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What is directional core drilling?

Directional core drilling is diamond drilling in which the path of the hole is designed in three dimensions and steered on purpose, so that it reaches a chosen target within an agreed accuracy (Coring Magazine). In mineral exploration it is used to reach deep or awkward targets, to branch daughter holes off a parent hole, and to bring a drifting hole back on line.

What does the term cover?

The phrase is used two ways. On this site it means any intentional steering of a diamond core hole, by wedge, downhole motor or steerable tool. Some suppliers use Directional Core Drilling, DCD, for one method only: a steerable core barrel that recovers core while the hole curves (IMDEX). Where that narrow method is meant, this site says steerable core barrel.

A few terms carry through every directional program. The parent hole, also called the mother hole, is drilled first. A daughter hole, or branch, starts from a point along it. The kick-off point is the depth where the hole is turned. Dogleg severity is the rate of bending, given in degrees per 30 m, and it is the limit on much directional work (Coring Magazine).

How is it different from horizontal directional drilling (HDD)?

HDD is a trenchless method for laying pipe, conduit or cable. A surface rig drills a shallow pilot arc from one surface point to another, reams it wider, then pulls the pipe through (Wikipedia, Directional boring). The hole is the product. In directional core drilling the product is rock core and a survey of where it came from, and the hole ends at a target at depth, not back at surface. In the Kevitsa mine area in Finland, for example, three steered holes reached 1000 m, 1100 m and more than 1200 m (Coring Magazine).

How is it different from oil and gas directional drilling?

The idea is shared, the scale and tools are not. In oil and gas the most common method is a bend near the bit in a downhole mud motor: the motor turns the bit while the drill string does not rotate, so the bit drills where it points. Rotary steerable systems steer while the whole string turns (SLB Energy Glossary).

Diamond drilling runs thin-walled wireline rods in small holes. Rod stiffness more than doubles from one size to the next, for example from NQ to HQ, so larger strings drill straighter but load up more in a curve (Boart Longyear). Even the numbers are written differently: in diamond drilling a vertical hole is -90 degrees and a horizontal one 0, while oil and gas counts 0 for vertical and 90 for horizontal (Coring Magazine).

When does an exploration program need it?

Deep targets. The deeper the hole, the more likely it is to deviate (Epiroc). Boart Longyear notes that a deep zone under cover can make many surface holes difficult and dangerous to drill, and describes a typical project as one 915 m mother hole with three or four legs to different targets (Boart Longyear).

Daughter holes from a parent hole. Copper Giant drilled daughter holes MD-059 and MD-060 from mother hole MD-057, kicking off at 269 m and 249 m, to test more targets from one platform (Copper Giant news release).

Correcting a hole that drifts. Directional drilling can bring a hole back on course when it strays from the planned path (Boart Longyear). Our page on correcting a hole covers the options.

Access limits. A target may sit under a pit bench or ground where no pad can be built (Boart Longyear). At Kevitsa the mine area had few suitable drill sites, and at Anglo American's Sakatti project wetlands are reachable by rig only in winter (Coring Magazine, Coring Magazine).

Infill drilling. Daughter holes can raise drill density inside a resource without new pads. Copper Giant used them to support converting Inferred resources to Indicated (Copper Giant news release). At Sakatti the same method steered six holes to follow a planned tunnel line for 3000 m of continuous core (Coring Magazine).

What limits it?

Rods. An NQ rod bent to the recommended maximum of 1.0 degree over its length presses on the hole wall with about 9 kN, which drives torque, rod wear and, in the worst case, fatigue failure (Boart Longyear). Curves therefore have to stay gradual. The rock still pushes the hole off its line while it is being steered, so every planned curve is an estimate until the survey confirms it (Coring Magazine).

Where Navilo fits

Navilo is developing a steerable full-face bit for N and H that screws onto the contractor's standard core barrel. It is non-coring and uses a point-the-bit principle established in oil and gas directional drilling, adapted for diamond drilling. 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. Performance figures are design targets that require testing. 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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