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How do you correct a diamond drill hole that is off target?

First survey the hole and project where it will end. If it will still reach the target within the agreed limits, keep drilling. If not, the choices are to wedge and redrill from a point above the error, to steer the hole back with a directional tool, or to move the rig and start again.

Why do diamond holes deviate?

Even a carefully planned hole is rarely straight. Epiroc lists weight on bit, rods and equipment, rig set-up, fractures and voids, bedding planes and changes from hard to soft rock among the causes. In banded ground the bit tends to turn into the softer layers; in fractured ground it follows the cracks. Depending on the rock, near horizontal holes can drift more than steep ones, and the deeper the hole, the more likely it is to deviate. A small error at the collar can put the hole several metres off at depth (Epiroc drillers' guide).

The rock. Anisotropy, meaning rock whose strength differs with direction such as foliated or layered rock, changes how the bit cuts and can push it off line. A review in the International Journal of Rock Mechanics and Mining Sciences covers the laboratory and field evidence for mineral exploration and site investigation holes (Brown, Green and Sinha, 1981). At Anglo American's Sakatti project, rock anisotropy, alternating soft and hard layers, fractures and broken intervals all drove deviation (Coring Magazine).

The rods and the bit. Wireline rods are slender. With a standard impregnated bit and steady drilling, the hole tends to form a slow helix set mainly by rod stiffness, and the string can buckle under load. Rod stiffness more than doubles from one size to the next, so larger strings drill straighter (Boart Longyear). Too much feed pressure bends the string and tilts the bit to one side (Epiroc drillers' guide).

How do revolutions per inch relate to deviation?

In Navilo founder André Hauderowicz’s drilling experience, the best results have come at 200–250 revolutions per inch of advance. This is an empirical working range from field practice, rather than a fixed rule for every bit and formation.

Revolutions per inch of advance, or RPI, relates rotation speed to penetration rate: divide revolutions per minute by advance in inches per minute. The metric counterpart is revolutions per centimetre. Epiroc explains that a recommended RPI range is only a starting point; the right balance depends on the bit and drilling conditions. Excessive rotation without matching penetration can polish the bit, while excessive weight on bit can cause deviation (Epiroc, Diamond Driller's Technical Book, pages 10–14).

RPI tracks the balance of rotation and advance; it does not measure hole straightness or replace surveys. Use it alongside the bit supplier's settings, ground conditions and the surveyed trend of the hole.

How is deviation measured?

A survey records inclination and azimuth at set depths, called stations. From those, methods such as minimum curvature give the path in three dimensions (Coring Magazine).

Modern survey tools pair accelerometers, for inclination, with magnetometers, for direction against magnetic north. Magnetometers give wrong readings inside steel casing or near magnetic minerals. There a gyroscope is used; it measures direction against a set reference or against true north, found from the rotation of the Earth (US EPA). North-seeking gyros such as the IMDEX OMNIx42 can survey inside the rods, including while core is being pulled (IMDEX).

Survey often. In its precautions for wedging, Epiroc advises keeping the turn within 1.5 degrees over a 15 m length to limit rod loading and the risk of broken or stuck equipment (Epiroc drillers' guide).

What are the options when a hole drifts?

Accept it. A directional hole only has to reach its target within acceptable limits of accuracy (Coring Magazine). If the projection still hits the zone, the cheapest correction is none. Any projection is approximate, because the natural trend of the hole keeps acting.

Wedge and redrill. A wedge set above the error deflects a new branch, and wedges are used for both minor and major corrections. Most give about 1.5 degrees, so a large error may need several wedges spaced along the hole. Wedges should not be set in broken ground; there the hole can be cemented and redrilled first (Epiroc drillers' guide).

Steer it back. A downhole motor or a steerable tool turns the hole in a smoother curve, and surveys guide it back to the planned path before normal coring resumes (Epiroc drillers' guide). At Sakatti, short directional runs corrected drift during near horizontal drilling (Coring Magazine). In Rajasthan, Terra Walker Drilling ran a downhole motor between about 350 m and 750 m, averaging 35 m to 70 m of correction per run (Coring Magazine). Our methods page compares the tools.

Restart. Moving the rig and collaring a new hole throws away the metres already drilled. Epiroc notes that each of the correction options can be a better choice, since a new hole can cost more and has greater environmental impact (Epiroc drillers' guide).

Can deviation be prevented?

Partly. Epiroc recommends an accurate rig set-up, no more feed pressure than needed, equipment in good condition, and a stabilized core barrel with reaming shells, which resists deviation at the cost of higher water pressure and torque (Epiroc drillers' guide).

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 already on site. It is non-coring: the crew drills the steering interval, then fits the standard coring bit and resumes coring. The tool is at prototype stage with no field data 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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