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BHA's numbered 1, 2, and 3 have small diameter collars. Those assigned numbered 3, 4, and 5, have intermediate size collars, and those numbered 6, 7, 8, 9, have large collars. BHAs numbered 8 and 9 have square collars and a roller reamer below the square collar. Only BHAs 5, 7, 8, and 9, are truly packed BHAs.

If drilling with a 12.25 inches bit, then 7 inches collars are small, 9.5 inches collars are intermediate, and 11 inches collars are large.

In practice the roller reamers protect the square collars from excessive wear rates. In hard rock roller reamers are required because blade stabilizers create excessive torque if the bit drills an under gauge hole.

It has been argued that square drill collars appreciably increase mud bills because they grind cuttings to an unremovable size before cuttings can pass by the square collars.

Square drill collars may increase drill string torque to an unacceptable level. It may be impossible to run a square collar after drilling a hole or a section of a hole with large round collars even if stabilizers were used. Most drillers drill out beneath the last casing string with squares for this reason. Often, it is impossible to run large round collars after drilling a hole with intermediate or small collars.

The packed-pendulum BHA has a packed assembly above the pendulum section. Its purpose is to assure that a packed BHA will follow a pendulum BHA. After the pendulum section is removed, reaming of the hole below the packed assembly may be required.

Fishing for large round collars and squares may be excessively expensive or impossible to recover.

Shock subs are not stiff and because of this they should not be run directly above the bit. A recommended location is above the first drill collar, if doglegs are a potential problem.

Stabilizers, reamers, large diameter collars, and square collars are expensive. ADVANTAGES

Packed BHA's are very popular because they serve all the purposes of BHA's but two: (1) they don't drill more vertical holes and (2) they don't drill directional holes. However, packed BHAs do the following very well:

1. protect the drillpipe in the drill string from excessive bending loads,
2. drill straighter holes,
3. reduce severities of doglegs, keyseats, and ledges,
4. assure that casing can be run into a hole,
5. increase drill bit performance,
6. reduce rough drilling, (rig and drill string vibrations).