Exploration Drilling Types

Geomechanics
    

Exploration Drilling Excellence

Geomechanics, a subsidiary of Geogroup, was conceived by Dave Rossiter in 1989 to service the geotechnical investigation and testing market. Over a decade later, Geomechanics commenced the gradual undertaking of branching out into exploration drilling and today, through our continued policy of training staff and investing in industry-leading exploration drilling machinery, Geomechanics is the leaders in its field. With that in mind, here is the list of available exploration drilling services. If you are looking for a suitable partner look no further than Geomechanics.

1. Percussion Drilling

  • Conventional Air Percussion Drilling
  • Reverse Circulation (RC) Drilling
  • Symmetrix Drilling

Conventional Air Percussion Drilling

Conventional Air Percussion drilling is used most frequently in the mineral and water exploration industry. The drill uses a pneumatic reciprocating piston-driven 'hammer' to drive a heavy drill bit into the rock. The drill bit is hollow, solid steel and has ~20 mm thick tungsten rods protruding from the steel matrix as 'buttons'. The tungsten buttons are the cutting face of the bit.

The cuttings or "chips" are blown up the outside of the rods by compressed air or a combination of air and foam and collected at surface.

Air Percussion produces lower quality samples because the cuttings are blown up the outside of the rods and can be contaminated from contact with other rocks. At depths of more than 200metres water can cause the outside of the hole to become clogged with debris, preventing the drill cuttings from escaping the hole. With the use of drill enhancing fluids and proper drilling techniques, experienced drillers can drill to depths in excess of 500m.

Depending on the density and weight of the rock being drilled, the use of multiple high-powered air compressors can enable drilling of deeper holes up to ~1250m.

Reverse Circulation (RC) Drilling

RC exploration drilling is similar to air percussion drilling except that that the drill cuttings are returned to surface inside an inner tube within the rods.

The most commonly used RC drill bits are 5-8 inches (12.7–20.32 cm) in diameter and have round metal 'buttons' that protrude from the bit, which are required to drill through rock and shale.

Experienced drillers continually 'sharpen' the buttons on their drill bits to prevent drilling slowing down and the rod string becoming bogged in the hole. Trying to recover the rods may take hours and in some cases weeks and the rods and drill bits are very expensive,

Reverse circulation is achieved by blowing air down the rods, the differential pressure creating air lift of the water and cuttings up the inner tube which is inside each rod. It reaches the bell at the top of the hole, then moves through a sample hose which is attached to the top of the cyclone. The drill cuttings travel around the inside of the cyclone until they fall through an opening at the bottom and are collected in a sample bag.

RC drilling is slower and costlier but achieves better penetration than conventional air core drilling. It is cheaper than diamond coring and is thus preferred for most mineral exploration work.

Depths of up to 500 metres are routinely achieved.

As for Air Percussion drilling water is also used, to reduce dust, keep the drill bit cool, and assist in pushing cutting back upwards. Sometimes the water is mixed with other substances the bring all the fine cuttings to the surface.

When the drill reaches hard rock, a collar is put down the hole around the rods which is normally PVC piping. Occasionally the collar may be made from metal casing. Collaring a hole is needed to stop the walls from caving in and bogging the rod string at the top of the hole. Collars may be up to 60 metres deep, depending on the ground, although if drilling through hard rock a collar may not be necessary.

Symmetrix Drilling

Symmetrix exploration drilling describes the process whereby a casing is installed as drilling proceeds. The sample passes up through the hammer as in Reverse Circulation drilling.

The casing shoe is welded to the casing. The pilot bit and ring bit are locked together and together they drill a hole large enough to allow the casing to be pulled into the hole. The pilot bit and ring bit rotate with the drill string while the casing shoe and casing do not rotate. After completing the hole, the pilot bit is unlocked by a slight reverse rotation of the drill string. The drill string and pilot bit are then retrieved through the casing.

This method of exploration drilling enables the drilling of holes at any angle (including horizontal), in any type of ground condition, and to depths beyond 100 meters.

Limitations

The main factor limiting the depth of percussion drilled holes is air pressure. Air must be delivered to the piston at sufficient pressure to activate the reciprocating action, and in turn drive the head into the rock with sufficient strength to fracture and pulverise it. As the hole gets deeper it’s volume increases and water pressure from ground water increases which means more air pressure is required to achieve operational pressures.

Dependant on the rock conditions, depths in excess of 500 m by reverse circulation drilling can become  cost prohibitive and approach the threshold at which diamond core drilling is more economical.

2. Rotary Core Drilling

Rotary Core Drilling utilises an annular diamond-impregnated drill bit attached to the end of hollow drill rods to cut a cylindrical core of solid rock. Fine to microfine industrial grade diamonds are set within a matrix of varying hardness to create a drill bit. The matrix hardness, diamond size and dosing can be varied according to the rock which must be cut. Holes within the drill bit allow water to be delivered to the cutting face. This provides lubrication, cooling, and removal of drill cuttings from the hole.

Drilling of 1200 to 1800 metres is common and at these depths, ground is mainly hard rock. Diamond rigs need to drill slowly to lengthen the life of drill bits and rods, which are very expensive.

Core samples are retrieved via the use of a lifter tube, a hollow tube lowered inside the rod string by a winch cable until it stops inside the core barrel. As the core is drilled, the core lifter slides over the core as it is cut. An overshot attached to the end of the winch cable is lowered inside the rod string and locks on to the backend, located on the top end of the lifter tube. The winch is retracted, pulling the lifter tube to the surface. The core does not drop out the inside of the lifter tube when lifted because a "core lifter spring," located at the bottom of the tube allows the core to move inside the tube but not fall out.

Once a rod is removed from the hole, the core sample is shaken out into core trays. The core is washed, measured and broken into smaller pieces using a hammer to make it fit into the sample trays. Once catalogued, the core trays are retrieved by geologists who then analyse the core and determine if the drill site is a good location to expand future mining operations.

3. Multi-combination Rigs

Rotary Core  rigs can also be part of a multi-combination rig. Multi-combination rigs are a dual setup rig capable of operating in either a reverse circulation (RC) and Rotary Core exploration drilling role (though not at the same time). This is a common scenario where exploration drilling is being performed in a very isolated location. The rig is first set up to drill as an RC rig and once the desired metres are drilled, the rig is set up for Rotary Core  drilling. This way the deeper metres of the hole can be drilled without moving the rig and waiting for a diamond rig to set up on the pad.

Limitations

Diamond exploration drilling can routinely achieve depths in excess of 1200 m. In cases where money is no issue, extreme depths have been achieved, because there is no requirement to overcome water pressure. However, circulation must be maintained to return the drill cuttings to surface, and more importantly to maintain cooling and lubrication of the cutting surface.

Without sufficient lubrication and cooling, the matrix of the drill bit will soften and the diamonds must remain firmly in the matrix to achieve cutting. The force exerted by the weight of the drill rods on the cutting face of the bit must also be monitored.