Types Of Bottom Hole Assembly (BHA)

In this article, we will talk mainly about the application for different types of the bottom hole assembly (slick BHA drilling, Pendulum Assembly & Packed BHA), its design & components. This article is a part of our drill string design guide.

The Slick BHA

This type of bottom hole assembly (slick BHA) consists of a Drill Bit and Drill Collar without Stabilizer. Slick BHA is suitable for formations that have mildly crooked hole tendencies. Slick Bottom Hole Assemblies are seldom. As there are no expensive components in this type of BHA, It is easier to fish such BHA.

Sometimes, we want to wash and ream (clean out) in some difficult holes. We choose to pull out of the current directional BHA. Then, we try to design a slick BHA but with the same configuration as the directional BHA. In other words, it will be a slick BHA, as you won’t add any BHA component. Still, you will have to use stabilizers with the same OD and position as in the previously used directional BHA. That is to keep the geometry of the cleaning assembly (which will be slick BHA with some stabilizers as same as the geometry of the directional bottom hole assembly).

The Pendulum BHA Assembly

The Pendulum BHA Assembly is one of the types of bottom hole assembly that we can use primarily to reduce or maintain hole deviation. The pendulum technique relies on the force of gravity to deflect the hole to vertically. The force of gravity, F sin I in the above figure, is related to the length of Drill Collars between the Drilling Bit and the point of tangency. The along-hole component of the force F is F cos I, which attempts to maintain the present hole direction.

The Pendulum BHA Assembly & Design

The pendulum type of bottom hole assembly BHA consists of a Drilling Bit, several Drill Collars to provide the pendulum force, and one or more Stabilizers. Lubinski presented charts for determining the location of the first Stabilizer from the Drilling Bit and the weight that can be applied on the Drilling Bit to maintain the best position of the Stabilizer is as high as possible, provided the Drill Collars below the Stabilizer do not contact or barely contact the wall of the hole.

The optimum position of the Stabilizer depends upon the size of the Drill Collars, the size of the hole, hole inclination, and Weight On Bit (WOB). So if we can maintain the weight On Bit (WOB) from the graphs constant, the hole deviation will not change. If the Weight On the Bit (WOB) increases, the hole angle will increase, and the opposite is true.

The Pendulum BHA Assembly Disadvantages

• The main disadvantage of the pendulum Bottom Hole Assembly BHA is that the Stabilizer must be placed within a few feet of a definite position obtained from the charts.
• Also, if we need to change the weight on bit, we will have to pull the BHA to change the position of the Stabilizer.

The Packed Bottom Hole Assembly

It relies on the principle that two points will contact and follow a sharp curve, while three points will follow a straight line, as shown. We can obtain three-point stabilization by placing three or more Stabilizers in the portion of the hole immediately above the bit.

Packed Bottom Hole Assembly BHA Types:

Mild Crooked Hole Packed BHA

We are using this bottom hole assembly type for formations that have mildly crooked hole tendencies that produce little or no deviation, such as hard and isotropic rocks. The three-point stabilization is provided at Zone 1, immediately above the Drilling Bit; at Zone 2, immediately above a short, large OD Drill Collar; and at Zone 3, on top of a standard-length Drill Collar.

Note: If a vibration dampener (shock absorber) is necessary, we should place it at Zone 2 for maximum effectiveness.

Medium Crooked Hole Packed Bottom Hole Assembly BHA.

We always utilize this type of Bottom Hole Assembly BHA for formations with medium crooked hole tendency, such as medium and soft formations. In this type of Bottom Hole Assembly BHA, we place a second Stabilizer at Zone-1 in order to provide increased Drilling Bit stabilization against deviation effects of the formation.

Severe Crooked Hole Packed BHA

Drilling engineers utilize this type of Bottom Hole Assembly BHA in drilling formations that have severely crooked hole tendencies, such as medium and soft formations, which show a great degree of dipping, fracturing, and variation in strength. In this type of Bottom Hole Assembly BHA, three Stabilizers are included in Zone-1.  

Forces Acting On Types of Bottom Hole Assembly

Bottom hole assembly (Drilling BHA) tendency is affected by the Forces Acting On Bottom Hole Assembly, which will drive it in a certain direction. These Forces must be understood to choose the most applicable Bottom Hole assembly BHA type.  Let’s move back to our main subject, Step By Step Learn How To Perform Full Drill String Design.