Water Well Drilling Rig Selection

Start from depth and casing diameter

Water well projects are defined by the final depth and the casing size you must install. A rig whose rated depth matches the borehole with margin — and whose hole range covers the casing plus annulus — is the correct class.

The two numbers are not interchangeable with "how deep can it go". A rig good for 600 m of 200 mm hole may be at its limit on 600 m of 400 mm hole, because the pullback, the rod string and the air or mud volume all scale with diameter as well as depth. Work out the hole size you must drill to set the casing first, then look for a rig whose published hole range brackets it with room to spare.

The arithmetic for casing is straightforward. A 250 mm casing with a 350 mm hole gives a 50 mm annular clearance; that is enough for DTH cuttings to clear in most formations, and enough for gravel packing where the design calls for it. Push the casing into a hole only 20 mm larger and the rig will spend its day reaming, and every metre will cost more than the one before. As a working rule, allow at least 50 mm of annulus on small wells and more where the formation is caving or the gravel pack is thick.

Depth rating also needs margin. Rated depth is normally quoted for a rod string the factory has tested, with a given rod diameter. If your borehole demands a heavier string for deviation control, the practical depth falls. Plan on 80–90 per cent of the rated figure as your working ceiling unless the manufacturer confirms otherwise for your exact string.

Our water well range (published parameters)

The table lists mounting type, hole diameter and maximum depth. Truck mounted units trade some depth for road mobility; crawler units favour stability and deeper holes; trailer units suit smaller, frequent jobs.

Our water well range (published parameters)

Reading the range by application

The ten models fall into three groups, and the group matters more than the exact model number.

Shallow and village supply. KW180 drills 140–350 mm to 180 m and KW360 drills 115–305 mm to 350 m. Both are truck mounted, which means they travel between villages at road speed without a low-loader. For a programme of many shallow wells spread over a district, mobility is the whole argument, and the depth rating is secondary.

Mid-depth production wells. CDC-450J at 105–350 mm to 450 m, CDC-600K at 105–350 mm to 600 m and CDC-600 at 90–850 mm to 600 m cover the common irrigation and municipal range. Note the wide hole range on CDC-600: the same carrier can open a large-diameter shallow hole or drill a smaller deep one to the same 600 m rating, which is the useful flexibility for a contractor taking mixed work.

Deep wells. CDC-800 at 140–350 mm to 800 m, CZT-1200 at 140–350 mm to 1200 m and KSC-1300 at 105–350 mm to 1300 m are the deep end. Three of them are worth comparing directly because they reach similar service depth from different carriers: CZT-1200 is a 1200 m crawler, KSC-1300 is a 1300 m truck. If the wells are deep and the sites are close together, the crawler's stability and lower ground pressure usually win. If the wells are deep and the sites are far apart, the truck's road speed changes the economics.

Trailer units. CDT-600 at 89–950 mm to 600 m and CDT-800J at 90–850 mm to 800 m are towed. They suit programmes where the customer tows the rig behind a tractor or truck, or where a single rig has to move between small jobs on farm tracks. They give up some drilling stability for that mobility.

For a direct comparison of the three mounting types on the same duty, see Water Well Drilling Rigs: Crawler vs Wheeled vs Truck-Mounted.

Mobility, water table and geology

If your sites are scattered, mobility decides the class before depth does. In hard rock, plan for a DTH hammer and a compressor; in soft overburden, mud rotary or casing driving may be needed.

Three site facts change the selection more than any other:

• Distance between wells. Under about 30 km, a crawler and a low-loader are usually cheaper over a campaign than a truck that cannot reach the difficult sites. Above that, road speed starts to dominate.
• Water table depth. A shallow water table (under about 50 m) usually means soft or unconsolidated overburden, where the top of the hole needs casing while the bottom is drilled by DTH. A deep water table usually means the whole section is hard rock, which changes the bit and hammer class rather than the rig class.
• Formation. Hard rock — granite, basalt, quartzite, limestone with high compressive strength — is DTH territory. Loose sand, gravel and clay need mud rotary, casing driving or an auger, and the rig must be able to handle the casing string as well as the drill string.

The drilling method drives the support equipment:

DTH work in water wells normally runs at 17–24 bar. Sizing that air correctly is a separate exercise — the air compressor sizing guide walks through it, and the high-pressure units such as LGCY-26/25 are the class used once hole size and depth push the demand up. A 5-inch hammer in the 135–165 mm range is quoted at 7.0–19.0 m³/min across 10–25 bar, and the compressor is chosen at the top of that band, not the bottom.

Geothermal wells use the same rigs with a different specification. Circulation temperature, casing programmes and grouting requirements change, and the geothermal drilling page sets out the differences. Municipal and irrigation supply is covered on the water well drilling page.

Selection steps

• Fix the well design: final depth, casing diameter and casing material, plus the hole diameter needed to install it with the annular clearance you want.
• State the geology by section — how many metres of overburden, how many of hard rock — and the expected compressive strength. Section-by-section geology, not a single description for the well.
• Choose the drilling method per section: casing drive or mud rotary through overburden, DTH in rock, and the hammer class for the rock diameter.
• Add up the support equipment: compressor capacity and pressure, casing string, hammer, bits, rods, pump. Support equipment usually costs as much as the rig, and it is the part most often under-budgeted.
• Match the rig class to the hole range and depth with margin, then to logistics: how far apart are the wells, what is the access like, and how much time will be lost moving.
• Check the practical details that decide whether a rig suits a site: minimum transport weight against local road limits, crane or rig-mounted handling for casing, and whether the crew can change a hammer at the collar without a second machine.
• Confirm spares and service. Hammer parts, bits, seals and hydraulic filters should be available within the lead time your contract allows, not on a six-week order.

Common mistakes

• Choosing on hole diameter alone. A wide hole range on a shallow rating is not the same capability as a narrower range on a deep rating, and both appear in the same table.
• Reading the top of the depth rating as a working figure. Ask what rod string the rating assumes, and what it becomes with the string you actually need.
• Forgetting the casing. The rig must handle the casing string as well as the drill string; a rig that drills the hole but cannot run the casing has not finished the job.
• Planning overburden and rock as one section. Many water well problems — stuck casing, caving hole, lost circulation — begin at the transition, so the transition is where the programme should be most precise.
• Under-budgeting the compressor. For a DTH water well, the compressor is a first-order part of the package, and the difference between 18 bar and 25 bar changes both penetration rate and bit life.
• Buying mobility you do not need, or needing mobility you do not have. Truck, crawler and trailer are decided by site distribution, not by preference.
• Ignoring transport limits. Minimum weight and transport dimensions, not drilling performance, decide whether a large rig can legally and practically reach the site.

Checklist

• Target depth and casing diameter. 2) Geology and expected hardness. 3) Number of wells and distance between sites. 4) Water table depth. 5) Truck access and road limits. 6) Support equipment: compressor, pump, casing.

Conclusion

A water well rig is selected by an arithmetic chain: casing diameter gives the hole diameter, the hole diameter with the rock type gives the method and hammer, the hammer gives the air, and the well programme gives the mounting type. The published table above is the last step of that chain, not the first. Work the chain in order, budget the support equipment alongside the rig, and keep the depth rating honest against the rod string you will really run. The per-metre consequences of those choices are set out in What Drives Your Drilling Cost per Metre.