Chapter 11Large-Diameter Wells

Stephanie FultonFebruary 27, 2014

Difference from other methods◦ Well storage previously assumed negligible◦ Must be taken into account

When is “large” diameter large? Two methods

◦ Fully penetrating well in a confined aquifer◦ Partially penetrating well in an unconfined

anisotropic aquifer

Large-Diameter Wells

Assumptions◦ Confined aquifer◦ Unsteady-state flow◦ Fully penetrating ◦ large-diameter well so storage cannot be neglected

Papadopulos’s (1967) Curve Fitting Method

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

Papadopulos’s (1967) Curve Fitting Method (cont)

Papadopulos Type Curves 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

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

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:

Remarks

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

Boulton-Streltsova’s Curve Fitting Method

Boulton-Streltsova’s Curve Fitting Method (cont) 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

Streltsova Type Curves

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):

Boulton-Streltsova’s Curve Fitting Method (cont)