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Chapter 11 Large-Diameter Wells

Chapter 11 Large-Diameter Wells. Stephanie Fulton February 27, 2014. Large-Diameter Wells. 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

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Chapter 11 Large-Diameter Wells

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  1. Chapter 11Large-Diameter Wells Stephanie Fulton February 27, 2014

  2. Large-Diameter Wells • 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 unconfinedanisotropicaquifer

  3. Papadopulos’s (1967) Curve Fitting Method • Assumptions • Confinedaquifer • Unsteady-state flow • Fully penetrating • large-diameter well so storage cannot be neglected

  4. Papadopulos’s (1967) Curve Fitting Method (cont) • 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

  5. PapadopulosType 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)

  6. Remarks • 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:

  7. Boulton-Streltsova’s Curve Fitting Method • 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

  8. 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-stateflow in an unconfined aquifer

  9. Streltsova Type Curves

  10. Boulton-Streltsova’sCurve Fitting Method (cont) • 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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