Wet hole drilling: water back-pressure and borewell air

What changes when a borewell hits water?

In dry rock, a DTH hammer exhausts into an open annulus and the air carries cuttings to the surface with little resistance. Once the bit passes a water-bearing fracture, water flows into the hole and starts to stand above the hammer. The hammer then exhausts into a column of water, and the compressor has to overcome that column before any air or cuttings come out at the top.

Three things happen at the same time. The pressure across the hammer falls, so each blow carries less energy and penetration slows. The return flow turns from dust to a heavy mixture of air, water and cuttings that needs more energy to lift. And the hole wall, now wet, can start to soften, slough or swell in weathered zones and clay bands.

Across Telangana and Andhra Pradesh, many borewells pass through weathered granite or basalt before reaching fractured hard rock, and the main water usually comes from fractures at depth. That is exactly where the hole becomes deep, wet and pressure-hungry at once, so the air package chosen for the dry top section is often not the one that finishes the hole.

Back-pressure: the 10 m rule of thumb

For a static column of fresh water, the pressure at the bottom is about 0.98 bar for every 10 m of vertical head. In practice drillers round this to 1 bar (14.5 psi) per 10 m. What matters is the submergence: the height of water standing above the hammer, not the total depth of the hole.

These are static figures for a clear water column. While drilling, the column is a mixture of air, water and cuttings, which is lighter than solid water, so the real back-pressure while circulating is usually lower. But at the moment you restart after a rod change, the column has settled and the compressor must lift something close to the full static head. That is why many crews find a wet hole “will not start” even though it was drilling a minute ago.

A simple way to think about the margin: the hammer works on the difference between the pressure arriving at it and the pressure in the hole around it. If the compressor delivers 17 bar at the rig and loses perhaps 1–2 bar through hoses, swivel and drill string, and the hole holds 8 bar of water, the hammer is working on what is left. As submergence rises, that difference shrinks and blow energy falls with it.

How to read the signs on the rig

• Discharge pressure on the gauge climbs as the hole deepens, without any change of hammer or bit.
• Penetration rate falls sharply after a water strike, more than hardness alone explains.
• Return flow pulses or surges instead of flowing steadily; slugs of water are thrown out between quiet spells.
• The compressor stays loaded at full output with no unloading, and safety valves may lift on high-pressure units.
• After a rod change, the hole takes a long time to “blow clear” or does not clear at all.
• Cuttings come up as fine slurry and the bit shows regrinding wear, a sign that chips are not leaving the bottom.

Record the depth of each water strike, how quickly the flow developed and the gauge readings before and after. Keep these as observations, not as a yield figure; the flow seen while air-drilling an open hole is not the tested yield of the finished well.

When to step up pressure, add a booster or change the method

There is no single bar-per-metre rule that replaces judgement, because the right answer depends on the hammer rating, the rig, the hole diameter and how much water is coming in. But the decision usually follows a clear sequence.

1. Check the machine is delivering. Before blaming the water, confirm the compressor reaches its rated pressure and flow. A tired airend, a sticking inlet valve or a derated engine looks very similar to a deep wet hole. See our borewell compressor performance diagnostics guide.
2. Estimate the submergence. Use the static water level measured after a pause, or the water-strike depth if the hole fills quickly. Convert at about 1 bar per 10 m.
3. Compare against your pressure band. If submergence back-pressure plus line losses approaches the compressor’s working pressure, the hammer has little left. That is the point to step up.
4. Step up pressure. Move from a 200–250 psig (13.8–17.2 bar) machine to a 250–300 psig (17.2–20.7 bar) or 300–400 psig (20.7–27.6 bar) class, within the hammer’s rated pressure.
5. Add a booster. A booster takes the air from a primary compressor and raises its pressure further, with special units reaching up to about 35 bar (507.6 psig). It suits contractors who already own a good primary machine and meet deep wet holes only on some jobs.
6. Review the method. For heavy inflow, very deep water or unstable ground, foam, a larger annulus, casing off the upper water zone or a different drilling method may be more sensible than adding pressure.

Remember that pressure costs energy. As a working figure, each extra bar of discharge pressure takes roughly 7% more compressor power, so running every hole at maximum pressure “just in case” burns fuel in the dry sections. Operating at the pressure the hole actually needs, and raising it only below the water, is usually the economical approach.

Flow still matters in a wet hole

Pressure alone does not clean a hole. The air must also travel up the annulus fast enough to lift water and cuttings. A larger bit in the same rod size creates a bigger annulus, which needs more free air delivery (FAD) to keep the uphole velocity up. When water enters, the mixture is heavier, and a package that cleaned a dry hole comfortably can leave cuttings settling once the water comes in.

This is why the typical bands pair pressure and flow: around 600–900 cfm at 200–250 psig for shallower and smaller work, 800–1,200 cfm at 250–300 psig for mid-range jobs, and 1,000–1,500 cfm at 300–400 psig for deep, large or wet holes. Matching the hammer’s air consumption at the expected pressure, and then adding margin for the annulus, is covered on our DTH hammer air requirement guide.

Keeping the hole stable once water comes in

Water in a borewell is not only a pressure problem. In weathered rock, laterite and clay layers, wet walls can collapse onto the hammer, trap the string or enlarge the hole until the air can no longer lift cuttings. Good practice includes:

• Casing the overburden and weathered zone before drilling into water-bearing rock, so that loose material cannot fall in.
• Keeping air on while pulling rods or making connections where the ground is unstable, so that the bit is not left sitting in settled cuttings.
• Avoiding long pauses at the bottom of a wet hole; blow clean before stopping.
• Watching for sudden changes in return colour or volume, which can signal wall caving.
• Not chasing more pressure in a hole that is collapsing; extra pressure can erode a soft wall further.

Using foam in wet or unstable holes

Foam is a mixture of water and a foaming agent injected into the air stream. It increases the carrying capacity of the returns, so cuttings and water are lifted at a lower uphole velocity than with dry air. It can also help hold a fragile wall together and reduce erosion.

Foam suits large-diameter holes, heavy water inflow and broken ground. It needs an injection pump on the rig, correct mixing, and a plan for handling the foam and slurry at the surface. Foam and water injection also reach the hammer, so check the hammer manufacturer’s guidance on lubrication and wet operation; some hammers need adjusted oil feed when running wet. Our page on DTH lubrication and air treatment interfaces covers the air-side checks.

What the compressor goes through in wet drilling

Deep wet holes keep a borewell compressor at full load and maximum pressure for long spells. In an Indian summer, with ambient temperatures of 40–45 °C, above 45 °C on open sites, and dust around the rig, that is the hardest duty a portable machine sees. Normal discharge temperature on an oil-injected screw is about 75–95 °C; sustained high pressure in hot weather pushes it towards the shutdown limit if coolers are dusty or oil is tired.

• Clean oil and engine coolers, and air intake filters, more often during deep-hole work.
• Check that the safety shutdowns for high discharge temperature, low engine oil pressure and high coolant temperature are working, not bypassed.
• Drain the separator receiver and check oil level and condition; water carry-back during sudden stops can contaminate oil.
• Use a non-return valve at the compressor outlet so that the hole cannot blow back into the machine when it stops.
• Keep to the service interval; on our hire jobs we change oil and service every 300 running hours.

Estimating water inflow from the rise between rod changes

Every rod change is a free inflow test. When the air is shut off, groundwater fills the annulus around the drill string. How fast it rises tells you roughly how much water the hole is making. That figure decides whether you can keep drilling on air, need foam, or should stop and case. The back-pressure this water adds, and the pressure table for it, are covered on DTH compressor selection. This section is about the flow of water, not its head.

Method

1. Use this only in stable ground. Where the walls are unstable, keep air running during rod pulls and connections, as advised above, and take readings from stable sections instead.
2. Before shutting off air, blow the hole clean and lift the bit off bottom.
3. When the air is off, start a stopwatch. Measure the water level in the annulus with a dip meter or a weighted tape at the start and again after a fixed time, for example 2 and 4 minutes.
4. Annulus area = π/4 × (hole diameter² − rod outer diameter²). With a working check valve in the hammer, water is kept out of the rods, so only the annulus fills.
5. Inflow = annulus area × rise ÷ time.

Worked example (illustrative)

The hole is 165 mm, the rods are 89 mm, and the water rises 6 m in the first 4 minutes after shutting off air.

• Hole area = π/4 × 0.165² = 0.02138 m². Rod area = π/4 × 0.089² = 0.00622 m². Annulus = 0.02138 − 0.00622 = 0.01516 m².
• Volume in 4 minutes = 0.01516 × 6 = 0.0910 m³ = 91 litres.
• Inflow = 91 ÷ 4 = about 22.7 litres/min = about 1.36 m³/h.

Inflow slows as the level rises, because the head driving water into the hole falls, so take the reading early. The early figure is the better estimate of what the hole makes while it is being blown dry. Two readings that fall sharply (say 6 m in the first 4 minutes, then 2 m in the next 4) point to a small fracture that drains quickly. Readings that stay steady point to a strong, connected zone.

This is a drilling-control figure, not a well yield. Record it with the depth and the time of each water strike, and keep the yield test for after development.

The same annulus figure also shows what each restart must clear. Forty metres of water in this annulus is 0.01516 × 40 = 0.61 m³, about 600 kg. The compressor must lift that slug before the hammer can work again. That is why a slow blow-clear after rod changes is one of the first signs of a wet hole.

Foam and surfactant injection: dosing basics

Foam is introduced on the page above as a method. This section covers how the dose is worked out on site. Water and foaming agent (surfactant) are pumped into the air line at the rig by a small positive-displacement injection pump.

These are starting ranges only. The foamer maker’s data sheet governs the actual dose.

Worked dose (illustrative)

Mist at 15 litres/min of injection water with 0.5% foamer:

• 15 × 0.005 = 0.075 litres/min = 75 mL/min of concentrate.
• 75 mL/min × 60 = 4.5 litres per hour.
• Over an 8-hour shift: 4.5 × 8 = 36 litres.

At 1% the same injection rate needs 9 litres per hour. Plan the drum count for the shift.

Three site effects change the dose:

• Formation water dilutes the foam. If the hole makes 1.36 m³/h (22.7 litres/min, as in the worked example above) and you inject 15 litres/min, the water in the return is more than doubled. The effective concentration falls from 0.5% to about 0.2%. Raise the concentration as inflow rises.
• Hard or saline groundwater, which is common in parts of Telangana and Andhra Pradesh, kills foam. Use a foamer rated for hard water, or raise the dose.
• Too much foam wastes chemical and can slow penetration. The target is a steady discharge, not a foam mountain at the collar.

How the hammer behaves in water

• Non-return (check) valve. When the air is shut off, the hammer’s check valve stops water and cuttings from flowing back through the bit into the hammer and rods. A worn or stuck valve lets slurry in. The next start is then slow, rough or impossible, and grit scores the piston and cylinder. If restarts get harder over a shift, inspect the check valve first.
• Flush before you hit. After a rod change, start the air with the bit just off bottom and blow until the returns run steady. Then drop the bit back down. Landing on a bed of settled cuttings under water blocks the bit air holes and leads to regrinding.
• Do not stop on bottom. Lift off bottom and blow clean before shutting off air, so cuttings do not settle round the bit.
• Lubricant washout. Water entering the air stream, including from mist or foam injection, washes rock drill oil off the piston and wear surfaces. The oil can also form an emulsion that does not protect. Use a rock drill oil that tolerates water. Check that oil film is visible on the bit shank at each rod change. Dose by the hammer maker’s rate for wet drilling; see DTH lubrication and air treatment.
• Foam in the hammer. Some hammers tolerate foam better than others. Excess foamer in the injected water can reduce hammer power. Follow the hammer maker’s guidance on injection.

Matching the response to what the hole is doing

Switching to mud is a change of rig set-up (pump, pits, bit), not a compressor adjustment. Raising compressor pressure does not fix caving ground. It usually makes it worse.

Wet hole operator checklist

The compressor-side precautions (cooler and filter cleaning, draining, shutdown checks and the outlet non-return valve) are covered earlier on this page. One point to add: connect the foam or mist injection pump downstream of the compressor’s non-return valve, never on the machine side, so injected water cannot reach the compressor.

1. At every rod change in stable ground: blow clean, lift off bottom, shut the air off, then time the water rise and log it with the depth.
2. Restart off bottom and wait for steady returns before landing the bit.
3. Check the oil film on the bit shank and adjust the lubricator if it is dry.
4. Check the foam concentration against the latest inflow figure.
5. Watch for tight rods, oversize chips and collar caving, which call for casing.
6. Stop and reassess if restart time keeps growing over successive rod changes.

How Reddy Compressors helps

For borewell rig owners we supply all types, all brands and all models of borewell compressors: new sales, used and refurbished machines, rentals, service and spare parts.

We work with rig owners, drilling contractors, irrigation and government water projects, and industrial and real-estate borewell work across Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India. As an independent multi-brand specialist we sell, rent and service borewell and DTH compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others.

• Review your hole depths, water levels, hammer and bit to see whether your present package has enough margin.
• Supply new high-pressure borewell compressors in the 200–400 psig bands, and arrange special units up to about 35 bar where the job needs it: see new borewell compressor sales.
• Hire high-pressure units from our own rental fleet for a deep-hole season or a single project.
• Test and service your existing machine with our team of more than 20 service engineers and technicians, so you know it is delivering before you spend on more pressure.
• Identify and supply spare parts from the nameplate and old part, with fit confirmed before quoting.

Our office is at Tarnaka, Secunderabad; call +91 99590 60718 or +91 92468 88812.

What to send us

• Compressor make, model, rated pressure and flow, and a nameplate photo.
• Hammer and bit size, rod size and rig type.
• Typical hole depth, depth of water strikes and the static water level if measured.
• Gauge readings in dry and wet sections, and the penetration rate before and after water.
• Whether you use foam or water injection, and whether the top section is cased.
• Whether you want to buy, hire, add a booster or have your present machine checked.

Frequently asked questions

How much pressure is lost per 10 m of water in a borewell?

A static column of fresh water adds about 0.98 bar for every 10 m of height, usually rounded to 1 bar or 14.5 psi. What counts is the height of water standing above the hammer, not the full depth of the hole. While circulating, the air-water mixture is lighter, so the working back-pressure is lower, but on restart after a rod change it approaches the static figure.

Why does my DTH hammer slow down after hitting water?

The hammer works on the difference between the air pressure reaching it and the pressure in the hole around it. When water stands above the hammer, that surrounding pressure rises and the useful difference falls, so each blow has less energy. The returns are also heavier to lift, so cuttings may be reground at the bottom. First confirm the compressor is delivering full pressure, then compare the submergence with your pressure margin.

When should I use a booster compressor for borewell drilling?

A booster makes sense when your primary compressor is in good condition and has enough flow, but deep wet holes need more pressure than it can give. The booster raises the delivered air to higher pressure, with special units reaching about 35 bar. It is often more economical than replacing a good machine if deep wet holes are only part of your work. Booster capacity must match the primary compressor’s output.

Does a deeper borewell always need a bigger compressor?

Not always. Depth on its own matters less than how much water stands above the hammer, the hole and bit diameter, and the hammer’s rated pressure and air consumption. A deep dry hole may drill well on a mid-range package, while a moderate wet hole with heavy inflow may need more pressure or flow. Size the compressor from the expected submergence, annulus and hammer data rather than depth alone.

Is foam better than more air pressure in a wet borewell?

Foam helps when the problem is lifting heavy water and cuttings or keeping a weak wall stable, especially in large-diameter holes. It increases carrying capacity at lower uphole velocity. It does not replace pressure if the water column is simply too deep for the hammer to work. Many contractors use both: enough pressure for the submergence and foam for cleaning and stability. Check hammer guidance for wet running.

Can I hire a high-pressure compressor just for deep wet holes?

Yes. Hiring suits a season or project where holes are deeper or wetter than your own machine can handle. Reddy Compressors rents borewell and high-pressure units from its own fleet, and the hire is planned around your hammer, depth and water conditions. Share your expected depths, water levels and rig details so the right pressure and flow class is proposed and mobilisation can be arranged.

What is a wet hole in borewell drilling?

A wet hole is a borehole that is making water. Groundwater enters from water-bearing fractures, stands above the hammer between rod changes and is carried up with the cuttings while drilling. It adds back-pressure, slows penetration, makes restarts harder and can soften the hole walls. Most Deccan hard-rock borewells become wet once they cut water-bearing fractures. The question is how much water is coming in, not whether there is any.

How can I measure how much water is coming into a wet hole?

In stable ground, use the rod change. Blow the hole clean, shut off the air, and measure how far the water rises in the annulus over 2–4 minutes. Multiply the annulus area by the rise and divide by the time. For a 165 mm hole with 89 mm rods, a 6 m rise in 4 minutes is about 23 litres/min, or about 1.4 m³/h. Treat this as a drilling-control estimate, not the final borewell yield.

How much foaming agent should be used in a wet borehole?

As a starting point, mist uses about 0.2–0.5% foamer in the injection water. Foam uses about 0.5–1%, and stiff foam 1–2% with polymer. The maker’s data sheet sets the final dose. At 15 litres/min and 0.5%, that is 4.5 litres of concentrate per hour. Increase the concentration when formation water rises, because inflow dilutes the foam. Use a foamer rated for hard water where groundwater is hard or saline.

Can water damage a DTH hammer?

Water itself does not damage a hammer, but its effects can. A leaking check valve lets slurry into the hammer and causes scoring. Water also washes off or emulsifies the rock drill oil, leaving the piston without lubrication, and landing the bit on settled cuttings causes regrinding. Keep the check valve in good order, use a water-tolerant rock drill oil at the correct rate, lift off bottom before stopping air, and flush the hole before restarting.

Related pages

• Borewell and DTH compressor library
• High-pressure borewell air
• Borewell compressor rentals and hire
• New borewell compressor sales
• Drilling depth and compressor selection
• Borewell compressor service and repairs