I have noticed that there are a lot of drillers who use the 6 inch reaming shell pretty exclusively, so in this blog I wanted to talk about the other sizes available and how to know when you should use another size. Reaming shells are used to maintain the diameter of the bore hole and to make sure other drilling equipment does not get stuck down the hole. The outside surface of the reaming shell has natural and synthetic diamonds embedded in a resistant matrix.
Reaming shells come in 6, 10 and 18 inches. How do you know when do you use each size?
6 inch reaming shells
The 6 inch reaming shell is the standard that is used for every kind of rock hardness or ground condition and is the minimum reamer requirement to provide minimum stabilization. Since the 6’’ has only one diamond pad, the vibrations from the rods and outer tube can be conducted easily and make deviation more likely.
When drilling is done in difficult ground conditions, such as swelling clay, a shorter reamer is a good choice as it can get through the ground without catching.
10 inch reaming shells
You need to consider a longer reaming shell once deviation becomes an issue. A 10’ inch reamer is a good compromise between the 6 inch and the 18 inch. The 10 inch has two diamond pads so it will provide better stabilization. A 10 inch reamer is a good choice for underground drilling where adequate space can be a challenge. It will do the job in shallower holes requiring stabilization and where an 18 inch reamer may be just a bit too much.
10’’ is also mainly used with 3m core barrels, so you can still pull your 3m inner tube + extension in one shot.
18 inch reaming shells
You should switch to an extended reaming shell when you have to keep your hole on direction and avoid deviation. The more diamond pads that you have on the reaming shell, the more control you will have on deviation and on hole diameter. The 18 inch gives you 4 pads. The front diamond pads will help you stabilize the core barrel and control the direction while the pads in the back will help maintain the diameter of the hole.
You can use an 18 inch reaming shell on the front end and a diamond type locking coupling on the back to make sure you have good stabilization of the core barrel. You can also use two 18 inch reamers when you have a longer core barrel (6 meters or more). It can act as a core barrel extension allowing you to get more core in the tube at one time.
Of course, the best way to deal with hole deviation is to take measures to prevent it. The main advantage of extended reaming shells is that they help you do just that. Geologists often make specific requests that a hole proceed in a very straight direction for a certain number of meters. Extended reaming shells are less costly and time consuming than using directional drilling equipment orwedges to try and get a hole back on the proper trajectory.