DRILLING BHA TYPES USED IN OILFIELD

Drilling BHA Types & applications

In general there are three types of Bottom Hole Assembly BHA: Slick, Pendulum and Packed BHA.

The Slick BHA

The slick BHA consists of a Drilling Bit and Drill Collars
without Stabilizer.

This BHA is suitable for formations which have mild crooked hole tendencies. Slick bottom hole assemblies are seldom used.

The Pendulum BHA

is used 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 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). If the Weight On Bit (WOB) obtained from the graphs is maintained constant, the hole deviation will not change. If the Weight On Bit (WOB) is increased, the hole angle will increase and the opposite is true.

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 the BHA must be pulled out 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 . A three-point stabilization is obtained by placing three or more Stabilizers in the portion of the hole immediately above the bit.

Mild Crooked Hole Packed BHA

This is used 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. If a vibration dampener (shock absorber) is  required, it should be placed at Zone-2 for maximum effectiveness.

Medium Crooked Hole Packed BHA.

This Bottom Hole Assembly BHA is used for formations which have medium crooked hole tendency such as medium and soft formations. In this type of Bottom Hole Assembly BHA a second Stabilizer is included at Zone-1 in order to  provide increased Drilling Bit stabilization against deviation effects of the formation.

Severe Crooked Hole Packed BHA

This type of Bottom Hole Assembly BHA is used for 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 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.

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