Jetting Gravel-Packed Wells

Jetting Gravel-Packed Wells

In some areas, such as where this writer drills, the surface waters are usually within 20 feet from the surface, underlaid by an impervious limestone or marl stratum, then one or two muddy sand and shell formations from five to 25 feet thick, followed by an impervious stratum, 150 to 300 feet thick. This is practically ideal for gravel packed wells.

Eight inch gravel packed wells with two inch slotted well screens are drilled for domestic supplies and irrigation work. Ten inch diameter gravel-packed wells with screens are drilled for public service and larger commercial-industrial applications. Some wells require casings, some do not. Also, some must be grouted while others need not.

Equipment used is the same as for tubular wells, plus an undercutting tool where required. A larger volume of water is usually needed and drilling compound is a must along with the proper screen and well pipe. Well casings, cement or clay grout, and grouting equipment are used as required. Where necessary, tremie pipes or pumping equipment are necessary for placing the gravel, plus a surge plunger, air compressor and pump. The gravel should be pre-chlorinated, graded and washed and of the proper size to suit the sands and stratum materials.

Sometimes it is necessary to drill a pilot hole through the stratum to obtain sand size and stratum location and thickness. This usually requires a machine operator, a driller, a combination pipe-fitter and helper for small wells, plus another helper for larger wells. The process is as follows.

Place the machine at the well site. Start a hole for the starter well casing. The drill pipe is raised and dropped repeatedly about two feet, again, while the operator turns the drill pipe as in the percussion-jet procedure.

While drilling, a close check is kept on the circulation water and samples must be collected at regular intervals and whenever there is a change in formation. These samples must be marked and recorded because this is the only time the log data can be obtained. As the driller is holding the drill, he can feel the grinding or "holding" of the drill in certain formations - a skill acquired with experience. It is necessary to examine the cuttings every few feet as well as the drilling mud removed from the ditch.

Once a sand water stratum is reached, it is advisable to keep the drill hole full of water to prevent loss of head pressure on the walls of the unlined drill hole. Also, do not thin the work water too much with clear water or the clean water will wash out the compound that is holding up the walls, causing the walls to cave in.

When the driller is satisfied that he has reached and penetrated the water stratum deep enough, the drill pipe is removed from the hole. Keep the drill hole full of water while removing the drill pipe. Too fast a removal will drop the water level in the well and perhaps cave in the sand stratum.

After removing the drill, attach the undercutter if it is being used. The undercutter can be fabricated from a piece of four inch pipe, three feet long, with a flush welded plate in the bottom and a threaded coupling on the top. A jet orifice is drilled in the center of the bottom plate. Four jet nozzles are welded through a six inch coupling at the center of the four inch pipe. These are placed two inches apart at the center and at 90 degree angles of center of the four inch pipe. A one-half inch by two inch bar extends on four sides from the outer edge of the six inch coupling to the bottom end of the pipe and to just below the threads on the top part. The size of the four nozzles equals the work pump delivery at high pressure. This forms high pressure cutting jets operating sideways and below to enlarge the drill hole and wash out the cuttings in the area to be gravel packed.