Optimizing Borehole Construction: Casing and Well Screens

As water is pumped out of a borehole, the water level in the hole falls. It may fall by an amount known as the ‘pumping drawdown,’ which eventually stabilizes for that rate of extraction. If the water level does not stabilize and continues to drop until the borehole is ‘dewatered,’ the hole is being over-exploited. In this discussion, it is assumed that boreholes are designed with the intention of maximizing yield and efficiency, the normal requirements for everything other than hand-pump-equipped holes.

• The maximum yield of a borehole is defined as that yield which the borehole can sustain indefinitely before drawdown exceeds recharge from the aquifer.
• Borehole efficiency is technically defined as the actual specific capacity (yield per unit of drawdown: say, litres per second per meter) divided by the theoretical specific capacity, both of which can be derived from a pumping test. Specific capacity declines as discharge increases.

Borehole casing
Boreholes are constructed by inserting lengths of protective permanent casing. These are lowered or pushed into the hole by the drilling rig to the required depth; the lengths of casing may be joined together by means of screw threads, flange-and-spigot, glueing, riveting, or welding. Casing normally extends up to the surface, with a certain amount (say 0.7 meter) standing above ground level. Lengths of casing may be obtained in mild steel, stainless steel, and plastic (such as UPVC, ABS, polypropylene, and glass-reinforced plastics).

Plastic casings are more fragile and deformable than steel casings (especially the screw threads), and so should be used mainly for low-yield and shallow boreholes. The casing should be capable of withstanding the maximum hydraulic load to which it is likely to be subjected, that is, about 10 kilopascals (kPa) for each meter that extends below the water level down to the maximum expected drawdown.

Steel casing is available in a variety of grades and weights. Low-grade casing can be used for shallow tube wells, but heavy-duty, high-grade steel should be used for deeper boreholes (especially those more than 200 meters deep) and when ground conditions hamper insertion (such as coarse gravel/boulder formations). Special types of casing that can resist aggressive waters are also obtainable, but stainless steel is the best means of combating corrosion. Casing is usually supplied in standard lengths already equipped with screw threads or other jointing methods.

Borehole well screens
When a borehole has been dug alongside a water-bearing zone, the casing installed in it must have apertures that allow water to enter as efficiently as possible while holding back material from the formation. These perforated sections are known as boreholes or well screens; they come in sizes and joints similar to casing, so can be interconnected with suitable plain casing in any combination, or ‘string.’ Screens can also be obtained with a variety of aperture (slot) shapes and sizes, from simple straight slots to more complex bridge slots and wire-wound screens made with V-cross section wire.

Screen slots should be of regular size, aperture, and shape because they might have to efficiently prevent all particles of a certain size from getting through. Plain plastic casing can be easily slotted with a saw or special slotting machine, but beware again of drilling contractors cutting irregular, messy slots in steel casing with grinders or oxyacetylene torches. The open area of factory-made plastic screens commonly exceeds 10% of total surface area, but rough-cut holes in mild steel casing rarely take up more than 2 or 3%. Screen slots should be slightly smaller than the average grain size of the aquifer fabric, and should allow water to enter the borehole at a velocity within the range 1 to 6 centimetres/second (0.01 to 0.06 meter/second). Entrance velocity is defined as the discharge rate of the well divided by the effective open area of the screen. Too high an entrance velocity may lead to screen incrustation, excessive well losses, and other damaging consequences of turbulent flow conditions.

Formula to calculate the open area of a screen:

Open area of screen per meter of screen in

cm2 = l*w*n/10

where l is length of slot in cm, w is width of slot in mm, and n is number of slots per meter length.

For example, a minimum screen open area of 100 cm2 provides, roughly speaking, the minimum entrance velocity for a yield of about 0.3 litres/second (about 4gallons/minute). In practice, additional screen lengths should be included to allow for variations within the aquifer (which is unlikely to be homogeneous) and in the borehole. The most efficient well screens are the well-known ‘Johnson screens’ – continuous-slot types manufactured with V-wire wound spirally around a cage of longitudinal support rods. The whole structure may be composed of stainless steel or low-carbon galvanized steel. These wire windings have been constructed such that the slots widen inwards, which significantly reduces rates of screen clogging.

The effective open areas of Johnson screens are more than twice that of conventional slots, which allows more water to enter per length of screen. Slot sizes of 0.15 to 3 mm, diameters of 1½” to 32″, and screen lengths of 3 meters and 6 meters are available. The different grades of screen are suitable for a variety of borehole depths; the ends are plain (for welding) or screw-threaded. Johnson screens allow yields of about 5 to 6 litres/second per meter length, so that a 6-metre-length
can give about 30 to 35 litres/second and a 12-metre-length twice as much. Most projects, and especially those involving shallow or low-yield boreholes, require only basic PVC casing and screens to be installed.

Gravel pack
After the casing and screen string have been inserted, natural material will tend to fall from the walls of the borehole into the annular space, forming a natural backfill or ‘gravel pack’ that helps to filter incoming water. The screen slot sizes should be such that only the finer content of this backfill is allowed into the borehole; this can be washed out during development, leaving the coarser portion behind to act as a filter. Thus, an aquifer is suitable for the development of a natural gravel pack if it is coarse-grained and poorly sorted, as many alluvial gravels are (a relatively rare situation).

A borehole drilled into an unstable aquifer formation, or into one that is well sorted, and with a high proportion of fines (which would be apparent from the drill samples), will require an artificial gravel pack around the screens. When the only screens available on site are of a slot size larger than the average grain size of the aquifer, then a gravel pack should be installed. Unfortunately, time and other constraints do not normally allow a detailed grain-size analysis of the aquifer fabric to be carried out in the field; so, a degree of intuition is required here. If there is any uncertainty, install an artificial gravel pack.

Artificial gravel pack
Ideally, an artificial gravel pack should consist of clean, rounded, quartz ‘pea’ gravel supplied in bags; grains the size of small household peas are generally suitable. Coarse, well-worn river sand is often ideal; the grains should be a little larger but no more than twice the screen slot size. Being smooth and spherical, the grains should run easily down into the annular space without clumping and leaving gaps of air (a little water often helps).

The standard practice is to produce a 3 to 4-inch wide annular space for the gravel pack (say, a 6″ screen in a 12″ hole); the casing/screen string must be centered in the borehole. Most boreholes are not perfectly straight, so the casing will almost invariably be in contact with the wall in some places unless it is centralized. This is achieved by using manufactured centralizers (such as flexible ‘wings’) or some other suitable alternative.

Before pouring gravel pack material into the annular space, which must be done smoothly and without haste, calculate the volume of annular space (it is reassuring to see that the correct volume of gravel has been installed). Again, an accurate log of borehole size changes is required here. Pouring gravel pack into a borehole with a high water level usually results in displaced water rising in the hole and overflowing. Water overflow will abruptly stop as the screen becomes covered by gravel. Continue to pour gravel until you are certain that the top of the pack is well above the top of the screen.

The essence of borehole design is deciding the combination of plain casing and screens to be inserted and the type of screen to be used, and whether a gravel filter pack (or thin formation stabilizer) is required. Table 3.2 attempts to summarize these decisions for a variety of ground conditions that are likely to be encountered during drilling. An ‘open hole’ design is one in which no screen or gravel pack is used in the area of the aquifer, but all boreholes that this writer has encountered have required casing, to at least stabilize the superficial soils or weathered zone. Open holes are suitable mainly for hand-pumps, because of the danger of a powerful motor pump sucking in debris even from a stable hard rock formation. If a stable formation is encountered some way below a water strike, reducing the penetration rate, some extra borehole depth, to act as an openhole sump, can be created a little more quickly by reducing drill bit size.