Numerical evaluation of the flowmeter test in a layered aquifer with a skin zone
In this study, we used a two-dimensional groundwater flow model to numerically evaluate the single flowmeter test in a series of two-layer confined aquifers for a fully penetrating well with a skin zone (i.e. a disturbed zone, filter-pack envelope, etc.). The skin zone causes errors in estimates of the vertical distribution of horizontal hydraulic conductivity by acting as a channel for cross-flow between layers of contrasting hydraulic conductivity. The bias in measured layer discharge rates, Q(i), is dependent on the ratios of the skin zone hydraulic conductivity to the layer hydraulic conductivities, K(s)/K(i), and on the ratios of the skin zone thickness to the thickness of the individual layers, (r(s) - r(w))lb(i). For any given K(s)/K(i), the cross-flow increased with increases in the skin zone radius relative to the well radius, r(s)/r(w). The error in estimated K(i) due to a skin zone of reduced hydraulic conductivity (K(s)/K(i) < 1) is greater than that of a skin zone of a correspondingly enlarged hydraulic conductivity (K(s)/K(i) > 1). As K(s)/K(i) increases, the ratios (K1/K1 and K2/K2 converge approximately to the ranges 0.9-0.95 and 1.0-1.25, respectively. As K(s)/K(i) decreases from value 1, the ratios of K1/K1 and K2/K2 diverge from these asymptotic ranges. In all simulated flowmeter tests, where K1/K2 = 10, k1 underestimates K1 and K2 overestimated K2.
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