Wellbore Skin Effect in Slug‐Test Data Analysis for Low‐Permeability Geologic Materials

Abstract

AbstractThe wellbore skin effect on slug‐test results was analyzed using numerical simulation and field tests for a well at progressive stages of development. The numerical simulation is based on a composite flow model that incorporates a zone of disturbed formation surrounding the wellbore. Field tests were performed on a water‐bearing clayey silt formation at a ground‐water remediation site in Wisconsin. Based on the numerical simulation, the radius of investigation was examined. The results show that the early‐time and late‐time data reflect ground‐water flow in the wellbore skin and undisturbed formation, respectively.Both the numerically simulated and the field slug‐test data define a downward concave curve on a semilog plot of time versus the logarithm of dimensionless head. For the Hvorslev (1951) and Bouwer and Rice (1976) methods, the late‐time segment of the simulated data yields estimates of hydraulic conductivity close to the value defined in the flow model. When a wellbore skin exists, the data curve in a plot of the logarithm of time versus the dimensionless head is shifted horizontally along the time axis. This shift leads to an inaccurate determination of hydraulic conductivity based on the Cooper et al. (1967) method. In the plots of time versus dimensionless head derivatives, the data curve geometry depends on the hydraulic properties of the wellbore skin. Consequently, the wellbore skin effect can be identified and eliminated using derivative‐based type curve methods.For low‐permeability materials, the effect of wellbore skin on estimates of hydraulic conductivity can be minimized through use of the late‐time data. However, proper well installation and development appears to be the most effective and practical solution.