Multilayer Well (MLW)

Units [L³/T]

A Multilayer well boundary condition is used to simulate abstraction or injection of water via a vertical well screen (or inclined) that possibly extends over a number of model layers or element groups. It is now also supported to simulate horizontal well screen.

While using physical properties of the well for input and data storage (e.g., screen top and bottom, well radius), internally during the simulation such Multilayered wells are represented combining an extraction or injection of water on the lowermost node (like a well boundary condition) and a linear discrete feature element along the screen interval with properties according to the well geometry.

The properties of a Multilayer well are defined in the Multilayer Well Editor dialog. The dialog is invoked by activating the boundary condition in the Data panel and double-clicking on Multilayer Well in the Editor toolbar. For or more details, please refer to Multilayer Well Workflow.

 

The meaning of this internal handling can be figured by using its physical counterpart: The well tubing along the screen is highly conductive, and the pump itself is located on the bottom of the well. Assuming a reasonably large well radius (and large hydraulic conductivity), water will enter the well pipe along the screened interval and will flow down to the pump through the pipe. There is no resistance between the aquifer and the well tubing, i.e., the screen is perfectly permeable

 

For the calculation of flow along the well pipe the Hagen-Poiseuille (cubic) law is applied, and appropriate parameters are derived from the radius set for the Multilayer well.

 

Multilayer wells in legacy models created with FEFLOW versions below 6.1 are converted automatically when such older models are opened. To ensure that results correspond to the results of previous versions for flow models, the well radius is set to 1.19389 m at conversion. For mass and heat transport models slightly different results can be expected at the Multilayer well as from version 6.1 on mass and heat transport within the well pipe are considered in a more physical way using a discrete feature element.

 

The subdivision of the total pumping or injection rate to the different layers/slices is a simulation result and depends on the surrounding material properties and the surrounding distribution of hydraulic head in the different slices. The Rate Budget panel does not show this subdivision, as for the simulation the abstraction is done from the bottom node of the well. The Flow Per Layer, however, can be shown as an attribute of the Multilayer Well in the View Components panel for a Slice view.

For Multilayer wells with low or no abstraction or injection a flow between layers with different hydraulic-head level can result water entering the well pipe in one layer and leaving the well in another one. Despite the fact that this process is realistic in its nature, it is crucial to apply the true well radius to obtain reliable results.

 

Due to spatial discretization in numerical modelling, the hydraulic head resulting from the simulation at the well nodes themselves highly depends on the size of the elements around the well location. Correct heads will only be obtained if the distance of the well node to the neighboring nodes is small enough. In regional models with typically coarser discretization around wells the modeller should therefore refrain from using hydraulic heads in wells for calibration. The Ideal Element Size can be shown as an attribute of the Multilayer Well in the View Components panel for a Slice view.