Core recovery

To collect the core of the rock drilled, a device known as the core barrel is used. Its length varies from 0.5 to 3m. There are two types of this:

1.  The single tube core barrel, and

2.  Double tube core barrel.

A single tube core barrel is suitable for homogeneous formations where the core is not eroded by flushing water and a solid core can be taken without risk of blockage in the barrel.

The connections of the diamond crown, the single-tube core barrel and the mud bucket (also called calyx) are shown in Fig. 3.10. The core lifter is placed within the bevel shell which has its inside conically shaped to receive the former. The core lifter is corrugated on the inner face and is a split ring. It occupies the wider portion of the bevel shell when drilling takes place so that it has little or no tendency to grip the core. After certain progress in drilling when the rods are lifted to take out the core, the split ring descends inside the bevel shell and grips the core. The latter may now be broken off by a twist and raised to the surface. The core lifter is replaced after about 250 m of drilling.

The larger particles of drill cuttings which the circulating water fails to carry upto the surface settle down in the mud bucket. Where supply of flushing water is plentiful, calyx is not necessary. The water under circulation is nearly 900 litres per minute.

In soft and friable rocks, the core is partially washed away due to the circulating water flowing past it. The rotation of the barrel greatly assists in grinding the core so that its recovery in a single tube core barrel is poor. To avoid these difficulties a double tube core barrel is used, specially where good core of soft rock is desired. In a double tube core barrel the inner tube which holds the core does not rotate uring drilling as it is suspended on ball bearings mounted in the block at the top of the barrel permitting the inner core barrel to remain stationary. Moreover, water does not flow past the core but in the annular space between the inner tube and the outer barrel and through channels near the bottom of the hole. A double tube core barrel improves drill bit economy and overall drilling performance. Core recovery is good in hard uniform rocks, but poor in loose, soft, friable or weathered rocks. Vibrations of drill rods result in poor recovery. In hard rocks, to achieve good core recovery the drill should be run at low speed and heavy pressure; in soft rocks, reverse procedure should be adopted. The combination of pressure on drill bit and its rotational speed should be such as to give vibration-free drill string during drilling.