Why the top of the hole causes most borewell problems
Many borewells in Telangana, Andhra Pradesh and the Deccan plateau start in material that is nothing like the granite or basalt below. Red soil, black-cotton soil, murram, sand, river gravel, highly weathered rock and loose boulders sit above the hard formation. This overburden can be a few metres or many tens of metres thick, and it varies from plot to plot.
Unsupported, these materials collapse into the hole, wash out, trap the drill string or bury the hammer. They also let surface water and contamination reach the aquifer. Casing solves both problems, but the way it is installed affects the bit sizes, the drilling method, the time per hole and the air package needed on the rig.
Casing decisions belong to the well design and the drilling contractor. The compressor must then support those decisions. This page explains the options in general terms so that the air duty matches the casing programme rather than a single depth and diameter quoted without context.
What makes a formation unstable?
- Loose granular material: dry sand and gravel run into the hole as soon as air or water disturbs them.
- Clays and black-cotton soil: swell when wet, become sticky and smear the hole wall, and can pack around the bit.
- Weathered and fractured rock: breaks into blocks that fall in behind the hammer.
- Boulders in a soft matrix: deflect the bit and can cause sudden loss of hole.
- Water-bearing zones: inflow destabilises the wall and turns dust into mud that air struggles to lift.
- Cavities and lost-return zones: the return air escapes into the formation instead of carrying cuttings to the surface.
Warning signs while drilling include returns stopping or turning to mud, the drill string tightening when pulling back, rising rotation torque, cuttings much larger than the bit produces, and a hole that has to be redrilled after each rod change.
The common casing approaches
There are three broad ways to deal with overburden. The right one depends on formation, depth to competent rock, final diameter and the rig’s capability.
| Approach | How it works | Suits | Air implications |
|---|---|---|---|
| Drill, then case (conventional) | A larger bit drills through overburden into rock; casing is lowered and seated; drilling continues with a smaller DTH bit | Overburden that stands long enough to set casing | Large top-hole diameter needs high flow for cleaning; pressure demand rises later with depth in rock |
| Casing advancement (casing while drilling) | A pilot bit with a reaming element opens a hole slightly larger than the casing, and the casing follows the bit down | Running sand, gravel, boulders, collapsing ground | Air must clean through the inside of the casing; flow and hammer pressure must suit the larger tooling |
| Mud or foam-assisted drilling | Drilling fluid or foam supports the wall and lifts cuttings until casing is set | Clays, wet unstable ground, where the method is suitable | Foam or mist injection changes flow needs and adds a water or foam pump to the system |
How casing advancement works, in general terms
Casing-advancement systems are sold under several trade names; ODEX is one of the best-known. All share the same idea: drill and case in one pass so the hole is never left unsupported.
Eccentric systems
An eccentric reamer swings out as the bit rotates, cutting a hole larger than the casing outside diameter. The casing, fitted with a shoe, follows the bit down. When competent rock is reached, reverse rotation folds the reamer in so the assembly can be pulled up through the casing, and drilling continues inside with a standard bit.
Concentric systems
A pilot bit works inside a ring bit that is locked to the casing shoe. The ring bit cuts the full diameter and stays at the bottom when the pilot is withdrawn, or is left in place. Concentric systems tend to hold straighter holes and are often preferred for larger diameters and bouldery ground.
The exact assembly, its air consumption, its permitted pressure and its compatibility with the hammer and rig must come from the tooling manufacturer’s documentation. A conventional open-hole set-up should not be assumed equivalent to a casing-advancement assembly.
What casing means for the compressor
Casing affects air demand in three ways that are often overlooked.
- Top-hole diameter drives flow. Annular velocity is flow divided by annular area, and area grows with the square of diameter. A 10-inch (about 254 mm) overburden hole needs far more air to lift cuttings than a 6.5-inch (about 165 mm) hole in rock. Large-diameter top-hole work often becomes a flow problem before it becomes a pressure problem.
- Casing advancement uses larger tooling. The reamer or ring bit, the larger hammer often used with it and the return path inside the casing all need adequate flow at the hammer’s working pressure. Under-airing leads to slow progress and a casing that refuses to follow.
- Depth below the casing adds pressure demand. Once casing is set and drilling continues in rock, the deeper and wetter the hole, the more pressure the hammer needs to work against the column of water and cuttings. This is where bands such as 250–300 psig (17.2–20.7 bar) at 800–1,200 cfm or 300–400 psig (20.7–27.6 bar) at 1,000–1,500 cfm become relevant.
A compressor package that is generous on pressure but short on flow may drill the rock section well but struggle in the overburden. One that is generous on flow but short on pressure may clean the top hole and then stall at depth. The duty statement should therefore list each section of the hole programme.
Write the hole programme section by section
| Section | Information to record | Why it matters for air |
|---|---|---|
| Overburden drilling | Expected thickness, material, bit diameter, method (open hole or casing advancement) | Sets the flow needed for cleaning the largest diameter |
| Casing | Material (PVC, MS or other), outside and inside diameter, length, seating depth | Defines the inside diameter the next bit must pass |
| Rock drilling | Bit diameter, hammer size, target depth, expected water strikes | Sets pressure demand and duty at depth |
| Any second casing or telescoping | Depth, diameters, reason | May require a change of tooling and air duty mid-hole |
| Completion | Final diameter, screens, gravel pack, pump size | Links back to the final bore and pump passage |
Mark which items are an approved design and which are an initial estimate. If a drilling log later shows the overburden was deeper than expected, the record should show the change and its effect on the air package.
What to record when instability appears
When the hole starts collapsing or returns change, record the depth interval, what the returns looked like, the pressure and flow at the time, any water inflow and what was done. The drilling specialist decides the response, which may be casing deeper, changing method or redrilling. There is no universal pressure increase or casing depth that fixes every case, and raising pressure blindly can wash out a weak zone further.
If the casing plan changes to complete the hole, reassess what follows: final diameter, pump passage, well development and yield testing. A change made to rescue a hole can also change how the finished well should be judged.
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.
Reddy Compressors sells, rents and services borewell and DTH compressors as an independent multi-brand specialist, working with ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes. We are not an authorised dealer of any brand, and we do not design wells; we match the air package to the casing programme your driller or consultant has set. We have tie-ups with all major and reputed brands.
- Duty review: we look at each section of your hole programme and confirm whether one compressor covers both the large-diameter top hole and the deep rock section, or whether a second unit or booster makes more sense.
- Hire: our own fleet of diesel portable and high-pressure compressors for overburden and deep drilling, from our base at Tarnaka, Secunderabad, with service across Telangana and Andhra Pradesh and supply across India.
- New supply: brand-new compressors for contractors and long-term projects.
- Service and spares: servicing, breakdown repair and parts identified from the nameplate, so a machine working hard in difficult ground stays productive.
Call +91 99590 60718 or +91 92468 88812 to discuss your casing and air requirements.
What to send us
- Expected overburden thickness and material, if known from nearby holes.
- Top-hole bit diameter and casing material, size and planned depth.
- Casing-advancement system details, if one will be used.
- Rock-section bit diameter, hammer size and target depth.
- Rig type and current compressor, if any.
- Site location and planned dates.
Frequently asked questions
Why does a borewell need casing in the top section?
The top of most borewells passes through soil, sand, gravel, clay or weathered rock that collapses into an open hole. Casing holds that material back, keeps the bore open for drilling and later for the pump, and stops surface water and contamination from entering the well. It is usually seated a short distance into competent rock so the joint is stable.
What is ODEX drilling?
ODEX is a well-known trade name for an eccentric casing-advancement system. A pilot bit with an eccentric reamer drills a hole slightly larger than the casing, and the casing follows the bit down, so loose ground never stands unsupported. At competent rock the reamer is retracted and pulled up through the casing. Air flow and pressure must suit the specific tooling used.
Does casing advancement need a bigger compressor?
Often it needs more flow, because the reamer or ring bit cuts a larger diameter and the cuttings must be lifted through the casing. The pressure required depends on the hammer used with the system. Check the tooling manufacturer’s air requirement and compare it with your compressor’s free air delivery at working pressure, not just its nameplate rating, before starting.
How deep should borewell casing go?
Casing normally goes through all the unstable overburden and is seated into firm rock. The actual depth varies from site to site and is set by the driller or well designer from the formation encountered, not by a fixed rule. If overburden turns out deeper than expected, the casing depth and possibly the bit sizes and air duty need to change.
Why does my hole keep collapsing while drilling with air?
Common causes are loose sand or gravel, swelling clay, fractured weathered rock or water inflow destabilising the wall. Excessive air at the bit can also wash out weak zones. Record the depth and what the returns showed, and discuss the response with the drilling specialist; the usual answers are casing deeper, using casing advancement or changing method rather than simply raising pressure.
Can one compressor handle both overburden and deep rock drilling?
Sometimes. Large-diameter overburden drilling needs high flow, while deep rock drilling needs high pressure. A compressor with enough free air delivery at the pressure needed at depth can cover both. Where it cannot, contractors add a second compressor or a booster. List both sections of the hole in your enquiry so the package is sized for each.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.