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Define Well Boundary Condition

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At this step, you can define well boundary conditions.

 

 

Quick Overview

 

Instructions:

Select a wells data object to be used as wells in the conceptual model

Pre-requisites:

Boundary Conditions are created

 

You have imported a wells data object, with screens and pumping schedule information.

 

Result:

A Conceptual Wells Boundary Condition object is created

Next Steps:

Create Finite Difference Grid;

Create Unstructured Grid; or

Create Finite Element Mesh

 

 

 

Before You Start!

Make sure you have imported a Wells Data object that contains well location, screens, and pumping schedule. Refer to Import Wells for more details

 

To add a new pumping well boundary condition, follow the steps below:

 

Click on the Define Pumping Wells button; the following window will appear

 

 

Select a pumping wells data object from the Data Explorer

Click the button to insert the data object into the Select Wells Data Object field. The wells should then show in the table.

 

Each pumping well must satisfy the following requirements in order to be deemed valid:

 

The pumping well must located within the simulation domain.

A screen must be defined for the pumping well

A pumping schedule must be defined for the pumping.

Wells that do not meet these requirements will not be included. For information on defining well data, i.e., screens, pumping schedules, please see "Well Table" section

 

Visual MODFLOW Flex will then add the boundary condition under the Boundary Condition node in the Model Explorer tree.

 

 

Changes to Wells in v.2015.1

VMOD Flex v.2015.1 utilizes a new conceptual approach to pumping wells; if you wish to utilize this format, you will need to convert your wells to this format.  For more details on this process, and the limitations, please see Converting "Old" Wells to "New" Wells Format

 

 

Calculation of Well Rates

 

 

Visual MODFLOW Flex supports wells with multiple well screens throughout the depth of the well-bore, and well screens that partially penetrate a model layer. However, it is important to note that the MODFLOW WEL package does not have any special considerations for multiple well screens or grid cells containing partially penetrating well screens. MODFLOW treats wells as flux boundary conditions, such that each grid cell intersecting a well screen is assigned a specified flux. In a situation where a well is screened across several model layers, Visual MODFLOW Flex uses the length of the well screen intersecting each model layer to determine the proportion of the total well pumping rate assigned to each well grid cell in the model, even though MODFLOW considers a cell to be screened over its entire vertical length, regardless of the length of screen assigned to the cell. The following equation is used to calculate the pumping rate for each grid cell

 

where:

Qi is the discharge from layer i to a particular well in a given stress period,

QT is the well discharge in that stress period,

Li is the screen length in layer i,

Kx is the hydraulic conductivity in the x-direction in layer i, and

Σ(LKx)i represents the sum of the products of screen length and hydraulic conductivities in the x-direction of all layers penetrated by the well.

 

This approach, in which a multi-layer well is represented as a group of single layer wells, fails to take into account the inter-connection between various layers provided by the well. One of the most significant problems related to this approach is that well grid cells are essentially shut off when the water table drops below the bottom of the grid cell (i.e. when the grid cell becomes dry). This automatically reduces the total pumping rate of the well, and may cause the water table to “rebound” and re-activate the well grid cell. This type of on-again-off-again behavior for the pumping well(s) causes the solution to oscillate, and may prevent the model from converging to a solution. In the event the model does converge to a solution, the model results may be misleading if one or more pumping wells have lower than expected total pumping rates.

 

Alternately, a multi-layer well can sometimes be simulated by means of a vertical column of high permeability cells with a screen at the bottom of the column. In this case, if the top part of the model becomes dry, the total pumping rate is unaffected. This also takes into account the vertical inter-connection between layers. The downside of this approach is that the conductivity contrasts can lead to convergence problems.

 

Both the MODFLOW MNW package and MODFLOW-SURFACT provide better representation of wells screened over multiple layers. MODFLOW-SURFACT is able to redistribute pumping rates to the remaining active grid cells if one or more cells in the screened interval goes dry, thereby more accurately simulating the real-world effects of partial overpumping of a well screened over multiple layers.

 

Multiple Wells in the Same Cell

 

Visual MODFLOW Flex supports multiple pumping wells in the same cell, a common scenario when grid cell sizes are large and wells are close to one another. When the .WEL package is created, and there are multiple wells in the same cell, you will see multiple entries for that cell; the well name will appear as a last parameter, and this will allow you to confirm that the correct wells have made it into the correct cells; an example is below:

 

in Layer 2, Row 40, Col 40, if this cell has 2 wells in model (named PW1 and PW2), in the WEL package this will appear as:

 

2  40  50  -100  #PW1

2  40  50  -250  #PW2

 


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