Evaluation of flowmeter-head loss effects in the flowmeter test
In this study, we develop an axisymmetric groundwater flow model to numerically simulate the hydraulic head loss across the flowmeter during a flowmeter test. Using it, we investigate the influence of flowmeter induced hydraulic head losses and filter pack effects on the calculated drawdown distributions in the vicinity of the pumping well and on the flowmeter test estimates of layer horizontal hydraulic conductivity, K(i), in three different aquifer-well systems: A homogeneous aquifer; a homogeneous aquifer with a filter pack surrounding the well; and a two-layer aquifer with a filter pack surrounding the well. We confirm that the K(i) estimates can be significantly biased due to the head loss across the flowmeter. This head loss induces a non-uniform flow pattern in the vicinity of the well, which violates the horizontal flow assumption invoked by flowmeter test interpretation methodologies. The effect of the hydraulic head losses alone (i,e. no cross-flow through the filter pack) can lead to K(i) estimation errors as great as a factor of three even in a homogeneous aquifer. The coupled effect of hydraulic head losses and cross-flow through the filter pack can lead to K(i) estimation errors of up to a factor of 10 or greater. In actual flowmeter tests, these head losses can be reduced by increasing the annulus of the flowmeter, by operating the flowmeter without a packer, or by choosing a small enough pumping rate.
Ruud, NC; Kabala, ZJ; Molz, FJ
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