Interpretation of the transient flow rate obtained from constant-head tests conducted in situ in clays

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Abstract
Hydraulic tests conducted in situ using a source condition of constant head are frequently employed to determine the hydraulic properties of low-permeability clays. In this paper, an analytical model is developed for analyzing the results of a constant-head test conducted under conditions where the influence of finite-thickness skin and partial penetration are present at the source well. The analytical model is derived by application of the Laplace transform method with respect to time and the finite Fourier cosine transform with respect to the vertical coordinate. The solution is used to produce type curves of dimensionless flow rate versus dimensionless time so as to investigate the influence of finite thickness skin and partial penetration on the results of constant-head tests. Results show that the presence of a skin zone of finite thickness having permeability less than the formation produces an inflection point in the type curves at which point the dimensionless flow rate tends asymptotically towards a steady value. For the case where the skin and formation permeabilities are similar, these type curves can be used to uniquely define both the skin and formation properties. Where the skin zone is very thin, the shape of the type curves mimics the curve for a uniform, fully confined medium, and thus only the properties of the skin are measured using type-curve analysis. Conversely, the type curves for the case where the skin zone is of greater permeability than the formation show a unique and interpretable shape for each skin thickness and ratio of formation to skin permeability over most practical values of hydraulic diffusivity. Additional effects due to partial penetration were found to be pronounced in piezometers completed with small screen lengths. Particular conditions where the conventional steady approximation can be used are also discussed. Key words : constant-head test, flow rate, skin zone, partial penetration.

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