Drilling productivity and cost per metre for borewells

Why fuel per hour is the wrong yardstick

Rig owners naturally watch diesel, because it is the largest bill they see every day. But a litre per hour figure says nothing about how many metres were drilled with it. A compressor that is too small for the hammer may use less fuel per hour while drilling far fewer metres, wearing bits faster through regrinding and losing time on holes that will not clean. Per metre, it can be the more expensive machine.

The fair comparison is cost per accepted metre: what it cost to produce a metre of finished hole that the customer pays for. That figure captures speed, fuel, wear and lost time together, and it is the one that decides profit on a contract priced per foot or per metre.

Define the metre before you count the cost

Before comparing machines, crews or seasons, agree what goes into the denominator.

• Accepted metres: drilled depth the customer accepts and pays for. Abandoned or redrilled holes cost money but add no accepted metres.
• Scope: decide whether casing, flushing, development and yield testing time are inside or outside the comparison, and keep it the same for every option.
• Period: use a month, a season or a set of holes, not a single good day.
• Boundary: compare the same items. A compressor-only cost should not be set against a full rig-and-crew cost from another supplier without saying what is missing.

What goes into the top line

How air sizing changes penetration rate

A DTH hammer’s penetration rate depends on the energy of each blow and how quickly the cuttings leave the bottom. Both depend on the air. Blow energy rises with the pressure difference across the hammer; cleaning depends on enough free air delivery (FAD) to keep the uphole velocity up in the annulus.

• Pressure: within the hammer’s rating, higher working pressure gives harder, faster blows. In wet holes, water standing above the hammer subtracts roughly 1 bar (14.5 psi) per 10 m from the useful pressure, so the margin matters most below the water table.
• Flow: if FAD is short for the hammer and annulus, pressure at the hammer falls and cuttings settle and are reground. Bits wear faster and metres per hour fall.
• Over-sizing: pressure costs energy. Each extra bar of discharge pressure takes roughly 7% more compressor power, so running far above what the hammer and hole need wastes fuel without adding speed.

Typical working bands for borewell air are 600–900 cfm at 200–250 psig (13.8–17.2 bar), 800–1,200 cfm at 250–300 psig (17.2–20.7 bar) and 1,000–1,500 cfm at 300–400 psig (20.7–27.6 bar), with special units up to about 35 bar (507.6 psig). Choosing the right band for your typical depth, diameter and water conditions is the biggest single lever on cost per metre. See drilling depth and compressor selection.

Worked example (illustrative only)

The figures below are round numbers chosen only to show the method. Use your own drilling records in their place. No prices are used: the comparison works in litres, metres and hours, so it holds whatever your local costs are.

A contractor compares two air packages on the same rig and ground over one day of drilling.

The steps are simple:

1. Accepted metres = drilling hours × average penetration rate (10 × 12 = 120 m; 10 × 18 = 180 m).
2. Fuel per metre = litres used ÷ accepted metres (300 ÷ 120 = 2.5; 380 ÷ 180 ≈ 2.1).
3. Time per metre = 60 ÷ penetration rate (60 ÷ 12 = 5 minutes; 60 ÷ 18 ≈ 3.3 minutes).
4. Apply your own fuel price, crew cost, rig ownership and wear rates to these per-metre figures.

In this illustration Package B burns more fuel per hour, yet it uses about 16% less fuel per metre and a third less crew and rig time per metre, with less bit and hammer wear. Every fixed cost of the day is spread over 50% more metres. Now add two hours of downtime to Package A for a hole that would not clean: metres fall to 96 while the crew and rig still cost a full day. That is the effect most rig owners underestimate, and it is why we size the compressor to the hole, not to the purchase price.

Downtime: the cost that does not appear on a bill

A stopped rig still pays its crew, its finance or hire and its site costs. The causes are usually predictable:

• Compressor high-temperature trips in summer heat from dusty coolers or tired oil.
• Engine power loss from choked air filters or poor fuel.
• Stuck or lost string in a hole that was not cleaned properly.
• Waiting for spares that could have been identified and stocked in advance.
• Waiting for a replacement machine when no hire option was arranged.

Keeping to the service interval prevents much of this. On our own hire jobs we change oil and service every 300 running hours. A simple daily log of hours, metres, fuel and stoppages lets you see where the money goes; our page on borewell rig reliability and drilling records describes what to record.

Buy, hire or use a reconditioned machine?

A useful test: if your machine would sit idle for long periods each year, or your typical hole needs a higher pressure band only on some jobs, hiring the larger package for those jobs may give a lower average cost per metre than buying it.

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 works with rig owners, drilling contractors, irrigation and government water projects, and real-estate and industrial borewell work across Telangana and Andhra Pradesh, and supplies compressors, 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.

• Help you choose the pressure and flow band that suits your hammer, depth and water conditions, so you are not paying for air you do not use or losing metres for air you do not have.
• Supply new borewell compressors, including brand-new machines for long-term projects: new borewell compressor sales.
• Hire borewell and high-pressure units from our own fleet: borewell compressor rentals and hire.
• Service and repair with more than 20 service engineers and technicians, and spares identified from the nameplate with fit confirmed before quoting.

Visit us at Tarnaka, Secunderabad, or call +91 99590 60718 or +91 92468 88812.

What to send us

• Your rig, hammer and bit sizes, and typical hole depth and diameter.
• Usual water strike depths and ground type.
• Present compressor make, model, pressure and flow rating.
• Your own records of metres per day, fuel use and downtime, if you keep them.
• Whether you want to buy new, buy used, hire or improve your current machine.
• Project location and expected duration.

Frequently asked questions

How do you calculate drilling cost per metre for a borewell?

Add every cost for a period, including fuel, bit and hammer wear, rods and consumables, compressor servicing, repairs, crew and site costs, and ownership or hire charges. Then divide by the accepted metres drilled in the same period. Use the same boundary for every option you compare, and use several holes or a full month rather than one day, so that good and bad days balance out.

Does a bigger compressor reduce cost per metre?

It can, when the present machine is short of pressure or flow for the hammer and hole. A correctly sized package drills faster and cleans better, so fixed daily costs are spread over more metres and bits wear less. But an oversized machine run at needlessly high pressure wastes fuel, as each extra bar takes roughly 7% more power. The goal is the right band, not the biggest.

How much diesel does a borewell compressor use per hour?

It depends on the machine size, working pressure, load and engine condition, so there is no single figure. Consumption rises with pressure and flow, and it falls when the machine unloads. Compare fuel against metres drilled, not hours run. Keep a daily record of litres issued and metres achieved to work out litres per metre for your own rig and ground.

Why is my drilling cost per metre going up?

Common causes are slower penetration from worn bits or a compressor short of flow, deeper water raising back-pressure, more downtime from breakdowns or waiting for spares, and holes that are abandoned or redrilled. Fuel price changes also matter. Look at metres per day, fuel per metre and hours lost together; one of them usually explains most of the rise.

Is it cheaper to hire or buy a borewell compressor?

Buying usually suits steady year-round drilling where the machine works most days. Hiring suits seasonal peaks, single deep-hole projects, or standby while your own machine is repaired, because you pay only while the machine works. Reconditioned machines can lower ownership cost if their condition is tested. Compare the options on cost per accepted metre over a realistic year of work.

How does downtime affect drilling cost per metre?

When the rig stops, crew, ownership or hire and site costs continue but no metres are added, so every hour lost raises the cost of each metre drilled. Many rig owners underestimate this. Regular servicing, clean coolers and filters, stocked critical spares and a hire backup arrangement cut downtime and often improve cost per metre more than small fuel savings.

Related pages

• Borewell and DTH compressor library
• New borewell compressor sales
• Borewell compressor rentals and hire
• Used and reconditioned borewell compressors
• DTH compressor sizing and duty confirmation
• Borewell compressor service and repairs