Normally, core is extracted inside the barrel by advancing the drill by its rotary action. When the depth of drilling increases (say beyond 300 m), withdrawing the entire heavy drill pipe is not only time-consuming but cumbersome; hence, a method has been developed to retrieve the core inside the barrel.
Suppose a solid granite area is drilled, the core would normally break at the drill bit when drilling is stopped, then a grabbing device is lowered by a wire to pull up the core. With the objective of cutting the downtime spent in hoisting and lowering of the drill string, the wire-line system of drilling was developed.
It is a special type of core drilling most commonly used for mineral exploration. In wire-line drilling in medium hard and soft formations, as are usually encountered in coal areas (Sandstone, Shale coal), it is seen that in deeper holes, more time is spent in hoisting and lowering of drill string, for taking out core sample after every 3-m run in comparison to the time spent in cutting the 3-m depth.
In order to remove the core from wire-line core barrel, an overshot assembly is lowered on the end of the wire-line. It gets attached to the back of the core barrel inner tube and the wire-line is pulled back to retrieve the core sample.
Only core sizes of NX and BX sizes are possible with the wire-line system.
Advantages of wire-line core drilling
- More time is available for drilling, hence speedier progress can be made.
- Hoisting the drill string needs to be done only when the bit is to be changed.
- Wire-line drilling is possible up to a depth of 1000 m.
- Best suited in coal formations drilling where depths involved are in the range of 200 m.
- Ordinary drilling equipment can be adopted to this system with necessary modifications.
- Wire-line rods are as per specifications as laid in “Q” series of DCDMA.
