Borewell development and cleaning after drilling

Why a newly drilled borewell needs development

When a DTH hammer drills through hard rock, it leaves fine rock powder in the fractures. In softer formations and weathered zones it can also leave a skin of crushed material on the wall of the hole. Cuttings that did not reach the surface settle at the bottom. If the well is handed over in this state, the pump draws silt, the impellers wear quickly and the apparent yield is lower than the formation can really give.

Development has three practical jobs:

• remove fines and loose cuttings from the hole, the casing and the slotted or open sections;
• open the fractures and water-bearing zones so water reaches the well with less resistance;
• settle the well so that later pumping does not keep pulling sand and silt.

A clear discharge for a few minutes is not proof that the job is finished. Development is complete when the discharge stays clear after repeated surging and the yield stops improving from one cycle to the next.

How air-lift development works

Air-lift is the most common development method on Indian borewells because the drilling compressor is already on site. Compressed air is injected into the water column through the drill rods or a separate air line. The air bubbles reduce the density of the water inside the pipe, so the heavier water outside pushes the mixture up and out at the surface, carrying fines with it.

The key idea is submergence: the depth of the air outlet below the standing water level compared with the total lift. The deeper the air outlet sits below the water, the more efficient the lift. With too little submergence the air simply bubbles through and lifts very little water. In practice the crew lowers the air line well below the dynamic water level and adjusts it as the water level falls during pumping.

The pressure needed to start an air-lift is set by the depth of water above the air outlet. Each 10 metres of water is roughly 1 bar (14.5 psi). Once water starts moving, the running pressure drops because the column is now lighter. This is why a compressor that drilled the hole comfortably usually has enough pressure to develop it, provided the air line is placed correctly.

Surging and back-washing

Steady air-lift alone tends to clean only the easiest paths. Surging works better. The crew shuts the air off, lets the water column fall back into the formation, then opens the air again so a slug of water is lifted hard. This back-and-forth movement loosens fines trapped in fractures and draws them into the hole. The cycle is repeated, with the air line moved up and down the open section so each zone is worked.

Compressor needs for development

Development is a lighter duty than drilling in terms of pressure, but it can run for many hours. The compressor must deliver steady flow without overheating, and the air must reach the bottom of the air line at a pressure that overcomes the water above it.

Typical borewell compressors in the 600–1,500 cfm range at 200–400 psig (13.8–27.6 bar) have far more pressure than most development jobs need. The point is control: the operator should be able to throttle the air and open or close it quickly for surging. A ball valve at the wellhead and a properly secured discharge pipe make this safe.

A machine that has just finished a long drilling run should be checked before development begins. Watch the discharge temperature: on an oil-injected screw compressor it normally sits about 75–95 °C, and a steady rise points to blocked coolers, low oil or a failing thermostatic valve. Safety shutdowns for high discharge temperature, low engine oil pressure and high coolant temperature must be working, not bypassed.

Step-by-step: a practical air-lift development sequence

1. Record the finished well. Note total depth, casing depth and diameter, open-hole sections, water strikes during drilling and the standing water level.
2. Set the air line. Lower the drill rods or a separate air pipe so the outlet sits well below the water level and near the zone to be worked. Do not rest it on the bottom where settled cuttings can block it.
3. Start gently. Open the air slowly until water begins to flow. Expect heavily silted discharge at first.
4. Surge in cycles. Close the air, allow the column to fall back, then open again. Repeat for several cycles at each depth.
5. Move through the open section. Raise the air line in stages and surge each zone, then return to the bottom to lift out what has settled.
6. Watch the discharge. Collect samples in a clear container at intervals. Note colour, sand content and how quickly it settles.
7. Measure yield. Once the discharge stays clear, measure flow with a container and stopwatch or a V-notch, and note the water level.
8. Stop when it stops improving. When repeated cycles give clear water and the yield is stable, development is done. Record the final observations.

Yield checks after development

An air-lift yield is a useful early indication, not a final rating. Air-lift draws water in surges and the flow depends on where the air line sits, so it can overstate or understate what a submersible pump will deliver over a long period.

Simple field checks

• Bucket and stopwatch: time how long the discharge takes to fill a container of known volume. Repeat several times and average.
• Weir or V-notch: for larger flows, a simple notch in a channel gives a steadier reading.
• Recovery: after stopping the air, watch how quickly the water level returns. Fast recovery suggests good connection to the aquifer.
• Clarity over time: a well that stays clear after repeated surging is ready; one that keeps producing sand needs more work or a review of construction.

For irrigation, municipal and industrial wells, the air-lift result should be followed by a proper pumping test with a pump, measured drawdown and recovery. That test tells the owner how hard the well can be pumped and which pump to install. Our page on borewell yield testing and production duty covers that stage.

Common problems and what they usually mean

Safety during air-lift and cleaning

Development runs at lower pressure than drilling, but the hoses still carry enough energy to injure. Use hoses rated above the compressor’s maximum pressure, fit whip checks on every joint, and never uncouple a line until the pressure gauge reads zero. Direct the discharge away from people, vehicles and the rig controls; slugs of water and stones can be thrown several metres. Keep the compressor upwind of the wellhead where possible so the air intake does not draw in spray and dust.

When to clean or re-develop an older borewell

Wells that have run for years often lose yield as fines, scale or iron deposits build up in fractures and around the pump. Air-lift cleaning can restore part of the lost flow when the problem is silt or loose deposits. It will not fix a well where the water table has dropped or the aquifer is exhausted. Before re-developing, compare the current static level and yield with the original records if they exist. Remove the pump, check for obstructions and confirm the casing is intact before lowering an air line.

Air-lift design: submergence, air pipe depth and air volume

An air-lift works well or badly depending on one number: the submergence ratio. It is the depth of the air injection point below the pumping water level, divided by the total length from the injection point up to the discharge.

Submergence ratio S = hs ÷ (hs + hlift), where hs is the running submergence and hlift is the height the water must be raised from the pumping level to the discharge.

Below about 40% the lift becomes weak and air-hungry; most practical air-lifts are set for 50–70%. The air pipe depth follows directly: injection depth = lift ÷ (1 − S).

Two pressures matter and they are different:

• Starting pressure is set by the water standing above the injection point before the lift starts, which is measured from the static water level. Allow about 0.098 bar per metre, plus hose and air line losses.
• Running pressure is lower, because once water is flowing the level in the well draws down to the pumping level and the column inside the eductor is aerated.

Air needed per m³ of water lifted

The useful work is lifting the water (ρ·g·h). The air does that work by expanding from injection pressure down to atmosphere, which gives at most p0·V0·ln(p1/p0) per volume V0 of free air. Real air-lifts convert only part of that energy, so:

Free air per m³ of water = ρ·g·hlift ÷ [η × p0 × ln(p1/p0)], with p0 = 1.01325 bar(a) = 101,325 Pa and p1 the absolute running pressure at the injection point. The efficiency η is an assumption, not a constant: values of about 0.35–0.45 are commonly assumed for a well-proportioned eductor at good submergence, and η falls sharply when submergence is low. The η values in the table below are assumed for illustration; confirm a real installation by measuring air and water flow.

The table assumes a pumping level 30 m below ground with discharge at ground level, so the lift is 30 m. Pressures are hydrostatic only (0.0981 bar per metre), and η is the assumed value in the fifth column. Deeper injection costs pressure but saves a great deal of air.

Worked example (illustrative)

A 165 mm borewell has a static water level of 15 m and an expected pumping level of 30 m. Discharge is at ground level, and the target is 20 m³/h of water through an eductor pipe.

1. Choose S = 60%. Lift = 30 m, so the injection depth is 30 ÷ (1 − 0.6) = 75 m below ground so running submergence hs = 75 − 30 = 45 m. Check: 45 ÷ (45 + 30) = 0.60.
2. Starting pressure. At start the water stands at the static level, so the column above the injection point is 75 − 15 = 60 m. 60 × 1000 × 9.80665 = 588,399 Pa = 5.88 bar(g). Add about 0.5 bar for the air line and hose, so you need roughly 6.4 bar(g) available to start.
3. Running pressure. 45 m × 0.0981 = 4.41 bar(g). That is 4.41 + 1.013 = 5.43 bar(a).
4. Ideal air. Lift work = 1000 × 9.80665 × 30 = 294,200 J per m³ of water. Expansion energy per m³ of free air = 101,325 × ln(5.43 ÷ 1.013) = 101,325 × 1.678 = 170,030 J. Ideal ratio = 294,200 ÷ 170,030 = 1.73 m³ of free air per m³ of water.
5. Real air. With an assumed η = 0.42 the ratio is 1.73 ÷ 0.42 = 4.12 m³ per m³. For 20 m³/h this is 20 × 4.12 = 82.4 m³/h of free air = 1.37 m³/min = about 48.5 cfm (1.373 × 35.315).

The answer is small. It shows that, with a proper eductor pipe, the air-lift itself is limited by pressure, not flow. The 600–1,500 cfm machines used for development, as described earlier on this page, carry far more air for other reasons. When air is blown down the drill rods with no eductor, the whole bore acts as the lift pipe, slip between air and water is high, and efficiency can be well under half the figures above. Surging and carrying out sand and chips also need high, short bursts of flow.

Development methods compared

Air-lift is the default for hard-rock borewells in Telangana and Andhra Pradesh, but it is one of several methods. In screened wells in alluvium or sandy aquifers, the choice matters more.

In practice contractors combine methods: air-lift to clean out, surge zone by zone, then air-lift again to remove what the surging loosened.

When is a well “developed”? Measurable end points

• Sand content. Take a known volume (for example 10 litres) at the end of each surge-and-pump cycle. Let it settle in a cone or bucket, then dry and weigh the sand. For water, sand in mg per litre is very nearly ppm by mass. Many drinking-water specifications look for a few ppm; irrigation wells usually accept more. Agree the figure before work starts.
• Turbidity. A portable turbidity meter reading in NTU is better than visual judgement. For a drinking-water well, IS 10500 is the reference: it sets an acceptable limit of 1 NTU and a permissible limit of 5 NTU where no alternative source is available. Those limits apply to the water supplied, so treat them as the target the developed well should approach, not a pass mark on a single sample. Turbidity should fall cycle by cycle and then level off.
• Specific capacity. Discharge divided by drawdown, for example m³/h per metre. Measure it at the same discharge after each cycle. Development is working while specific capacity rises. It is complete when two or three consecutive cycles agree within about 5–10%.
• Recovery. After air is shut off, a developed well recovers to static level faster and more consistently than it did on the first cycle.

“Water looks clear” is not an end point. Record the cycle number, time, discharge, drawdown, sand and turbidity in one table. That record is far more useful to the pump installer than a single “yield” figure.

Compressor selection: development versus drilling

The same machine often does both jobs, but the duties are different. Knowing which limit applies stops you hiring the wrong size.

A high-pressure drilling compressor will do development work. Set its pressure regulation to what the air-lift actually needs, and keep a pressure gauge at the well head as well as on the machine. If starting pressure is not enough, the usual fix is to inject the air higher, then lower the air pipe in stages as the water column is lightened, rather than hiring a bigger machine. For the drilling side of the comparison, see drilling depth and compressor selection.

Air-lift hazards in numbers: blowout, casing collapse and air entry

• Sudden unloading (blowout). When the starting pressure finally breaks through, the whole water column above the injection point can leave the well in seconds. At 60 m of submergence in a 165 mm bore, that is π/4 × 0.165² × 60 = 1.28 m³ of water, roughly 1.3 tonnes. Size the discharge deflector or bend for that slug, and open the air valve slowly so it leaves as a surge rather than a single slug.
• Casing collapse. Air-lifting lowers the water inside the casing while groundwater outside stays near static level. Every 10 m of difference puts about 1 bar of external pressure on the casing. Thin PVC casing and screens have limited collapse strength, so check the maker’s collapse rating against the largest drawdown you will create, and stage the drawdown.
• Air in the formation. In a screened well, air injected below or across the screen can enter the aquifer, reduce permeability near the well and give false yield readings. Keep the injection point inside blank casing above the screen.

How Reddy Compressors helps

Reddy Compressors, based at Tarnaka in Secunderabad, supplies all types, all brands and all models of borewell and DTH compressors to rig owners, drilling contractors, farmers and project teams: new sales, used and refurbished machines, rentals, service and spare parts. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply compressors, rentals and spares across India.

• Rental for development and cleaning: high-pressure diesel compressors from our own fleet for drilling, development or well rehabilitation, hired by the day, week or month. See borewell compressor rentals and hire.
• New and used machines: we supply new, used and refurbished borewell compressors from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others as an independent multi-brand specialist.
• Service and repair: our 20-plus service engineers and technicians check coolers, oil, controls and safety shutdowns so the machine holds temperature through long development runs. On our own hire jobs we change oil and service every 300 running hours.
• Spare parts: filters, separators, valves and hoses identified from the machine’s nameplate and fit confirmed before we quote.

What to send us

• Well depth, casing depth and diameter, and standing water level if known
• Whether you need the compressor for drilling, development only, or cleaning an old well
• Site location (district and village), access for a towed compressor and expected duration
• For an existing machine: make, model, serial number and a nameplate photo
• Any symptoms, such as overheating or low pressure, with gauge readings

Frequently asked questions

How long does borewell development take after drilling?

It depends on the formation, depth and how much fine material the drilling left behind. Some hard-rock wells clear within a short session; wells in weathered or fractured zones can need several hours of surging. The useful rule is to keep cycling until the discharge stays clear and the measured yield stops improving between cycles, rather than stopping at a fixed time.

What pressure compressor do I need for air-lift development?

The compressor must overcome the water column above the air outlet, which is roughly 1 bar (14.5 psi) for every 10 metres of submergence, plus losses in the hose and pipe. Most borewell drilling compressors rated 200–400 psig have ample pressure. Flow and the ability to control the air for surging matter more than maximum pressure on most development jobs.

Why does my borewell still give muddy water after development?

Persistent muddy water usually means an unstable zone, a collapsing wall, a poor casing seal or development that only worked part of the open section. Try moving the air line through each zone and surging rather than running steadily. If it still does not clear, stop and review the well construction before investing more compressor hours in it.

Is an air-lift yield the same as the real borewell yield?

No. Air-lift yield is an early field indication and can differ from what a submersible pump will sustain, because flow depends on air-line depth and the surging pattern. For irrigation, industrial or municipal wells, follow it with a pumping test that measures drawdown and recovery over time. That result should decide pump size and setting depth.

Can an old borewell with low yield be cleaned with a compressor?

Often, yes, when the yield loss comes from silt, loose deposits or fines blocking fractures. Remove the pump, check the casing and depth, then air-lift and surge in stages. Cleaning will not help where the water table itself has fallen or the aquifer is depleted, so compare current water levels with the original records before deciding.

Can I hire a compressor only for borewell cleaning?

Yes. Reddy Compressors hires diesel compressors from its own fleet for drilling, development and rehabilitation work across Telangana and Andhra Pradesh, with supply elsewhere in India where feasible. Send the well depth, water level, site location and expected duration, and we will suggest a suitable capacity and the hoses and fittings needed for the job.

What is air-lift development of a borewell?

Air-lift development means injecting compressed air into a new or old well through a pipe set below the water level. The aerated column is lighter than the water outside, so water, fines and drilling chips are pushed up and out. Repeated cycles of air-lifting and surging clean fractures or screens and settle the formation around the well. Development is finished when sand content, turbidity and specific capacity stop improving between cycles, not simply when the water first looks clear.

How much air does air-lift development need per m³ of water?

It depends mainly on submergence. For a 30 m lift with about 60% submergence, expect roughly 4 m³ of free air per m³ of water through a proper eductor pipe. At 40% submergence it rises to about 8 m³. In the illustrative example above, 20 m³/h needs only about 48 cfm, but about 6.4 bar(g) to start. Blowing down open drill rods without an eductor is much less efficient, and surging needs extra flow in bursts.

What submergence is needed for an air-lift to work?

Aim for 50–70% running submergence. That means the injection point is at least as far below the pumping water level as the water has to be lifted. Below about 40% the air-lift becomes erratic and uses a great deal of air. Above 70% it is efficient, but starting pressure rises: about 1 bar for every 10 m of water above the injection point, measured from the static level, plus line losses.

How do I know when borewell development is complete?

Take readings at the end of every cycle at the same discharge: sand in a measured sample, turbidity in NTU, and specific capacity (discharge divided by drawdown). Development is complete when sand and turbidity have levelled off at an agreed figure and specific capacity agrees within about 5–10% over two or three cycles. Write the readings into the well record, because the pump installer needs them.

Related pages

• Borewell and DTH compressor library
• Borewell compressor rentals and hire
• Borewell yield testing and production duty
• Wet hole drilling and water ingress
• Borewell compressor service and repairs