Core drilling techniques – what you need to know
Core drilling is a technique used to bore a cylindrical hole into a material like solid metal or the earth. This technique dates back to 3000 BC when the ancient Egyptians invented it. It has several applications like penetrating holes through materials as hard as stone or concrete, mineral exploration. This technique is useful for the places where the minerals are several hundred feet below the surface of the earth.
There are many techniques of core drilling to sink a borehole into the ground. Each technique has various capabilities of the depth of the holes drilled. All these techniques can be widely classified into two categories based on the tasks performed:
- Core drilling technique which produces rock chips: When there is solid mineral accumulated in the earth’s core, one would need to extract it by extracting the rock. This is where this technique is utilized.
- Core drilling technique which produces core samples: Places where you need to inspect a core to check its contents, this technique is used.
Both these techniques have the same methodology just different drill blades.
There are several different types of drilling methods as listed below:
Auger Core Drilling: This is done with a helical screw which is driven into the earth with rotation and a part of the earth is lifted out by the blade of the screw forming the required borehole. Auger drilling is restricted mainly to soft and unconsolidated material and weak weathered rock. Also, it is cheap and a quick method.
Rotary Air Percussion Core Drilling: This is also known as air percussion drilling. It is used most frequently in the mineral and water exploration industry. The core drilling instrument used in this uses a pneumatic reciprocating piston-driven ‘hammer’ to energetically drive a heavy drill into the rock.
Cable tool core drilling: They are mainly used traditionally for drilling water wells on many farms. Most of the large diameter water supply wells including the deep wells completed in bedrock aquifers are constructed using this drilling method. It is also known as ballistic good drilling. Cable tool core drilling is simple to operate as well as is cheaper than the rest of the core drillers.
Reverse circulation core drilling: This method is just like the process of air core drilling in which drill cuttings extracted are returned to surface inside the rods. Reverse circulation core drilling has utilized large rigs that can drill cylindrical holes as deep as 500 meters with ease. It is cheaper than diamond core drilling and is preferred for mineral exploration. But it is costlier than other techniques. But again, it achieves better penetration. It is air powered and sometimes or water powered to reduce dust.
Diamond core drilling: This technique utilizes an annular diamond impregnated drill bit which is attached to the end of hollow drill rods in order to cut a cylindrical core of solid rock. The type of diamonds used are fine or micro-fine industrial grade diamonds and are set within a matrix of varying hardness. It is much slower than reverse circulation core drilling but can cut through extremely hard rocks easily. Of course, due to the diamonds, this technique is extremely expensive as well as very accurate.
Direct Push Core Drilling: This technique utilizes several types of core drilling rigs and core drilling equipment which advances a drill string by pushing or hammering or applying pressure instead of rotating the drill string. The drillers use hydraulic cylinders and hydraulic hammers. This technique is mainly used for producing core samples. The advantage is that it can help produce a large number of high-quality samples quickly and cheaply.
All these types of core drilling make the process easier. But proper precautions need to be taken to avoid grave injuries during the process. Core drilling machines can be hazardous if not handled by experienced personnel.