Unstructured Upggriding and Transmissibility Upscaling for Preferential Flow Paths in 3D Fractured Reservoirs
Authors
S. Vitel and L. Souche
Journal/Article/Conference
SPE Reservoir Simulation Symposium, Houston, TX, USA
Abstract
Most existing upscaling methods attempt to evaluate effective
permeabilities of coarse-scale gridblocks, so that the upscaled
model locally reproduces the behavior of the fine-scale grid,
under a set of boundary conditions. When applied to fractured
reservoirs, this approach shows several drawbacks. First, it
assumes the existence of a representative elementary volume
(REV), which size is constrained by the practical needs for an
efficient simulation. Yet, no REV exists for fractured systems
which are characterized by a wide variety of fracture sizes.
Second, the dynamic behavior of the model is unknown far
from the applied boundary conditions. Third, this approach
tends to underestimate the impact of steep pressure gradients
that may occur between fracture and matrix media.
The presented method overcomes all three limitations by
upscaling transmissibilities, so that the coarse-scale model
preserves the same pressure response as the detailed
geological model at a set of arbitrarily chosen observation
points. A discrete fracture network and a corner-point grid are
first jointly discretized using a dual approach (pipe network).
Nodes of the pipe network represent either discrete fractures
or matrix blocks. Pipes stand for matrix-to-matrix, fracture-tofracture
and matrix-to-fracture connections. Then, upgridding
and upscaling are simultaneously performed, without
imposing any boundary conditions: nodes are iteratively
removed by applying electric simplifications (series, parallel,
star-mesh transformations) until only the selected observation
points remain. This process introduces new connections that
may link nodes that were initially not connected, thus better
modeling features such as super-K or large-scale fractures.
This tends to convert a large sparse system into a smaller but
fuller one; therefore parts of the network need to be lopped off
before informing a flow simulator. Pipes holding the lowest
transmissibilities are decimated and the remaining
transmissibilities are updated accordingly in an optimization
procedure. Flow simulation results obtained for several data
sets on upscaled models are in good accordance with those
obtained before upscaling, and show appreciable
improvements compared to conventional structured local
approaches.
BibTeX Reference
@INPROCEEDINGS{Vitel_SPE2007,
| AUTHOR
|
=
{S. Vitel and L. Souche}
,
|
| TITLE
|
=
{Unstructured Upggriding and Transmissibility Upscaling for Preferential Flow Paths in 3D Fractured Reservoirs},
|
| BOOKTITLE
|
=
{SPE Reservoir Simulation Symposium, Houston, TX, USA},
|
| YEAR
|
=
{2007},
|
| MONTH
|
=
{26-28 February},
|
}
