Cuttings transport and borehole cleaning in DTH drilling

What cuttings transport means in DTH drilling

In DTH drilling, the air that drives the hammer leaves through the bit and turns upward, travelling through the annular space between the drill pipe and the borehole wall. This return air has a second job: it must carry the rock chips and dust broken by the bit all the way to the surface.

The hammer breaks the rock; the airflow removes it. If removal falls behind, the bit works on a bed of broken material instead of fresh rock. Energy is wasted regrinding chips into fine powder, the bit and hammer wear faster, and in the worst case the string becomes stuck. A rig can reach a point where a stronger hammer no longer drills faster because hole cleaning, not rock breaking, has become the limit.

How annular (uphole) velocity is calculated

The working idea is simple. Annular velocity is the volume of air flowing upward divided by the annular area.

• Annular area = π/4 × (hole diameter² − drill pipe outside diameter²)
• Annular velocity = air flow ÷ annular area

Two points matter in practice. First, area grows with the square of diameter, so a modest increase in hole size needs a much larger airflow to keep the same velocity. Second, the calculation is an ideal: real holes are rarely perfectly round, washouts enlarge sections, and the air is at a different pressure and density at the bottom than at the top. The figure is a guide for comparing set-ups, not a measured downhole condition.

For dry-air drilling, common practice aims for uphole velocities in the region of 15–25 m/s (roughly 3,000–5,000 ft/min), with heavier, larger chips needing the upper end. Foam drilling can work at much lower velocities. These are typical guides and the hammer and bit supplier’s advice should take priority.

A worked example

Take a 6.5 inch (165 mm) hole drilled with 3.5 inch (89 mm) drill pipe and a compressor delivering 1,000 cfm, which is about 28.3 m³/min or 0.47 m³/s of free air.

• Annular area = 0.785 × (0.165² − 0.089²) ≈ 0.0152 m²
• Velocity on a free-air basis ≈ 0.47 ÷ 0.0152 ≈ 31 m/s

Now enlarge the hole to 8 inch (203 mm) with the same pipe and air. The area rises to about 0.026 m² and the velocity falls to roughly 18 m/s, close to the lower end of the dry-air guide. The same compressor has gone from comfortable to marginal simply because the bit got bigger.

Note that this uses free-air flow. Near the bottom of a deep or wet hole the air is compressed by the back-pressure, so its actual volume and velocity there are lower than the surface figure suggests. That is one reason cleaning often gets worse with depth even when nothing at the surface has changed. Use the example to compare options, and always state which flow basis a calculation uses.

Why bore size and pipe size change everything

This is why the same compressor that cleans a 6.5 inch hole well can struggle on a larger diameter, and why matching drill pipe size to bit size is a cleaning decision as well as a strength decision. It also explains why borewell compressor flow bands rise with hole size: typical bands run from 600–900 cfm at 200–250 psig (13.8–17.2 bar) up to 1,000–1,500 cfm at 300–400 psig (20.7–27.6 bar).

What decides whether a chip is lifted?

A chip moves up only if the air is moving faster than the chip tends to fall back. That falling tendency, the slip or settling velocity, depends on the chip’s size, density and shape. Fine dust is lifted easily. Large, dense, angular chips from hard granite need much stronger flow. Wet chips clump together and clay becomes sticky, so a flow that cleans dry granite may not clean wet mixed ground.

Penetration rate matters too. The faster the bit drills, the more rock per minute the return air must carry. A powerful hammer in good rock can generate cuttings faster than a marginal airflow can remove them.

Water, mist and foam injection

When dry air alone is not enough, liquid is added to the air stream.

• Mist drilling: a small amount of water is injected, mainly to suppress dust and help bind fines so they are carried out. It adds some weight to the return stream.
• Foam drilling: water with a foaming agent is injected to create a stable foam in the annulus. Foam supports chips that would settle in air alone, lifts water out of the hole and can clean well at lower velocities, which helps in large-diameter, deep or water-producing holes.

Injection needs to be engineered rather than improvised. It changes pressure losses, mixture weight and hammer conditions, and it usually means the hammer needs more oil. Foam quality and injection rate matter: too little gives no benefit, too much adds cost and handling problems on site.

Groundwater entering the hole changes the job most. The air must now lift a column of water as well as cuttings, and the water’s back-pressure on the hammer increases with depth below the water level. This is often when a contractor needs a higher-pressure compressor or a booster, not just more cfm.

Signs of poor borehole cleaning

• Returns at the surface become intermittent, surge, or stop for periods
• Cuttings are very fine powder instead of chips, showing regrinding
• Penetration rate falls while compressor pressure and flow look normal
• Rotation torque rises or becomes erratic
• Pipe is hard to pull back or tight on lifting
• Material falls back into the hole on a pipe change and has to be redrilled
• Bit wear increases, especially on the gauge and the face

These signs can also come from low pressure at the hammer, a worn hammer or hose restrictions, so the cleaning question should be checked alongside the air supply.

How to improve hole cleaning

1. Confirm the compressor is delivering its rated flow and pressure at the rig, not just at its own gauge.
2. Check the annular velocity for the actual bit and pipe sizes, and consider a larger pipe if the gap is wide.
3. Blow the hole clean before each pipe change, holding the bit just off bottom.
4. Use mist for dust and fines, and foam for large holes, water-producing zones or unstable returns.
5. In deeper wet holes, review whether more pressure, a booster or a second compressor is needed.
6. Record depth, returns, water and penetration so changes can be linked to cause.

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 sell, hire and service borewell and DTH compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes as an independent multi-brand specialist, from our office at Tarnaka, Secunderabad. Our service teams cover all of Telangana and Andhra Pradesh, and we supply compressors, rental units and spares across India, with brand-new machines available for long-term projects.

For a cleaning problem, we help you work out whether the compressor’s flow suits your hole and pipe size, check that the machine is delivering its rated output, and advise on higher-flow or higher-pressure options, including hire units for deep or wet jobs. We can also service compressors that have lost output and supply the spares to restore them.

What to send us

• Bit diameter, drill pipe outside diameter and target depth
• Compressor make, model, rated cfm and pressure, with a nameplate photo
• Formation type and depth at which water is struck
• Whether you use mist or foam, and how returns behave
• Penetration rate before and after the problem started

Frequently asked questions

What is annular velocity in borewell drilling?

Annular velocity is the upward speed of the return air in the gap between the drill pipe and the borehole wall. It is calculated as the air flow divided by the annular area, which depends on the hole diameter and the drill pipe outside diameter. It decides whether cuttings are lifted to the surface or fall back to the bit, so it is central to hole cleaning.

How much uphole velocity is needed to clean a borehole?

For dry air DTH drilling, common practice aims for roughly 15–25 m/s (about 3,000–5,000 ft/min), with larger, denser chips needing the higher end. Foam drilling can clean effectively at much lower velocities. These are typical guides only; actual needs depend on chip size, formation, water and depth, and the hammer and bit supplier’s guidance should take priority.

Why are my borewell cuttings coming out as fine powder?

Very fine powder in place of chips often means the cuttings are not being removed quickly enough and the bit is regrinding them. Likely causes are too little airflow for the hole and pipe size, low pressure at the hammer, water in the hole or a worn bit. Check delivered air at the rig and the annular velocity before changing other equipment.

When should I use foam in DTH drilling?

Foam is useful in large-diameter holes, deep holes, water-producing formations and where returns are unstable, because it supports cuttings that air alone would drop and helps lift water. It needs the right foaming agent, injection rate and equipment, and usually more hammer oil. Mist is more suited to dust control and binding fines in drier conditions.

Does a bigger compressor always clean the hole better?

More flow raises annular velocity, so it usually improves cleaning, but it is not the only factor. Pipe size, hole diameter, washouts, water ingress and hose losses all matter. In deep wet holes the limit is often pressure rather than flow, because the air must overcome the water column. The right answer comes from matching both flow and pressure to the hole.

What are the signs of poor hole cleaning?

Typical signs are intermittent or surging returns, very fine cuttings, falling penetration with normal compressor readings, rising or erratic torque, tight pipe when lifting, and material falling back on pipe changes. Bit wear may also increase. Because low hammer pressure and hose restrictions can produce similar symptoms, check the air supply at the rig alongside the cleaning arrangement.

Related pages

• Borewell and DTH compressor library
• Drilling depth and compressor selection
• Wet-hole drilling and water ingress
• New borewell compressor sales
• Borewell compressor rentals and hire
• High-pressure borewell air