Unformatted text preview: then defined by
, (35)
Given the lengths of the two well segments of a multinode well associated with a given node, the celltowell conductance for that node can be calculated as:
(36)
where the subscripts 1 and 2 refer to the segment number.
The method used to calculate the total length of a well
and the length of the two well segments associated with each
node of the multinode well (Lw) is derived from that developed by Konikow and Hornberger (2006b). The method can
account for a well that has multiple discrete sections of well
screen. The following working assumptions and rules are
made for nonvertical wells: Model Features and Processes 35 Figure 36. Schematic crosssectional view to the
west of part of a MODFLOW grid containing a multinode well that changes orientation from a vertical
well at the surface to a horizontal well in model layer
2. Both cells “j,i,k” and “j,i+1,k+1” contain two well
segments that are not aligned in the same direction. Figure 37. Depiction of orientation and alignment of a nonvertical well on the basis of two characteristic angles. A, top (plan) view showing
example of well oriented at 135degree angle (q) from the xdirection measured at the cell node “j,i,k” connected to the top of the well. B, side
(crosssectional) view in the vertical plane containing the well showing the tilt angle (w) of approximately 70 degrees as measured from the
vertical plane passing through the top node of a well (or of a well section). • Where a multinode well is open to the aquifer, the
open interval of the well is assumed to pass through the
blockcentered node in the finitedifference cell; any
change in direction or orientation of a well can only
occur at the location of the node.
• A well is assumed to fully penetrate each fully saturated cell in which a well node is located (that is, it
crosses or touches two cell faces in addition to passing
through the node).
• Where a well node is located in an unconfined (convertible) cell, the upper limit of the open interval is defined
by the elevation of the water table. If the open intervals
are defined by specified elevations, these will further
constrain the position and length of the open interval. • The location, orientation, geometry, and elevations of
the well and its open intervals are defined on the basis
of (and consistency with) the initial conditions specified for the model. These will not change with time
even though the saturated thickness and relative node
location in unconfined cells may change with time.
• Each node of a multinode well has two linear segments associated with it; each segment connects the
blockcentered node with a cell face through which the
borehole enters or exits the cell.
• Nodes for each multinode well must be listed in sequential order in the input data file, starting with the first
node closest to the wellhead at the land surface and ending with the deepest node furthest from the wellhead. 36 Revised MultiNode Well (MNW2) Package for MODFLOW GroundWater Flow Model
• The length of the well between two successive nodes
of a multinode well is defined as the distance between
those two nodes. If two nodes are not adjacent to each
other (such as well nodes “j,i,k” and “j,i+1,k+2” in
figure 38), it is assumed that the well is not open to that
part of the aquifer represented by the intervening cells.
• The first segment of the first node of a multinode well
is always assumed to be vertical and extend upwards
to the land surface (as in figures 38 and 39). Thus, if
the first node of the well is not in the uppermost active
layer of the model at that location in the grid, where the
well passes through higher layers, the casing is closed
and the well is not connected to the aquifer (fig. 39).
• The second (last) segment of well screen (or open
interval) associated with the final end node of a multinode well is assumed to have an identical length and
orientation as the first segment of that well node, with
symmetry about the cell node (figs. 39 and 40). For
the relatively simple case illustrated in figure 39, the
cumulative length of the two segments of open interval
in the fifth well node would be equal to the width of
column 5.
• If a node of a multinode well is unconfined and the
watertable elevation is below the top elevation of
the well screen, then the length of the open interval is
based on its top coinciding with the water table. The
length of the well above the water table is considered
to be dry, noncontributing, and therefore a component
of the closed interval of the well. In a transient simulation, both the open and closed lengths of the well
within a cell can change every time step if the watertable elevation and saturated thickness change (fig. 40). Well Screen Variability
It is possible for wells to have complex sets of multiple
well screens (open intervals), or variable diameter boreholes,
or skin properties that vary with depth. The revised MNW2
Package has added a limited capability to deal with such
complexities in vertical wells, although the model is inherently
limited to one set of composite well properties to be associated
with each node of a multinode well and i...
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This document was uploaded on 01/20/2014.
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