The control lines are defined in the model file (*.net) using the KONT data type. The KONT data type is independent of the model type (2D or 3D).

The lines can extend from a 2D area into a 3D area; however, they must not run along the 2D/3D boundary.

In order to obtain rates over an area boundary section, it is better to add up the inflow rates (for potential boundary conditions) of the corresponding nodes (attribute BILK) instead of control lines, as these are more accurate.

The control lines must always run along the sides of the element so that the velocities and flow areas of two elements are always available for the calculation. The areas are averaged, as the differences are generally not that great. The user can choose whether the velocities are averaged or whether the value of the element on the left or right should be used. The accuracy of the volume calculation depends on the FE discretisation and the discontinuity of the elements.

The following points should also be noted when using the KONT attribute:

• The calculation of the control lines is activated in the main menu of the respective flow module as soon as the KONT attribute is present in the model file.

• The results for the individual nodes and the total volume of the control line are written to the output file of the flow calculation. The value of the control line in the model file corresponds to the number of the line in the output file.

• If a closed control line is created in SPRING, e.g. around a well, the last (= first node) node must be inserted manually in the model file so that the line is closed.

• Control lines can also be assigned via the element diagonals. However, during a later mesh refinement, an "unclean" line routing occurs at these points and a warning is issued in the flow calculation. Manual or renewed assignment of the KONT attribute is required here.

• With the SITRA module, the calculation is only possible via the element edges

• The sign of the control line rates at the individual sections depends on the assignment direction: A flow across the control line from RIGHT to LEFT in the assignment direction is defined as positive, a flow across the control line from LEFT to RIGHT in the assignment direction is defined as negative:

• In three dimensions, the rates are calculated over the depth, starting in the layer to which the KONT attribute was assigned, up to the model base.

The following chapters describe specific examples of the use of control lines.

Use of control lines to determine the inflow to an extraction well (KNOT)