SURFACE CORE DRILLING

HOW IT WORKS?

The goal of exploration drilling is to gather ground samples for a better understanding of the soil structure and mineralisation in a determined area.

 

Application Used for very accurate surface solid sampling extracted from deep, hard rock environments for stratigraphy and mineral content analysis
Sample quality High core sample with very reliable results
Drilling speed penetration Slow speed penetration depending on ground conditions normally 25 – 30m per shift
Hole depth Great depth hole can be achieved depending of on the capabilities of the drilling rig, but typically 3000 m
Hole size 46 to 146 mm
Flexibility Taking into account the ground geological characteristics and choosing accordingly the right range of tools, allow to be very flexible with this method of drilling
Cost High, because of drilling speed rate limitations and expensive tailor made drilling consumables made to suit the rig.
Time Time consuming as the sample has to be retrieved and core drilling is done penetration is slowly to preserve the life of the parts and also because of the rock hardness
Technique A drill is required to produce the rotation and force needed to drive a string of tools consisting of a bit which rotates at high speed, cutting through the fresh rock, core barrel assembly, drill rods through the ground, all the while retrieving a solid core sample. Flush is also used required for lubrication, cooling, and removal of drill cuttings from the hole.
Challenges The main challenge related to core drilling is providing a large inner diameter through the drill pipe to allow easy retrieval of core barrels

 

Drill Structure

Diamond core drilling is so called because it uses a ‘diamond bit’. This drill bit is can composed of group of small, industrial grade diamonds set into a metallic, soft matrix, or natural diamonds. As the ground is drilled, this matrix will wear away and expose more diamonds on the impregnated bits, but will just wear the diamonds on the natural diamonds.

diamond core drilling illustration

This is then attached to a core barrel which in turn is attached to a drill rod, which is around normally10 foot or 3 metres in length, and then more sections of pipe can be attached to the top of this so a greater depth can be drilled. The depth that is drilled to is estimated by the number of rods attached to the top of the drill rod.

The wireline core barrel has an inner-tube which allows the recovery of the sample core without taking out the core barrel, this is achieved by sending an overshot down the center of the drill pipe, latching on to the inner-tube and pulling the inner-tube to the surface to take out the sample.

Inside the drill rod, a core inner tube is attached to a cable via a latching mechanism. The core tube is lifted to the surface using the cable, so the solid core can be removed.

There are two primary types of diamond drilling-rotary conventional drilling and wireline drilling. In Conventional drilling the core barrel has to retrieved every run to recover the sample, this makes the drilling process very slow. Rotary drilling is used primarily for bore hole drilling, whereas wireline drilling is used for solid core sampling is more economical

There a five standard tube sizes associated with wire line drilling as follows:
AQ (Hole diameter: 48mm)
BQ (60mm)
NQ (75.5mm)
HQ (96mm)
PQ (122.6mm)

The drill size used depends on the desired core diameter and the desired depth of drilling, and the wider greater the diameter of the tube, the more power that is required to drive the drilling.