Abstract
Wells are seldom modeled explicitly in large scale finite difference reservoir simulations.
Instead, the well is coupled to the reservoir through the use of a well index, which relates
wellbore flow rate and pressure to grid block quantities. The use of an accurate well index
is essential for the detailed modeling of nonconventional wells; i.e., wells with an arbitrary
trajectory or multiple branches. The determination of a well index for such problems is com-
plicated, particularly when the simulation grid is irregular or unstructured. In this work, a
general framework for the calculation of accurate well indices for general nonconventional
wells on arbitrary grids is presented and applied. The method entails the use of an accurate
semianalytical well model based on Green’s functions as a reference single phase flow so-
lution. This result is coupled with a finite difference calculation to provide an accurate well
index for each grid block containing a well segment. The method is demonstrated on a number
of homogeneous example cases involving deviated, horizontal and multilateral wells oriented
skew to the grid. Both Cartesian and globally unstructured multiblock grids are considered.
In all these cases, the method is shown to provide results that are considerably more accurate
compared to results using standard procedures. The method is also applied to heterogeneous
problems involving horizontal wells, where it is shown to be capable of approximating the
effects of subgrid heterogeneity in coarse finite difference models.
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