Abstract : A modified form of the MODFLOW-2000 code provides a tool to estimate a hydraulic conductivity distribution near a borehole. The modified code allows the effects of wellbore screen intervals, screen clogging, and disturbed zone skin to be taken into account. The primary application has been in an investigation of electromagnetic borehole flow meter measurement interpretation. MODFLOW-2000 was modified to simulate a two-dimensional cylindrical geometry. The two dimensions are vertical and radial with angular symmetry. The geometry is similar to the RADMOD preprocessor to MODFLOW, but is more flexible in that the grid spacing in the radial direction may be non-regular and the hydraulic conductivity of a layer need not be constant. This modification includes sensitivity calculations so that parameter estimation can be performed. To support parameter estimation using flow meters, a new measurement category of internal flow observations was introduced. Internal flow observations occur at locations that are fully contained within the model domain. The standard MODFLOW-2000 code is limited to flow observations across the model domain boundaries. The new internal flow measurement observations are compatible with the normal MODFLOW-2000 Cartesian geometry and with the cylindrical geometry modifications. A third modification moves the node location of a cell containing the water table from the center of the cell to the location of the water table. The vertical position of the node dynamically follows changes in the water table position. This change improves the simulation of water table dynamics for both cylindrical geometry and Cartesian geometry simulations. Together these modifications provide a tool for modeling pumping from a wellbore with multiple screened zones in a layered aquifer with an azimuthally uniform disturbed zone. If the wellbore is included, the pumping rate can be applied to the upper node of the wellbore.
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