Chapter 11 Large-Diameter Wells

Report
Stephanie Fulton
February 27, 2014
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Difference from other methods
◦ Well storage previously assumed negligible
◦ Must be taken into account
 When is “large” diameter large?
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Two methods
◦ Fully penetrating well in a confined aquifer
◦ Partially penetrating well in an unconfined
anisotropic aquifer
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Assumptions
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Confined aquifer
Unsteady-state flow
Fully penetrating
large-diameter well so storage cannot be neglected
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Similar to other methods (Theis equation)
except for the well function
Well function F(u,α, r/rew) accounts for the
size of the well
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For 1/u and α = (10-1, 10-2, 10-3), select a value for r/rew
using look-up tables in Annex 11.1
◦ α is a function of well radius and storativity
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For long pumping times, F(u,α, r/rew) can be approximated
with the Theis equation well function W(u) (Equation 3.5)

Early drawdown data yields unreliable results
◦ Data curve can be readily matched with more than
one type curve but estimated S values differ by an
order of magnitude
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
Transmissivity (KD) is less sensitive to the
choice of type curve
Large-diameter wells are often partially
penetrating, in which case another solution is
needed.
◦ Drawdown reaches a max when t > DS/2K
◦ Drawdown can be estimated using an equation
analogous to Equation 10.7:
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Unconfined, unsteady-state flow
Homogeneous, anisotropic, uniform thickness
Partially penetrating large-diameter well
Well diameter is not small so well storage cannot
be neglected
SY/SA > 10
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Type A curves
◦ Early-time drawdown
◦ Boulton and Streltsova (1976) developed a well
function describing the first segment of the S-curve
typical of unsteady-state flow in an unconfined
aquifer
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Type B curves
◦ Late-time drawdown
◦ Curves result from Streltsova’s equation for a small
diameter, partially penetrating well in an
unconfined aquifer
◦ Applicable for long pumping times when the effect
of well storage is negligible
◦ Modifed form of the Dagan solution (1967):

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