Environmental Monitoring by Thomas Kwader, Ph.D., P.G. Owner, Qwater Well Developer and WorldWide Drilling Resource ® Hydrogeologist Using Electrical Resistivity Logs to Determine Well Screen Intervals Wells with screens set in permeable sand and gravel zones are probably the most common type of well completion used to obtain water samples or supply water for drinking purposes. Wells completed without screens into consolidated rock are another popular type of well completion when the formations are capable of maintaining an open borehole. This article discusses the drilling methods used to select the best producing zone while drilling by the mud rotary method. The mud rotary method circulates the formation drill cuttings back to land surface with the mud generated while drilling. The goal is to select a “water bearing zone” that will produce enough water to meet our needs. Screened zones are generally determined by observing the drill cuttings brought to land surface and observing the column of mud flow to and from the drill bit. Sometimes, it is difficult to determine if a formation is highly permeable or the formation is “above” the water table - and “dry”, which causes a “loss” of water and drilling fluid into the formation. Another tried-and-true method of determining relative permeability in a borehole is using wire line to measure electrical resistivity or formation resistivity. High formation resistivity is generally indicative of good water quality and relatively high permeability. Electrical resistivity logging generally involves lowering a lead electrode on a wire line which measures formation as the electrode passes by the formation. A low formation resistivity is indicative of low permeability and/or low water quality. Resistivity measurements are usually recorded on a paper graph, with depths, as the sonde is lowered into the borehole. The illustration shows representative responses to different conditions encountered in a sand and clay environment. Tom Tom Kwader may be contacted via e-mail to michele@worldwidedrillingresource.com www.starironworks.com 257 Caroline Street Punxsutawney, PA 15767 800-927-0560 • 814-427-2555 Fax: 814-427-5164 SERVING THE WATER WELL INDUSTRY Serving the Drilling Industry 20 DECEMBER 2020 WorldWide Drilling Resource ® ENV
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