces may be different for the individual layers (slanted faults). The fault is in this case a three-dimensional surface. The flow through the fault is calculated by automatic connection of the juxtaposed layers and a new technique is applied to handle the flow term. Vertical faults are special cases of the general solution.

For any part of the reservoir grid, subdomains, so called windows, can be defined with different grid patterns inside. Windows can be switched in and out at any stage during the simulation run. The windows will be calculated with smaller time steps and in parallel (on different CPU-s). Windows can be used for updating the geology or for using different realizations obtained from stochastic modeling, for introducing local radial grids, for constructing special grids around horizontal (inclined) wells, etc.

The grid is constructed interactively and supported by a graphical user interface in an X-windows environment. Based on the underlying geological information (maps, crosssections) the user defines the area of interest, the overall extension (size of the aquifer), the number of simulation layers and their relations to the facies. Within the productive area local grid refinement leads to greater horizontal and vertical resolution. As a next step, faults will be imported and modeled with an adaptive grid. Then the windows and well models will be introduced. This step can be done at any stage of the simulation work. The aquifer can be resized independently from the productive area. The parameterization of the grid and of the windows is automatic. All procedures are guided by error checks and the user is supported by an extended help library. The system is developed for large-scale applications. A simulation model consisting of several hundred thousand blocks, more than one reservoir in a model, hundreds of faults and about thousand wells is currently being set up.

The concept will be demonstrated in a full-scale example. The obvious advantages of flexible gridding, fault modeling, windowing technique and dual timestepping allow a simulation model to be employed for continuous reservoir management and not only for reservoir studies.