# Special Region Definition Methods¶

Different methods are available in PFLOTRAN to define a space portion within the reservoir (volume, surface or point), depending on the type of grids.

For structured grids the following methods are available:

For unstructured_explcit grids only the following method is available

These methods are described one-by-one in the paragraphs below.

## Surface REGION by lower and upper coordinate bounds¶

Surface regions can be defined once the upper and lower coordinates bounds are known. This method is only available for structured grids. Reservoir boundaries for structured grids, can be defined with the following instructions:

REGION <region_name>: specifies the region name
COORDINATES
<x_min, y_min, z_min>: specify the minimum values for the x, y and z coordinates
<x_max, y_max, z_max>: specify the maximum values for the x, y and z coordinates
END
FACE <face_location>: [WEST, EAST, SOUTH, NORTH, BOTTOM, TOP]
END


face_location indicate either be the west, east, south, north, bottom or top facets in the grid blocks intersected by the surface. The user must be sure that the surface defined by coordinates, and the face location identify an actual domain boundary, and not an internal surface. For example, when defining a boundary located at the west of the reservoir, the surface must have a constant x value, corresponding to the lower I logical coordinate bound, and use face_location=WEST.

The example below shows the definition of a west boundary (or a portion of it) of a reservoir.

REGION west
COORDINATES
0.0 0.0 0.0
0.0 2000.0 120.0
/
FACE WEST
/


## Volume REGION by IJK coordinates¶

This method can be used when the IJK coordinates (I,J,K) of the reservoir portion to use to create a region are known. The method, which relies on IJK coordinates can be applied to structured grids only, using the BLOCK keyword as follows:

REGION <region_name>: specifies the region name
BLOCK <I_min, I_max, J_min, J_max, K_min, K_max>
END


I_min, I_max, J_min, J_max, K_min, K_max define the lower and upper bounds of the IJK coordinates that define the region.

See example below:

REGION inj1
BLOCK 1 28 10 15 1 6
END


Note that this BLOCK definition of a region cannot be used with GRDECL files (which are unstructured in PFLOTRAN terminology).

## Boundary REGION by specifying face location¶

When using the structured grid type, and an entire side boundary is requested, the present method can be adopted using the CARTESIAN_BOUNDARY keyword, as follows:

REGION <region_name>: specifies the region name
CARTESIAN_BOUNDARY <face_location>: [WEST, EAST, SOUTH, NORTH, BOTTOM, TOP]
END


face_location can either be the west, east, south, north, bottom or top boundary of the reservoir.

See example below:

REGION south_boundary
CARTESIAN_BOUNDARY SOUTH
END


Note that this BLOCK definition of a region cannot be used with GRDECL files (which are unstructured in PFLOTRAN terminology).

## REGIONs by external files¶

In the case of unstructured grids the regions can be conveniently defined using external files generated by other software packages. This applies to unstructured_explicit grids, to both volume and surface regions.

REGION <region_name>: specifies the region name
FILE <region_file_name>: specifies the name of the external file to load
END


The external file where the region is defined can be a list of cell ids or faces depending if the region is a volume, a surface or an array of points.

See example below:

REGION inj2
FILE inj2.txt
END


External file region may be used to generate two different type of regions:

• Volume regions, i.e. group of grid blocks, which defines a portion of the reservoir where to apply source/sink terms or an observation, see Volume REGION defined by external files.

• Surface regions, i.e. a group of cell facets, which define a reservoir boundary where to apply a boundary condition, see Surface REGION defined by external files.

### Volume REGION defined by external files¶

The file that defines a volume region is a list of natural ids, each identifying the id of the grid block as reported in the explicit unstructured grid.

The file extension is .txt, and it does not have any header.

An example is:

REGION inj2
FILE volume_region.txt
END


An example for the file volume_region.txt above is:

volume_region.txt:
34
36
45
56


### Surface REGION defined by external files¶

The file that defines a surface region is a list of faces, each specifying the same id of the boundary cell the face is attached to, the face centroids coordantes, and the face area. The file has an header that specify the number of connections that describe the surface. It may be use to define a reservoir boundary where to apply a boundary condition.

CONNECTIONS <number_of_connections>
<cell_ids_fc_1> <fc_1_x> <fc_1_y> <fc_1_z> <fc_1_area>
.......................
<cell_ids_fc_i> <fc_i_x> <fc_i_y> <fc_i_z> <fc_i_area>
.....................
<cell_ids_fc_n> <fc_n_x> <fc_n_y> <fc_n_z> <fc_n_area>


This type of file are usually generated using software able to select boundary cells. Below an example that show its use.

Region definition:

REGION east_boundary_region
FILE east_boundary_region_file.txt
END


Contents of east_boundary_region_file.txt:

CONNECTIONS 18117
27826 202.1343344 12099.998 -1690.488061 477.999283
37213 201.5764552 12099.998 -1691.66311 477.999283
46616 202.1343344 12099.998 -1692.878647 477.999283


Example of use in a BOUNDARY_CONDITION:

BOUNDARY_CONDITION east_boundary_condition
FLOW_CONDITION vertical_equilibration
REGION east_boundary_region
END