The method can account for a well that has multiple

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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 cell-to-well 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 multi-node 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 cross-sectional 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 135-degree angle (q) from the x-direction measured at the cell node “j,i,k” connected to the top of the well. B, side (cross-sectional) 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 multi-node well is open to the aquifer, the open interval of the well is assumed to pass through the block-centered node in the finite-difference 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 multi-node well has two linear segments associated with it; each segment connects the block-centered node with a cell face through which the borehole enters or exits the cell. • Nodes for each multi-node 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 Multi-Node Well (MNW2) Package for MODFLOW Ground-Water Flow Model • The length of the well between two successive nodes of a multi-node 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 multi-node 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 multi-node well is unconfined and the water-table 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 multi-node well and i...
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