Bottom hole assembly

A bottom hole assembly (BHA) is a component of a drilling rig. It is the lowest part of the drill string, extending from the bit to the drill pipe. The assembly can consist of drill collars, subs such as stabilizers, reamers, shocks, hole-openers, and the bit sub and bit.

The characteristics of the BHA help to determine the borehole shape, direction and other geometric characteristics.

The BHA is used to help the drilling process; the proper selection of the right BHA would go a long way in ensuring high ROP and thus help drill quickly and efficiently. This would lead to lowered drilling costs.

Rotary assemblies are commonly used where formations are predictable and the rig economics are an issue. In such an assembly the weight of the drill collars gives the BHA the tendency to sag or flex to the low side of the hole, collar stiffness length and stabilizer diameter and placement are engineered as a means of controlling the flex of the BHA. this will bring about the desired hold, build or drop tendency.

The ability to vary the directional tendency of the assembly comes from varying the weight on the bit. A fixed assembly has only one directional tendency. The weight on bit allows you to tune that tendency.

The bottom-hole assembly (BHA) can be:

Bottom-hole assemblies are also described as

This assembly is used to build angle. The assembly usually incorporates a near bit stabilizer. Behind this would be a selection of drill collars and Heavy Weight Drill Pipe (HWDP). The length of the section AFTER the near bit stabilizer would determine the extent of the angle build rate.

After the appropriate length of pipe after the near bit stabilizer you would place a drill string stabilizer.

In short the longer the gap between the near bit and the drill string stabilizer the greater the angle building rate. Care must be taken to not have this section too long as the pipe may sag too much and rub against the borehole wall. This could result in key seating and pipe digging into the borehole wall.

This assembly is usually used to maintain borehole angle. This assembly is very rigid allowing little movement of the bit.

Such an assembly would mean that the stabilizers are closely packed: a near bit stabilizer within 0–30 feet of the drill bit, and two more spaced 30 and 60 feet beyond it. If a short drill collar is used, then the stabilizers can be even closer together.

The shorter the distance between the stabilizers means that the drill collars bend less and the weight on bit (WOB) pushes directly on the bit, hence maintaining the angle.

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Wikipedia