A Natural Modeling Approach to Pumping Wells in Visual MODFLOW Flex

April 24, 2015

The Waterloo Hydrogeologic software development team is constantly making improvements based on the feedback and suggestions from our worldwide users. Recently, our focus has been on redesigning the pumping wells boundary condition in Visual MODFLOW Flex. We chose to emulate the behavior from Visual MODFLOW Classic which provides a natural approach of defining the well discharge rate over the entire screen interval, and independent of the numerical grid. With this “conceptual” approach, a single well can contain multiple screens, fully penetrating screens, or partially penetrating screen intervals over the depth of the wellbore.

The USGS MODFLOW “basic” WEL package treats pumping wells as a Flux (Type 2) boundary condition, such that each grid cell intersecting a well screen is assigned a specified flux. The .WEL package does not make any special considerations for screens spanning multiple layers. When a well is screened across several model layers, Visual MODFLOW uses the length of the well screen intersecting each model layer and the conductivity of those cells to automatically calculate the proportion of the total pumping rate assigned to each well grid cell in the model. 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.

Visual MODFLOW Flex will use the same logical approach to handling wells.

Visual MODFLOW Flex is well-known for its conceptual modeling capabilities, which includes the treatment of wells, and adopting the similar “conceptual wells” approach from Visual MODFLOW Classic will be a great fit. Pumping wells are defined at X,Y co-ordinates, and not assigned to specific grid cells. This provides flexibility in changing the numerical grid and using well locations (points) as grid node generators for MODFLOW-USG.

Pumping well data are shared between the conceptual and numerical models so that modifications to the conceptual model will translate into the numerical model. Pumping wells are presented in an easy-to-use database view showing the well location, screen geometry, and discharge rate. Since the rate is assigned to the screen, you don’t need to worry about figuring out how much discharge is proportioned to each model layer; Visual MODFLOW Flex does this calculation for you automatically.

In addition, if you are running a transport simulation, point source concentrations for an injection well are defined/edited on a per screen level, an approach that more closely represents actual field conditions.

Further improvements to wells that will enhance the “ease-of-use” include:

  • Import pumping schedule data in relative times (starting at 0 days), in addition to absolute dates (eg. 04/22/2015)
  • Setting individual wells as active/inactive in the database view, and having this setting translated to the .WEL package
  • Adding one or more new wells with the click of a mouse
  • Enhanced visualization of the well screen in cross-section view (by row/column), and in 3D view
  • Flexible labeling and rendering options in 2D and 3D; common example include coloring the well by its discharge rate, which may be useful to see “at a glance” which areas in your model have wells pumping the highest for a particular time interval.
  • Display the well name in the .WEL package, to correlate the conceptual well to a specific MODFLOW grid cell.