Sometimes, it is good to get back to basics and review the fundamentals of what we do. In this blog, which you can consider a primer, I will be reviewing some of the basics of core drilling, including methods and process.
There are two ways to do core drilling: conventional and wireline. The main difference is how you recover the core sample once the inner tube is full. In conventional drilling, the entire drill string needs to be pulled out of the bore hole for each drilling cycle. This means that all the drill rods must be removed from the hole and the threads must be taken apart and rejoined for each core sample.
In wireline core drilling, a piece of equipment called an overshot is sent down the hole to release and retrieve the inner tube. The overshot and inner tube are then brought up to the surface using a wireline hoist, allowing the rods and core bit to remain in the hole. A new inner tube is then lowered down and locked into place. The wireline method is more common these days as it is more efficient and safe.
Three major operations make up the core drilling cycle: the actual drilling into the ground (or the advancement of the bit), retrieving the core sample from the core barrel using one of the 2 methods listed above, and replacing the equipment with a clean, lubricated inner tube assembly which will allow you to resume the drilling cycle. These 3 actions make up a drilling cycle. After each drilling cycle, the bore hole is made deeper by the length of the core barrel.
While the drilling cycle is underway, drilling fluids are flushed down into the drill string. These fluids will be under pressure and will flush past the face of the drill bit as it drills, cooling it and bringing the drill cuttings back up to the surface. When the drill bit starts to wear out, it will no longer cut well. It will need to be replaced and the entire drill string will need to be removed from the bore hole in both conventional and wireline methods. Choosing the right core bit for the type of ground becomes very important in order to get the longest bit life and reduce downtime.
The core sample
The core is the ultimate goal of this process and there are products that can be added to drilling fluids to make core recovery easier and extend the life of drilling equipment. For example, these products can help solidify the sample, stabilize boreholes, lubricate equipment, prevent rust and reduce torque.
After each drilling cycle, the core sample is stored in a core box and logged with the date and time and specific location of the drilling. The driller should also take note of the depth, core recovery and any losses. A good driller will know to the inch, the location of his drill bit. This information is important for the geologist and will help them locate mineral deposits and determine whether the quantities are large enough for mining.
Many of you may already be familiar with the above but it never hurts to review basic information. For those who may be new to diamond drilling, or who simply have questions, you can always contact our technical team. They have a wealth of experience to share.