IMPACTS OF POSITIVE SKIN EFFECTS ON BOREHOLE FLOWMETER TESTS IN A HETEROGENEOUS GRANULAR AQUIFER

In heterogeneous granular aquifers, an important issue affecting the a nalysis of pump test data is whether or not positive skin effects exis t. Positive skin effects include head losses and changes in radial flo w patterns caused by a local reduction of aquifer permeability near a well, In order to investigate the impact and formation of positive ski ns, field tests were performed at seven pairs of closely-spaced wells located in the vicinity of a paleochannel at Columbus Air Force Base ( CAFE), Mississippi, In the paleochannel a 1-2 m thick zone of high-g c hannel lag deposits lies beneath a 2-3 m thick zone of fine-grained ch annel fill deposits, Each well pair consisted of a well,vith and witho ut a gravel pack, At the well pairs outside of the paleochannel bounda ries, borehole flowmeter tests produce similar field results, At the w ell pairs inside of the paleochannel boundaries, however, borehole flo wmeter tests produced very different results, Examination of the test results indicate that positive skins exist at wells without gravel pac ks that intersect the paleochannel, During well installations, positiv e skins appear to be produced by the downward mixing of the fine-grain ed deposits into the channel lag deposits, Notable positive skin effec ts include increased specific capacities, increased total drawdowns, a nd changes in the distribution of horizontal now to a pumped well, Amo ng the key implications regarding borehole flowmeter tests in heteroge neous aquifers similar to that at CAFE are: (1) wells should be instal led with gravel packs; and (2) transmissivity values should be calcula ted using the Cooper-Jacob straight-line method rather than the Cooper -Jacob equation.