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Wedge, downhole motor or steerable tool: how do they compare?

Diamond drillers steer a hole three ways: a wedge set in the hole deflects the bit, a downhole motor with a bent sub points the bit, or a steerable tool on the rod string curves the hole as it drills. They differ in who runs them, whether they recover core during the curve, and how smooth a curve they leave. Each has its place, and many programs use more than one.

What does a wedge do?

A wedge is a steel ramp set in the hole that forces the next bit off the old path. Wedges correct deviation, drill around stuck rods or barrels, and start daughter holes from a parent hole. Most produce about 1.5 degrees of deflection, and Epiroc advises setting them at least 15 m apart so the rods are not overloaded (Epiroc drillers' guide).

The common types, as that guide describes them:

  • By-pass or Hall-Row wedge. Set at the bottom of the hole or on cement, oriented, and left in the hole. It can take up to four trips to set.
  • Retrievable or Clappison wedge. Oriented, then removed after the deflection, leaving no steel. The cut is one size smaller than the hole and has to be reamed out. TIME Limited lists its retrievable wedges as reusable and set directly off the bottom of the hole (TIME Limited).
  • Casing wedge. Set in the casing string, retrievable or left in place.
  • Epiroc Prism wedge. A permanent wedge in N and H with a two-stage lock: it locks at any depth without cement, then locks the facet direction. Epiroc states it can be set, oriented and anchored in one trip (Epiroc), with 1.5 to 2 degrees of deflection depending on ground (Epiroc Prism manual).

Limits. Wedges should never be set in broken or fractured ground. Very hard siliceous rock can make the bit cut the wedge instead of the rock, and in soft talc-like rock the bit may slide past with little change. Epiroc sums wedging up as something most drillers can do, but time-consuming, costly and less accurate (Epiroc drillers' guide).

What is navi drilling with a downhole motor?

Navi drilling uses a downhole motor, driven by the water pumped down the rods, to turn a non-coring bit while the rods do not rotate. An adjustable bent sub points the bit in the chosen direction, surveys track the new path, and once the hole is on line the motor comes out and coring resumes. It leaves a smoother curve than a wedge, and most drillers can run it (Epiroc drillers' guide). Boart Longyear uses motors with full-hole wedges for sharper angles and a continuous curve (Boart Longyear).

Limits. No core is recovered during the motor run. A motor needs a bigger pump for the higher fluid pressure, better mud cleaning, and more technical support than wedges (Boart Longyear). Penetration is much slower while the hole is being turned than in standard coring, so time and day costs weigh more (Devico).

What is a steerable core barrel?

A steerable core barrel curves the hole and recovers core at the same time. The Epiroc guide describes a drive shaft running through an offset bushing, with expanding pads holding a fixed orientation (Epiroc drillers' guide). IMDEX states that its DeviDrill fits the N-size wireline string without extra pumps or rods, recovers a 31.5 mm core, and gives a typical dogleg severity of 5 to 9 degrees per 30 m (IMDEX). Aziwell's AZIDRILL, as used at Kevitsa, collects and orients a B-size 36.4 mm core, with curves usually planned at 4 to 9 degrees per 30 m (Coring Magazine).

Limits. The core is smaller than the standard core, and at Sakatti it was often more fractured (Coring Magazine). It is usually offered as a service with trained technicians, which Epiroc notes makes it more expensive (Epiroc drillers' guide).

What is a rotary steerable tool?

In oil and gas, a rotary steerable system drills directionally while the whole string keeps rotating from surface, so there is no need to slide a motor (SLB Energy Glossary).

Whether a tool recovers core depends on its design. Check the specific tool when core is needed from the steering interval.

How to choose

The questions that decide it are plain. Is core needed from the curve? How sharp is the turn, and how far? Who will run it, the drill crew or a service team? What is the ground like where the deflection starts? Epiroc's own answer is that any of these can be better than moving the rig and starting a new hole (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. It is non-coring, and it uses a point-the-bit principle established in oil and gas directional drilling, adapted for diamond drilling. It 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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