Well Construction

Water Well Casing & Screen: How to Choose for Boreholes

By Windrilling Technical Team – Updated October 2026 – 12 min read

A driller in West Africa sent me a photo last season. Four-inch PVC casing. An eight-inch hole. He wanted to know why his pump would not go in.

Nothing in his order was wrong. The casing was sound. The screen was sound. He had measured the hole himself. Every part passed inspection on its own.

That is the trap with casing and screen. The parts are easy. The intersections are hard. A casing string that is perfect in the catalogue fails the moment it meets a pump, a gravel pack or a collapsing layer of sand.

I have quoted casing on enough boreholes to know the order you decide things in matters more than the catalogue you decide from. So this guide follows the order of a real job: hole, casing, screen, gravel, seal. If you get to the end and every choice still fits the choice before it, the well will last.

What Casing Is Actually For

Casing does three jobs, and they pull in different directions. It holds the hole open. It isolates the water you want from the water you do not. And it has to leave room for a pump, a screen and a rising main.

• Structural. It stops loose ground from closing the hole.
• Hydraulic. It keeps shallow, dirty or salty water out of the aquifer you tapped.
• Mechanical. It carries the screen, the pump and the pipe that lifts the water.

Once you see casing as a container for a pump, the sizing argument stops being academic. The pump decides the casing. The casing decides the hole. Work it backwards and you rarely get it wrong.

Sizing From the Pump Outwards

Start with the production pump body diameter. Then leave clearance. Then pick the casing. Then pick the hole. That is the whole method.

Pump body OD	Casing ID you need	Nominal casing	Minimum hole for gravel pack
75 mm (3 in)	Roughly 90-100 mm	4 in class	150-200 mm
100 mm (4 in)	Roughly 115-130 mm	5-6 in class	200-250 mm
150 mm (6 in)	Roughly 170-200 mm	8 in class	250-350 mm

Treat those bands as planning ranges, not rules. Two things matter more than the exact number. First, a submersible pump has a cable clamped to its side, and the cable eats clearance. Second, the pump needs cooling flow past the motor, so a pump jammed into a casing runs hot and dies early.

The hole diameter is set by the gravel pack, not by the casing. If you plan to gravel pack, the annulus is the design driver. If you do not, you can drill closer to the casing OD. That single decision changes your rig, your air requirement and your cost per metre, which is why I always ask about it before quoting a machine. Our water well drilling rigs are specified against hole diameter and depth for exactly this reason.

PVC, uPVC or Steel?

Most shallow and mid-depth water wells are cased in PVC. It is cheap, it does not corrode and it is light enough to handle without a crane. Steel is stronger, which matters when the hole wants to squeeze, but it costs more and it needs corrosion protection in aggressive water.

Material	Strength	Corrosion	Cost	Where it fits
PVC / uPVC	Low to moderate	Very good	Lowest	Shallow to mid-depth wells, stable and mildly unstable ground
Steel	High	Needs coating or thicker wall	Highest	Deep wells, squeezing ground, large diameters
Stainless screen on steel or PVC body	Moderate	Very good	High	Fine sand, long service life, potable water

Whatever you choose, check that it is rated for potable water. Some recycled PVC pipe carries no such approval, and the difference is invisible on site.

Screen Types: Slotted, Louver and Continuous Slot

Screen type	Open area	Sand control	Best for
Slotted PVC	Low to moderate	Moderate	Coarse sand, gravel-packed wells, low cost
Louvered	Moderate	Moderate	Thick aquifers where strength matters
Continuous-slot wire-wound	High	Best	Fine and uniform sand, high yield wells, long life

Open area is not a marketing number. It decides how much water enters the well and how much head loss you pay for. A screen with half the open area needs more drawdown to give the same yield, and more drawdown means more pumping cost forever.

Slot Size: The Part Most People Get Wrong

Slot size is not chosen by feel. It is chosen from a sieve analysis of the formation, or from the sieve analysis of the gravel pack you are installing around the screen.

The old rule of thumb from water well practice is that a gravel-packed screen should retain most of the gravel. In plain terms: the slot should be small enough to hold back the bulk of the filter pack, and large enough to pass water without choking. Slot openings are usually expressed in millimetres or in thousandths of an inch.

Formation / pack	Typical slot opening	Note
Fine uniform sand with gravel pack	Around 0.5-1.0 mm	Pack does the filtering, screen holds the pack
Medium sand with gravel pack	Around 1.0-2.0 mm	Most common water well case
Coarse sand and gravel, no pack	Around 2.0-3.0 mm	Only where the aquifer is naturally graded
Fissured rock	Slotted liner or open hole	Screen is for stability, not for sand control

Two cautions. First, an oversized slot is a slow disaster: the well pumps sand, the pump wears, and the gravel pack slowly empties. Second, an undersized slot is a fast one: yield drops, the pump starves, and the operator blames the rig. If you have no sieve analysis, spend the money on one before you order screen. It is cheaper than a re-drilled well.

Gravel Pack: When It Helps and How to Size It

A gravel pack is a graded filter placed between the screen and the formation. You need one when the aquifer is fine, sorted sand that would otherwise flow into the well. You do not need one in competent rock or in well-graded gravel.

The sizing logic has two steps, and the order matters. First size the gravel against the formation, using the finer part of the formation grading curve as the reference and picking a pack several times coarser than that. Then size the screen slot against the gravel, so the screen retains the pack. Do it the other way round and the pack will not hold the formation, no matter how neat the slot looks.

Ask any competent well designer for the exact multipliers and you will get slightly different numbers, because the right answer depends on how uniform the sand is. That is normal. What is not normal is installing a pack with no grading data at all.

Sealing the Annulus and Protecting the Water

Between the casing and the hole there is a gap. Water travels in it. If you leave that gap open, surface water, shallow dirty water and anything spilled at the well head can run straight down the outside of your casing into the aquifer.

• Above the screen: place a bentonite or cement seal so the annulus is blocked, not merely filled.
• At the surface: build a sanitary seal and a proper well head so runoff cannot pond at the casing.
• Below the seal: backfill with clean, graded material, and never with drill cuttings.

Grouting is the part of a well nobody photographs and everybody regrets. It is also the cheapest insurance against a contamination problem you can never clean up. The same discipline applies during the drilling stage; our step-by-step guide to the water well drilling process covers where the casing and seal sit in the sequence.

Running the String Without Losing It

Casing running is a crane job in slow motion, and it fails in slow motion too. The classic mistakes are simple.

1. Centralizers keep the string off the wall. Space them so the string hangs near the middle of the hole, especially through the screen interval.
2. Lower slowly. A dropped joint cracks screens and splits threads.
3. Check the bottom. If the hole is not clean to grade, the string will hang and you will force it.
4. Fit a casing shoe on the leading joint. It guides the string and protects the thread, and it is cheap compared with a failed run. We stock them as a standard item in the drilling tools range.
5. Do not rotate or hammer a PVC string to get it down. It will not survive the favour.

A Selection Workflow for the Next Job

1. Confirm the target yield and the pump size you expect to install.
2. Set the casing diameter from the pump, with real clearance for the cable.
3. Decide whether the aquifer needs a gravel pack. This sets the hole diameter.
4. Take a sieve analysis of the formation, or specify a graded pack with test data.
5. Size the gravel against the formation, then size the slot against the gravel.
6. Choose the screen type by sand uniformity and required open area.
7. Design the annular seal and the surface sanitary seal before you drill.
8. Write all of it on the drilling programme, not on the back of your hand.

Five Mistakes That Cost the Most

I have seen all five of these on real sites, and I have made two of them myself.

1. Casing sized before the pump. The order is the mistake, not the number.
2. Slot size guessed. No sieve analysis, no defence when the well makes sand.
3. Open annulus. Surface water finds the easiest path down, every time.
4. No centralizers. The screen ends up against the wall and half of it never sees water.
5. Cuttings used as backfill. It looks tidy and it ruins the seal.

Questions I Get Asked Most

Can I use the same pipe for casing and screen?

Yes, if you are in competent rock and only need a stable liner. For sand aquifers, no. Casing is meant to be closed, the screen is meant to be open, and confusing the two is how wells make sand.

How far above the screen should the casing stop?

The screen should face the producing interval, and the casing should blank off everything above it. Getting the screen opposite the wrong layer is a design error you cannot fix from the surface.

Does the rig choice affect the casing programme?

Yes, and more than most buyers expect. Hole diameter, depth and whether you need a mud programme all come from the casing and gravel design. That is why we ask for the casing programme before we recommend a water well drilling rig. A mud pump sized for the wrong annulus will either fail to clean the hole or wash it out.

What comes after the casing goes in?

Development and testing. Casing a well is not finishing it. The well has to be cleaned until it runs clear, then tested to prove what it will actually deliver. That is a separate job with its own methods, and it deserves its own guide.