Water Well Journal
Issue link: http://read.dmtmag.com/i/440561
to the substantially lower cost compared with abandoning the well and drilling a replacement. The relatively large diameter of the pump required the liner be 16 to 18 inches in diameter, with 18 inches preferred. Prior to ordering liner material, a casing dummy (18 inches outside diameter, 50 feet long) was lowered to the bottom of the well on a cable to ensure an 18-inch liner could be installed. Using a cable to run the casing dummy allows for direct observation of deviation or deformation that might pre- vent installation of a liner of a diameter equal to that of the casing dummy. After successful completion of the dummy test, the liner was ordered. High-strength low-alloy steel (HSLA, ASTM 139 A 606 Type 4) was selected for the liner material. This alloy is nine times more corrosion resistant than mild steel and is significantly less expensive than stainless steel. In light of the loss of original filter pack, a blank section 20 feet long was planned for installation from 300 to 320 feet bgs for the pump intake position to minimize pumping of sand. The annulus between the 18-inch liner and the inside diam- eter of the existing casing was only 0.625 inch, so there was no room for weld couplings or louvers on the liner. This re- sulted in the selection of double-row mill-slot, slot opening 0.125 inch, with machine beveled ends on each joint of liner to aid in welding. The small annulus between the casing and liner precluded the installation of a tremie pipe for placement of filter pack, and free-falling natural gravel pack from surface would likely cause bridging of the gravel pack, resulting in an incomplete filter pack. Therefore, 5 mm (0.196 inch) diameter glass beads were chosen for the filter medium, as they were about 150% larger than the slot size of the liner. Glass beads are chemically inert and highly resistant to breakage. Of even greater importance to this well rehabilita- tion is the fact glass beads are nearly perfect spheres of the specified diameter, which allows them to flow freely with lit- tle likelihood of bridging during installation and minimize well loss during operation. After landing the liner, a wooden frame was constructed around the top of the well. An 18-inch bullnose was placed over the top of the liner, and the glass beads (packaged in 20 kg [44 pound] bags) were poured down the annulus. A total of 3 metric tons of beads were installed in 45 minutes with no difficulty as shown in the lead photo. It was not necessary to wash the beads down the annulus with water, as they flowed freely. Well development was accomplished using a double surge block (swab) while airlifting. Total development required after installation of the liner and glass beads was 25 hours. Figure 3 is a screen capture from the video survey conducted after development. The glass beads are clearly visible through the mill slots, and exhibit a uniform distribution. Figure 4 shows the original well construction and the reha- bilitated well. The original well had a specific capacity of 44 (pumping 2000 gpm with 46 feet of drawdown). After reha- bilitation, the specific capacity is 79 (pumping 1500 gpm with 19 feet of drawdown). Clearly, the rehabilitation effort was successful. 26 January 2015 WWJ waterwelljournal.com GOING GLASS from page 25 Figure 1. Corrosion damage at 263.3 feet bgs. Cobbles from the alluvial native ground are clearly visible in the center of the photograph. Louvers are approximately 3 inches wide. Figure 2. Corrosion damage at 352.1 feet bgs. Louvers are noticeably corroded, and no filter pack is visible. Figure 3. Screen capture from video survey conducted after development. Note the uniform distribution of glass beads visible through the mill slots.