In this article we will talk mainly about the application for different types of bottom hole assembly (slick bha drilling, Pendulum Assembly & Packed BHA) its design & components. This article is a part of the our drill string design guide.
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In general there are three types of Bottom Hole Assembly BHA In drilling:
- Slick BHA Type
- Pendulum BHA Type
- Packed BHA Type
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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 which have mild crooked hole tendencies. Slick bottom hole assemblies are seldom. As there are no expensive components on this type of BHA, It is more easier to fish such BHA.
Sometimes when you want to wash and ream (clean out) in some difficult holes. We choose to pull out of the current directional BHA and to try to design a slick BHA but with the same configuration of the directional BHA. In other words it will be a slick BHA, as you won’t add any BHA component but you will have to use stabilizers with the same OD and the same position of any part of directional BHA previously drilled this hole so that to keep the geometry of 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 which we can use primarily to reduce or maintain hole deviation. The pendulum technique relies on the force of gravity to deflect the hole to vertical. 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 best position of the Stabilizer is as high as possible provided the Drill Collars below 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, size of 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 Bit (WOB) increased, 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 there is a 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
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.
There are three different types of bottom hole assembly packed BHA:
Mild Crooked Hole Packed BHA
We are using this bottom hole assembly type for formations which have mild crooked hole tendency 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 Drill Collar; and at Zone 3, on top of a standard length drill 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 which have 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 which have severe crooked hole tendency 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 include in Zone-1.
Forces Acting On Types of Bottom Hole Assembly
Any types of bottom hole assembly (Drilling BHA) Tendency is affected by the Forces Acting On Bottom Hole Assembly which will drive it into a certain direction. These Forces must be understood to be able 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