6-1. Setting Casing. In rotary-drilled water wells, you can set the casing in the borehole after you finish drilling. However, you must set the casing and grout it near the surface. This prevents the upper portion of the borehole from caving in.
For water wells drilled in rock aquifers, you can place the casing through the unconsolidated strata and into the rock to get a tight seal. However, this method does not ensure a tight seal. You can improve the seal by using the following procedures:
- Step 1. Drill a borehole about 10 feet into the rock.
- Step 2. Flush the hole with clean water.
- Step 3. Fill the rock interval with grout, and immediately set the casing into the hole. Let the grout set around the bottom of the casing.
- Step 4. Drill through the grout, plug in the lower casing, and progress into the aquifer.
- Step 5. Complete an open-hole well or install a smaller casing and screen inside the outer casing.
NOTE: You cannot pour grout through drilling fluid to properly seal the casing.
6-2. Selecting Casing. Well casing is plastic, wrought iron, alloyed or unalloyed steel, or ingot iron. Well-completion kits have either plastic (Figure 6-1) or steel casing. You must know the properties of other casings because you may have to use them. When selecting the casing, consider the stress factor during the installation process, the corrosive element of the water, and the subsurface formation.
If you use casings other than those in the well-completion kits, you must specify the weight per foot of pipe you want to use. The tables in A100-66, American Water Works Association Standard for Deep Wells (AWWA), present data on the steel and wrought-iron pipes used as permanent well casings. Joints for permanent casings should have threaded couplings or should be welded or cemented so they are watertight from the bottom of the casing to a point above ground. This precaution will prevent contaminated surface water and groundwater from entering the system. Table 6-1 lists hole diameters for various sizes and types of well casings.
- Step 1. Clean the borehole by allowing the fluid to circulate with the drill bit close to the bottom so cuttings will come to the surface. You may carry the borehole deeper than necessary so that if any material caves in, the material fills the extra space below the casing depth.
- Step 2. Attach a coupling to the top of the casing. Suspend the casing by either attaching a hoisting plug to the coupling, using a sub (adapter) if necessary, or by placing a casing elevator or a pipe clamp around the casing under the coupling.
- Step 3. Lower the first length of casing until the coupling, casing elevator, or pipe clamp rests either on the rotary table or on another support placed around the casing. If lifting with a sub, unscrew the sub on the first length of casing and attach it to the second length of casing. If lifting by elevators or pipe clamps, release the elevator bails from the casing in the hole, and attach it to another elevator casing or pipe clamp on the second length of casing. Lift it into position, and screw it into the coupling of the first casing length. (Lightly coat the threads of the casing and coupling with a thin oil. Screw the lengths tightly together to prevent leaking.) Remove the elevator or other support from the casing in the hole and lower the string that is supported by the uppermost coupling.
- Step 4. Repeat the procedure for each casing length you install. If a cave-in occurs, attach a swivel to the casing with a sub, and circulate the drilling fluid through the casing to flush out the hole and wash the casing down. After the casing is set on the bottom of the hole, drill through the casing into the aquifer.
b. Single-String Method. With this method, you install the casing and screen (that have been joined) in a single assembly. See paragraph 6-6a for installation.
c. Wash-In Method (Jetted Wells). With this method, you advance the borehole for an expedient jetted-well construction. See paragraph 9-2b(1) for installation.
d. Driven Method (Driven Wells). Install the casing as with the borehole, the cable-tool, or driven-point well method. See paragraph 9-3 for installation.
e. Uncased-Interval Method. In rock, you normally leave the lower portion of the borehole uncased because water emerges from irregular functions in the borehole wall. Therefore, for the well to function properly, you must be careful when locating the bottom of the cased portion in relation to any impermeable zone. Once you establish the depth, drill down and set the casing. Then drill a smaller hole to full depth and proceed with development.
6-4. Grouting and Sealing Casing. Once grout is mixed, it starts to set. Therefore, place the mixture immediately after mixing it. You must have freshly mixed grout continuously to meet requirements. Portland cement meets most grouting requirements. (You can use a quick-hardening cement to save time.) For proper consistency, use no more than 6 gallons of water per 94-pound sack of cement. If you need a large amount of grout, add 1 cubic foot of fine or medium sand for each sack of cement. Add a few pounds of bentonite or hydrated lime per sack of cement for a better flow. For small jobs and if no equipment is available, use a 55-gallon steel drum as a mixing tank. Put 20 to 24 gallons of water in the bard and slowly add 4 bags of cement while stirring or jetting the water. Use as many mixing barrels as the job requires. If a concrete mixer is available, mix batches and dump them into a storage vat for future use.
You can force grout into place using pumps or air or water pressure. In some cases, you can place grout using a dump bailer. If you use the tremie methods, you will need one or more strings of pipe with small diameters. Other equipment you may need to place grout are a mixing tank, hoses, and a feed hopper.
a. Dump-Bailer Method. With this method, you can place grout simply and with a minimum amount of equipment. On the dump-bailer, the bottom valve opens and the operator can unload grout at a specified location. This method works best when you need to grout only the lower portion of the casing. Use the following procedures to place grout:
- Step 1. Run the casing in the hole and mix enough grout to fill the lower 20 to 40 feet of the hole. (A quick method of mixing the grout is to put the required amount of water in a barrel and circulate the water, using a high-pressure jet while adding cement.)
- Step 2. Place the grout inside the casing with a dump-bailer.
- Step 3. Lift the string of casing 20 to 40 feet off the bottom, depending on the amount of grout placed. The lower end of the casing should be below the top of the grout. Fill the casing with water and cap the top.
- Step 4. Lower the casing to the bottom of the hole to force most of the grout up the annular space outside of the casing. Do not uncap the top of the casing until the grout is set.
- Step 5. Drill through the cement that has hardened in the lower end of the casing and continue drilling to the required depth. (Green cement will increase the viscosity of the drilling mud.)
If you anticipate difficulty in filling the casing with water when it is lifted in the borehole, you can add water on top of the grout without lifting the casing. Calculate the volume of grout in the casing. Fill the casing to the top with water. Connect a pump so that you can force in additional water. Pump water into the casing, measuring the volume pumped, until you put in a quantity equal to the volume of grout. This will force all or most of the grout out of the lower end of the casing. You can place a wad of burlap on top of the grout before filling the casing with water to keep the fluids separated.
b. Inside-Tremie Method. With this method, you place the grout in the bottom of the hole through a tremie pipe that is set inside the casing (Figure 6-2). The grout will either descend naturally or you may have to pump it through the pipe. Make sure that the tremie pipe is at least 1 inch in diameter. With this method and any other method where you place the grout inside the casing, make sure that water or drilling fluid circulates up and around the casing before you start grouting. To check this, cap the casing and pump in water. If the water comes to the surface outside the casing, you can start grouting. Use the following procedures to complete the tremie method:
- Step 1. Continue pouring grout until it appears at the surface around the casing. Once the grout reaches the desired depth, it will fill the space around the outside of the casing.
- Step 2. Suspend the casing about 2 feet above the bottom of the borehole during the operation. Once the grout is in place, lower and seat the casing in a permanent position. Remove and clean the tremie pipe. The check valve prevents the grout from moving back into the casing.
- Step 3. The grout should set and harden in about 24 to 72 hours, depending on the cement you use. Drill out the packer and continue drilling the well below the grouted casing.
c. Outside-Tremie Method. With this method, you use a tremie pipe to deliver grouting outside the casing. This method is not recommended for depths greater than 100 feet. You can use this method if the space between the casing and the borehole wall is large enough to contain a 1-inch tremie pipe. Use the following procedures to complete grouting using this method:
- Step 1. Lower the pipe to the bottom. Make sure that the lower end of the casing is tightly seated at the bottom of the borehole.
- Step 2. Mix a sufficient quantity of grout and pump it through the tremie pipe or let it descend naturally (Figure 6-3). As the grout is placed, lift the tremie pipe slowly, but keep the lower end submerged in the grout.
- Step 3. Fill the casing with water as the grout is placed to balance the fluid pressure inside and outside the casing. Doing so prevents grout from leaking under the bottom of the casing.
You can quickly place grout near the surface using a tremie or by dumping the grout in the hole around the casing and puddling it with pipe lengths. However, this procedure only provides a surface seal around the casing and cannot be used as a normal grouting method.
6-5. Selecting Screens. The military uses continuous-slot screens (Figure 6-4) when drilling wells using the rotary method. You make the screens by winding triangular sections around a skeleton of longitudinal rods. Join the triangular sections and rods securely wherever they cress. The screens are constructed of either PVC or stainless steel and are packed in the well-completion kits. Some important factors to remember when selecting screens are that they--
- Must produce a sand-free well of less than 2 ppm.
- Should prevent minimum head loss.
- Should be of a commercial grade. (Screen sizes are in increments of 0.005 inch.)
- Should lend themselves well to development (allow for a two-way flow).
(1) PVC Screens. These screens must beat least 8 inches in diameter. Continuous-slot PVC screens are in the 600-foot well-completion kits packed in boxes containing four 20-inch-long sections that can be joined together (Figure 6-5). The kits should contain enough sections to assemble up to 50 feet of continuous-slot, 8-inch well screen.
You can use the PVC casings to construct an alternative screen. You saw or mill horizontal slots in sections of the 20-foot-long casing. You cut six rows of slots down the casing. The slots are 1/2 inch apart at a width of 0.025 inch. You can construct PVC screens in the field by cutting slots in PVC casing with saws. The following lists saws and approximate slot sizes:
- Hacksaw, about 0.035 inch.
- Handsaw, 0.050 to 0.080 inch.
- Circular saw, 0.100 to 0.125 inch.
These makeshift screens are not as efficient as the continuous-slot screens because of the relatively low open area per foot of screen. However, you should use these screens when the kit contains an insufficient number of continuous-slot screens. The alternative screens are the same as the casing sections. You can place the screens intermittently up the well if you screen more than one aquifer or waterbearing strata. If you screen multiple intervals, you must place gravel packs around the screens and backfilling between the screens. If sufficient gravel-pack material is available, use it continuously to above the top of the screen.
(2) Stainless-Steel Screens. These screens are usually 6 inches in diameter and come in 10- and 20-foot sections. You can join the sections to make longer pieces. Most stocked screens have 0.025-inch openings that are suitable for medium-sand formations. Screens with other slot openings are available and may be needed for special installations. Various end fittings are available so you can use different installation methods. See paragraphs 6-6a through d for uses of these end fittings.
(3) Pipe-Base screen. You make this screen by wrapping a trapezoidal-shaped wire around a pipe base that has drilling holes evenly spaced. A pipe-base screen is strong and suitable for deep wells. The screen has two sets of openings. Outer openings are between adjacent turns of the wrapping wire and inner openings are the holes drilled in the pipe base. The percentage of open area per foot of screen (usually low) governs the efficiency of the screen. These screens come in 3- and 4-inch pipe bases for field operations.
b. Lengths. Use the electric logging units in the well-completion kits to determine the location, depth, and thickness of an aquifer. Screen length should not exceed the thickness of thin aquifers. You should screen the bottom third of unconfined aquifers and 75 to 80 percent of confined aquifers. However, for interlayered fine and coarse beds, consider the thickness of the coarse strata when determining screen length. Set the screen in the coarse strata. If you have a choice of slot sizes, consider the percentage of the open area or intake area of the well screen to determine length. Use the following calculations to determine the amount of screen to use during drilling operations:
(1) Surface Area.
SA = p (OD)(12)
SA = surface area, in inches per foot.
OD = outside diameter of the screen, in inches.
(2) Open Area.
OA = open area as a percentage.
SO = slot opening, in inches.
WD = wire diameter, in inches.
(3) Total Area.
TA = (SA)(OA)
TA = total area, in square inches.
SA = surface area, in inches per foot.
OA = open area, as a percentage.
(4) Transmitting Capacity.
TC = (TA).31
TC = transmitting capacity, in GPM per foot of screen (based on one-tenth foot per minute of velocity).
TA = total area, in square inches.
(5) Screen Length.
SL = screen length required, in linear feet.
PR = pumping requirement, in GPM.
TC = transmitting capacity, in GPM
The following example uses the above equations to determine the amount of 12-inch screen required at one-tenth foot per minute of velocity with a slot opening of 0.040 inch, a wire size of 0.092 inch, and a pumping requirement of 800 GPM:
- Increasing the screen diameter to increase the yield or capacity of a well. The increase is not proportional.
- Doubling the diameter of a well screen increases the capacity of the well by about 10 percent.
- Using a larger diameter screen if the aquifer is thin or the pump is large.
- Using a long screen with a smaller diameter in a thick aquifer, for better performance.
- Increasing the length, not the diameter, of the screen to increase the yield of a well in a thick aquifer.
d. Slot Sizes. You should understand the function of slot size in well construction. When possible, choose the screen slot size to fit the gradation or grain sizes of the aquifer. Sand and gravel interaction greatly affects the development of the formation around the screen. Small openings limit well yield. Also, small slot sizes produce high velocity in the water passing through the screen. In time, scale or incrustation tends to form on the screen. If the openings are too large, you may have to develop the well more than usual, or you may not be able to clear the well of sand.
Screen slots that are sized to retain the coarsest one-third to one-half of sand or gravel of the aquifer work best in a naturally developed well. About two-thirds of the sand in the layer behind the screen should pass through the slots. A screen set across both coarse and fine strata may need sections with different slot sizes. If you have to pack the screen with gravel artificially, make sure the slot openings correspond to the size of the gravel you use. Screen openings that retain about nine tenths of the gravel work best.
6-6. Installing Screen. You can use several methods to install screens in rotary-drilled wells. To set well screens, you will use the screen hook and casing elevators. You use the hook to engage a bail in the bottom of the screen to suspend the screen on either the sand line or hoist line while lowering the screen into the well (Figure 6-6). Do not pull the screen with the hook after the formation has closed in around the screen. If you are installing a screen using the telescoping method and you must seal the casing, you will need rubber or neoprene packers. All screen-setting methods require accurate and complete measurements of pipe, screen, cable length, and hole depth.
a. Single-String Method. With this method, you install the casing and screen as one assembly. Figure 6-7 shows a single-string assembly equipped with fittings for the washdown method. You can omit the washdown fittings if the hole stays open at the bottom. Use the following procedures to install the assembly:
- Step 1. Attach couplings to the top of the casing and screen section. Install a sand trap to the bottom of the string.
- Step 2. After running the entire string into the borehole, use casing elevators to carry most of the weight of the string until the formation collapses in around the screen.
- Step 3. Run the drill pipe inside the casing to the bottom of the screen, and pump water into the well to displace the drilling fluid.
- Step 4. Raise and lower the drill pipe to wash the full length of the screen.
- Step 5. Wait for the formation to settle around the screen. Proceed with well completion and development.
For deep wells and wells requiring surface casing, you can use the following modified telescoping procedure:
- Step 1. Grout the surface casing in the borehole before you drill the well to the final depth.
- Step 2. Use the single-string method inside the surface casing, while bringing the inner casing to the surface.
- Step 3. After placing the gravel pack around the screen and impervious backfill on top of the grovel, grout the entire annulus between the inner and outer casing to the surface.
A disadvantage of the single-string method is the weight of a long string of casing on top of the screen. When the screen touches bottom, it becomes a loaded column that can easily buckle because of its slenderness. When the screen reaches the correct depth, you can prevent buckling by supporting the screen on casing elevators until the formation material collapses around the screen and supports it laterally.
- Step 1. Sink the well casing to the full depth of the well, and clean out the hole to the bottom of the pipe with the bailer.
- Step 2. Assemble the closed bail plug in the bottom of the screen. Screw one or more packers to the top of the bail plug.
- Step 3. Lower the screen inside the well casing using the sand line. After setting the screen on the bottom, pull the casing back far enough to expose the screen in the aquifer and to a position where the packer is still inside the casing.
- Step 4. Use a casing ring and slips with two hydraulic jacks to pull the pipe. If the screen moves upward as you pull the pipe, lower the drill bit or other tool inside the screen to hold the screen down.
- Step 5. Hold the casing with pipe clamps until either the hole caves around the casing and grips the pipe or until you can place grout around the casing and the grout sets, after pulling the casing to its permanent position.
- Step 6. Bail out the drilling mud so that the sand and gravel of the formation will close in around the screeen.
- Step 1. Sink the well casing into the aquifer to a depth slightly below the desired position from the top of the well screen. Fix the casing in place by grouting or other means.
- Step 2. Mix drilling mud and fill the casing. Using a bit that will pass through the casing, drill into the aquifer below the casing to make room for the length of well screen to be exposed.
- Step 3. Lower the screen into position, ensuring that the rubber or neoprene packer remains inside the casing near its lower end when the screen is on bottom. If the hole is too deep, drop gravel into the hole to the correct height.
Use the closed bail plug and the packer top-end fittings to support the screen and sand trap (Figure 6-8). With this method, the diameter of the screen must be smaller than the casing, since the hole drilled for the screen will be no larger than the inside diameter of the casing. Also, make sure that the packer fitting at the top of the screen is the proper size to seal inside the casing. The drilling mud must be heavy and thick to prevent the open borehole from caving in, and the mud must be completely removed from the aquifer during development.
d. Washdown Method. This method works best if the aquifer is composed of fine to coarse sand with little or no gravel. The screen fittings you need for this method are a washdown or self-closing bottom. Figure 6-9 shows a fitting you can use when using a telescoping method to set the screen through the casing. Set the casing from the surface to slightly below the depth where you will install the top of the screen. Screw a section of wash pipe into the left-hand female thread of the self-closing bottom and attach the bottom to the screen or sand trap with the wash pipe projecting through the screen. Lift the entire assembly by the wash pipe and lower the screen inside the casing. Add sections of wash pipe until the bottom of the screen is near the lower end of the casing.
Use the following procedures for the washdown method:
- Step 1. Connect the top of the wash pipe to the kelly and start the mud pump. Circulate water (not drilling mud) down the wash pipe.
- Step 2. Let the screen move down as circulation continues and material washes. Take measurements and stop the descent of the screen when the packer is near the lower end of the casing. If drilling mud is still washing out of the well, continue pumping water until most of the mud is displaced.
- Step 3. Stop the pump and let the aquifer close in around the screen. When the formation develops friction on the outer surface of the screen, turn the entire string of wash pipe to the right to unscrew the left-hand joint at the bottom.
- Step 4. When the wash pipe is free, pump in more water. Raise and lower the string several times so that the lower end travels the full length of the screen. This action will wash out more drilling mud and some fine sand from the formation.
- Step 5. Start development work. Remove the wash pipe and continue the development work.
e. Bail-Down Method. With this method, you need special end fittings for the screen. Figure 6-10 shows an assembled bail-down shoe in the bottom of the screen. The bail-down shoe has a special nipple that has right-and left-hand threads and a coupling with right-and left-hand threads. Figure 6-11 shows a shoe with a guide pipe that extends below the screen.
Use the following procedures for the bail-down operation:
- Step 1. Start the operation after you sink the well casing to its permanent position. The casing's lower end should be slightly below where you install the top of the screen.
- Step 2. Assemble the bail-down shoe, special nipple, and special coupling in the bottom of the screen.
- Step 3. Screw a length of premeasured pipe into the right-hand half of the special coupling. This pipe, which will extend up through the screen, is called the bailing pipe or conductor pipe.
- Step 4. Screw one or more packers to the top of the screen.
- Step 5. Lift the whole assembly by the bailing pipe and lower the screen inside the well casing. Add lengths of bailing pipe as the screen descends until it reaches the bottom of the borehole.
- Step 6. Mark off the length of the screen on the bailing pipe that projects above the casing, using the top of the casing as the reference measuring point. Run a bailer or sand pump inside the bailing pipe and start bailing sand from below the shoe.
- Step 7. As you remove sand from below the shoe by the bailer, the combined weight of the screen and the string of bailing pipe will cause the screen to move downward. Attach additional weights to the bailing pipe, if necessary.
- Step 8. Monitor the progress of the work carefully, and stop the operation when the screen reaches the desired depth. The packer should be near the lower end of the casing, but still inside the casing. Accurate measurements will avoid sinking the screen too far.
- Step 9. Drop a weighted and tapered wooden plug (Figure 6-11) through the bailing pipe to plug the special nipple on the bail-down shoe. When the plug is in place, unscrew the left-hand threaded joint at the upper end of the nipple by turning the entire string of bailing pipe to the right. Remove the bailing pipe and proceed with well development.
If you use a different type of bail-down shoe, the left-hand threaded connection for the bailing pipe may be in the opening in the shoe or it may be in the packer fitting at the top of the screen. In either case, use the same procedures as above for operating the bailing down, plugging the bottom, and removing the bailing pipe.
Under certain conditions, you can bail down a well screen without using a bail-down shoe. The bailing pipe is not connected to the screen. You fit the pipe's lower end with a flange or coupling large enough to press on the packer at the top of the screen. The weight of the bailing pipe rests on the screen. You fit the lower end of the screen with an open ring or a short piece of pipe. Be very careful when using this method because the screen is not connected to the bailing pipe and you cannot control the screen's movement from the surface. Careful measurements will prevent sinking the screen too far. This method should be limited to fairly short screens. Plug the bottom of the screen by putting a small bag of dry concrete mix in the bottom and tapping the concrete lightly with the drill bit or other tool.
6-7. Placing Gravel. The most important criteria for grovel pack (artificial sand filters) are correct grain sizes and screen slot opening. Grading should be in proper relation to the grading of the sand in the aquifer. You could have trouble if you use gravel that is too coarse. Coarse, uniformly graded filter sand (about 1/8 inch) makes the best gravel pack for most fine-sand aquifers. You should use fine gravel (1/4-inch maximum size) to pack aquifers consisting of medium or coarse sand. Use a screen with openings that cover about 90 percent of the gravel pack. The following is a field method for producing a filter material or gravel pack from a sand and gravel deposit for a medium sand aquifer.
- Make two sieves with lumber. Cover one sieve with 1/4- to 3/8-inch hardware cloth. Cover the other sieve with window screen.
NOTE: A layer of hardware cloth under the screen provides extra strength to the sieve.
- Discard all material that will not go through the hardware cloth but that will go through the window screen. Save the materials that the screen retains for analysis and logging purposes.
a. Open-Hole Placement. Where drilling mud keeps the borehole open, you can install a gravel pack using the positive-placement method. This method is the most common and best suited to military field operations. Use the following procedures for this placement method:
- Step 1. Drill a large diameter borehole the full depth of the well.
- Step 2. Set a smaller diameter screen and casing centered in the large diameter borehole.
NOTE: Basket-type centering guides work best.
- Step 3. Fill the annular space around the screen with properly graded gravel.
- Step 4. Fill the borehole with gravel well above the top of the screen. Gravel works downward as sand and silt are removed from the formation around the gravel pack by subsequent development.
Development work must be thorough when you drill the borehole using the rotary method because the mud cake on the borehole wall is sandwiched between the gravel pack and the face of the formation. You must break up the mud cake and bring it up through the gravel into the well. Any mud cake not removed reduces the efficiency and yield of the completed well. To ensure that you remove all of the mud cake, limit the thickness of the gravel envelope around the screen to a few inches. A common mistake is to drill a very large borehole and use a small screen, making the gravel too thick for satisfactory results.
Another common mistake is to try and place gravel pack into a small annular space, such as 1 inch. The gravel pack usually bridges at a coupling and does not get down around the well screen. A 2-inch annular space is minimum; 3- to 5-inch spaces are best. Remember, the annular space is the difference between the outside of the casing and the wall of the borehole with the casing centered in the hole. In most cases, you must also consider the outside diameter of the couplings.
b. Tremie Placement. You can use a tremie pipe when placing gravel-pack materials. The fine and course particles should not separate, as in the open-hole placement, when the aggregate settles through the drilling fluid in the well. Lower a string of 2-inch (or larger) pipe into the annular space between the inner and outer casings. Feed the gravel into the hopper at the top of the pipe. Feed water into the pipe with the gravel to avoid bridging the material in the pipe. The pipe raises as the gravel builds up around the well screen. The tremie system is practical for placing the gravel pack in shallow to moderately deep wells.
c. Bail-Down Placement. With this method, you can place a gravel pack as you install the screen. Feed the grovel around the screen, it will go downward with the screen. The bail-down shoe used is somewhat larger than the screen so that gravel being added will follow down and around as the screen sinks in the formation. Figure 6-12 shows this operation. Development work is an essential part of this method. Screen openings must be larger than the grain size of the aquifer so enough aquifer sand will pass through and the gravel pack will replace the sand around the screen.
d. Double-Casing Placement. With this method, you place gravel and use a temporary outer casing (Figure 6-13). With this method, you pull back the casing as you pour gravel into the space. This method is somewhat similar to pull-back screen installation.
a. Formation Stabilizer. When you do not use a grovel pack you can place formation-stabilizer material to help prevent deterioration of the annular space outside the screen. Fine, loose strata may cave into that space, enter the screened interval, and degrade the well. The decision to use this material usually occurs during the well-construction process. In unstable formations, consider stabilizing wherever the annular space is more than 2 inches thick.
Grain size is important since you will develop the aquifer naturally, and as much as half of the stabilizing material could flow through the screen. The grain size should average slightly coarser than that of the aquifer and should be well distributed. Widely used formation stabilize are concrete or mortar sand. Use the following procedures when placing formation stabilizers:
- Step 1. Center the screen at its final position in the open section of the borehole, using a centralizer if needed.
- Step 2. Place the stabilizer material by dumping and tamping it down the hole or by using a tremie. Raise the level of stabilizing material above the top of the screen and add additional material as development progresses.
- Step 3. As you develop the well, the level of the stabilizing material drops as the material is pulled into the screen. You must replace this material to maintain the level above the top of the screen.
b. Unscreened Well. In competent rock, you usually tap the aquifer through numerous, irregularly spaced fractures. Once cleared of mud and rock fragments, the fracture stay open and the intake interval functions efficiently for a longtime. You should not need a screen in such a rock well. If you anticipate an unscreened-well design, you must be particularly attentive to the location of the top of the unscreened intake interval and its relation to the position and thickness of any impermeable layer.
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