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Impacts of Positive Skin Effects on Borehole Flowmeter Tests in a Heterogeneous Granular Aquifer
Author(s) -
Young Steven C.
Publication year - 1998
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/j.1745-6584.1998.tb01066.x
Subject(s) - palaeochannel , borehole , aquifer , geology , permeability (electromagnetism) , hydrology (agriculture) , geotechnical engineering , geomorphology , groundwater , chemistry , biochemistry , structural basin , membrane , fluvial
In heterogeneous granular aquifers, an important issue affecting the analysis of pump test data is whether or not positive skin effects exist. Positive skin effects include head losses and changes in radial flow patterns caused by a local reduction of aquifer permeability near a well. In order to investigate the impact and formation of positive skins, field tests were performed at seven pairs of closely‐spaced wells located in the vicinity of a paleochannel at Columbus Air Force Base (CAFB), Mississippi. In the paleochannel a 1–2 m thick zone of high‐K channel lag deposits lies beneath a 2–3 m thick zone of fine‐grained channel fill deposits. Each well pair consisted of a well with and without a gravel pack. At the well pairs outside of the paleochannel boundaries, borehole flowmeter tests produce similar field results. At the well pairs inside of the paleochannel boundaries, however, borehole flowmeter tests produced very different results. Examination of the test results indicate that positive skins exist at wells without gravel packs that intersect the paleochannel. During well installations, positive skins appear to be produced by the downward mixing of the fine‐grained deposits into the channel lag deposits. Notable positive skin effects include increased specific capacities, increased total drawdowns, and changes in the distribution of horizontal flow to a pumped well. Among the key implications regarding borehole flowmeter tests in heterogeneous aquifers similar to that at CAFB are: (1) wells should be installed with gravel packs; and (2) transmissivity values should be calculated using the Cooper‐Jacob straight‐line method rather than the Cooper‐Jacob equation.