Accurate reservoir simulation using flexible grid.

In reservoir simulation, finite difference method has long been used, which requires small grid size around a well to account for rapid pressure change. With finite difference grid for Cartesian coordinate, reducing grid size around a well results in large aspect ratio at cells far from the well and in decreasing accuracy. To deal with this problem, local grid refinement method has been developed and used.Another difficulty in Cartesian coordinate is the grid orientation, which greatly affects flow of fluids because pressure gradient can be calculated only in the direction of the grid. In the case of horizontal well simulation, gridding is dominated by the direction of the horizontal well, which makes existing finite difference approach nearly impossible to simulate.One solution to these problems is to use flexible grid. We have developed “VERDI/2D”, which is a prototype of two-dimensional, three-phase reservoir simulator using flexible grid. In VERDI/2D, control-volume finite element (CVFE) method is used to discretize flow equation. Voronoi grid method is used to generate grid by overlaying sub-grids (cylindrical or Cartesian) on background Cartesian grid at any location and any angle.The validity of VERDI/2D was proved by comparing calculation results with those of widely used commercial simulator “ECLIPSE” of Intera Information Technologies Ltd.