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Does a directional daughter hole cost less than a new hole from surface?

Often, but not always. A daughter hole reuses the parent hole down to its kick-off point, so those metres are not drilled again and the rig does not move. Against that stand slower drilling while the hole turns, directional tools or service charged by time, extra surveys, and the risk of losing the branch. The answer depends on how deep the kick-off is and how far the target lies from the parent.

What is saved?

Metres. The metres above the kick-off point are shared. At Copper Giant's Mocoa project, daughter hole MD-058 left its mother hole at 273 m and ended at 721 m (Copper Giant news release), so, by subtraction, 273 m of its length was drilled once for two holes. At SolGold's Alpala project, the company reported that daughter holes off single parent holes had saved over 12,000 m of drilling (SolGold, 2018).

Rig moves and pads. Directional drilling avoids moving the rig to new sites and drilling several surface holes (Boart Longyear). At Torex Gold's Media Luna infill program, Aziwell reports 117 targets reached from nine drill sites, with fewer metres per target to cut cost and water use where water came in by truck (Aziwell).

Time. SolGold reported that deviated holes from existing parent holes were saving up to three weeks and 500,000 USD per hole (SolGold, 2017). Those are one company's figures on one deep porphyry project, not a rule.

What is added?

Rig time charged by the hour. Where normal coring is priced per metre, directional work can add hourly charges. The Western Australian government's drilling cost guidance lists navi drilling, wedging, reaming and gyro surveys as active standby, billed by the hour rather than by the metre (Government of Western Australia). Penetration is much slower while a hole is being turned than in standard coring, so day costs weigh more (Devico).

Directional service and tools. Steerable core barrels are usually supplied as a service with trained technicians, which Epiroc notes makes them more expensive; a downhole motor is cheaper and simpler but recovers no core; wedging is time-consuming and costly (Epiroc drillers' guide). Motors also need a bigger pump and better mud cleaning (Boart Longyear). Treat the service day rate as its own line in the budget, next to the metre rate. No neutral public source gives a typical figure, so ask for quotes.

Surveys. Budget for orientation and check surveys using the time required by the chosen survey method. Gyro hire and surveys count as direct drilling costs in the same Western Australian guidance (Government of Western Australia).

What are the risks?

A curved hole loads the rods. An NQ rod bent to the recommended maximum of 1.0 degree over its length presses on the wall with about 9 kN, which can bring high torque, rod wear and fatigue failure (Boart Longyear). Branches can be lost. At Kevitsa a core barrel stuck in broken rock and the branch had to be abandoned; the crew cut a new branch from the bend and still reached the target (Coring Magazine). A wedge adds friction and the risk of losing rods, and it should not be set in broken ground (Epiroc drillers' guide).

How to compare the two for one target

Put both options on the same sheet:

  1. Metres from surface for a new hole, times the metre rate.
  2. Metres from the kick-off point for the daughter hole, times the metre rate.
  3. Hours for the directional interval, wedge setting and surveys, times the hourly or service rate.
  4. Rig move, pad, water and permits for the new hole.
  5. An allowance for a lost branch or a redrill.

The deeper the kick-off point and the closer the target to the parent hole, the more a daughter hole tends to win. The contractor at Kevitsa describes planning each hole for the ratio between saved metres and time spent steering (Coring Magazine). Planning software such as HolePlan can lay out both paths; Navilo recommends it and has no partnership with it.

Where Navilo fits

Navilo is developing a steerable full-face bit for N and H that screws onto the contractor's standard core barrel and is designed to run with the rigs, rods and pumps the crew already has. It is non-coring and at prototype stage; no field data and no prices are published. H trials are planned for the first quarter of 2027, with N trials planned during 2027. Strong demand for H has made it the priority, with N development following. More on how it steers and the pilot program.

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