WorldWide Drilling Resource

25 WorldWide Drilling Resource ® AUGUST 2015 Using Borehole Geophysical Logs to Select Your Screen Interval Selecting the correct depths to set your screens is critical when well yield (maximum gallons per minute) is important. There are many methods for determining which zones will yield the most water, depending on the type of drilling and the local geology. If the goal of the well is only five gallons per minute and there are thick zones of coarse sand and gravel, it is a much simpler task. If the geology of the site is not well known, or the customer would like to obtain as much water as possible, consider using borehole geophysical logs (BHGLs). There are many types of BHGLs available. However, to keep it simple and relatively inexpensive, I recommend an electrical resistivity log and a natural gamma log. These logs can be run on a single probe in a single trip (in and out) of the borehole. These two logs are often run on a steel cable with a single electrical wire inside to conduct electrical current to the probe. The equipment can be rented, purchased, or contracted through a logging service company. Shallow wells (a few hundred feet or less) can be logged using a small portable handheld winch assembly (rented or purchased); whereas on deeper wells it is advisable to contract with a wireline service company (query borehole logging). Natural gamma and electrical resistivity logs measure two distinct physical parametersof geologic parameters as follows: Natural Gamma is simply a Geiger counter which measures natural gamma radiation of the formation encountered in the borehole. “High gamma” radiation is often associated with clays and organics which generally are not desirable aquifer type material and generally are not screened. Gamma logs can be run in casings, cement grout, and drill rods if necessary. Resistivity Logs measure the electrical resistance of the formation adjacent to the borehole. Clay and other low-permeability materials generally have low “bulk” resistivity. Clean sands, limestones, and dolomites often show “high” resistivity and produce larger volumes of water. BHGLs are often used to pick the exact depth and thickness of the producing zones and determine if the transition between the zones is transitional over many feet, or if the contacts of the producing zones are “sharp” or transitional to aid in depth of screen placement. BHGLs are a valuable tool, and with experience can take the guesswork out of placing screens - and save time and money. Tom Tom Kwader may be contacted via e-mail at michele@worldwidedrillingresource.com Environmental Monitoring by Thomas Kwader, Ph.D., P.G. )0&"%1 "6)&4 ,, ,, 2)&0 -%3120*"+ )0&"%1 "-% 312., )0&"%1 "-3'"$230&0 .' -&02*" !&+%&% 0*++ */& 3"+ !"++ 0*++ */& !*0&+*-& 0*++ .% "1*-( .,/.1*2& !*0&+*-& 0*++ .%1 .-*$ 0*++ */& "1*-( *21 &0$311*.- "1*-( *21 0*++ .++"01 *1)*-( ..+1 3#1 2"02&0 .%1 "-% 2"#*+*7&01 0*++ */& -$ )*0% 2 8 !*-12&% ).-& 8 8 "5 2., %0*++/*/&*-$ $., 8 444 %0*++/*/&*-$ $.,