The bottom hole assembly (BHA) consists of the drill bit and downhole tools attached to the bottom of the drill string. There are three main types of BHAs: slick, pendulum, and packed. A slick BHA contains no stabilizers, while a pendulum BHA uses the force of gravity to redirect the wellbore vertically. A packed BHA utilizes three points of contact to drill straight sections and two points of contact to follow sharp curves. The document discusses the components and functioning of different BHA types.

1. MODULE-8
BOTTOM HOLE ASSEMBLY
DEVIATION WITHOUT DEVIATING TOOLS
By
GAURAV HAZARIKA
Assistant Professor
School of Petroleum Technology
Pandit Deendayal Petroleum University
DRILLING ENGINEERING II

2. BOTTOM HOLE ASSEMBLY
The lower portion of the drill string, consisting of the bit, bit
sub, a mud motor (in certain cases), stabilizers, drill collar,
heavy-weight drill pipe, jarring devices ("jars"), and crossovers
for various thread forms. It must provide force for the bit to
break the rock (weight on bit), survive a hostile mechanical
environment, and provide the driller with directional control of
the well.

3. BOTTOM HOLE ASSEMBLY
Often the assembly includes a mud motor, directional
drilling and measuring equipment, measurements-while-drilling
tools, logging-while-drilling tools and other specialized devices.
In general, there are three types of BHA
• Slick
• Pendulum
• Packed BHA

4. Slick BHA
A slick BHA may be defined as one in which no stabilizer is
used. The drill string simply consists of bit, drill collars and drill
pipe. Thus, this type of BHA is suitable only for mild crooked
hole formations. Slick BHAs are seldom used except to drill to
kick-off point in directional wells.

5. Pendulum BHA
BHA using the pendulum technique
are primarily used to reduce hole
deviation, as exemplified by the
drop-off section of an S-type
directional well.

6. Pendulum BHA
• Used to drill soft, unconsolidated formations
• Technique relies on the principle that the force of gravity may be
used to deflect the hole back to vertical.
• The force of gravity F (shown below) is related to the length of drill
collars between the drill bit and the point of tangency called 'active
length' of drill collars.
• Increasing the active length means that the vertical force is
increased and the hole is quickly deflected back towards the
vertical.

7. Pendulum BHA
• The actual force which brings about
the deflection towards vertical is
FSinI which acts perpendicular to
the well-bore axis.
• FCosI maintains the present hole
direction & WOB.
• Thus Increment of FSinI to increase
deviation

8. Pendulum BHA
• Maximum pendulum force can be obtained by placing the first string
stabilizer as high as possible above the bit so that the active length of
the drill collars will not touch the walls of the hole.
• However, if the distance from the bit to the first stabilizer is made too
large, there is a danger that the 'active length' of the drill collars will
contact the low side of the hole, thus reducing the pendulum effect.
• Wilson pointed out that a second stabilizer 30 ft. above the first string
stabilizer should be used to reduce the lateral force on the first
stabilizer.

9. Fulcrum BHA
BHA using the fulcrum (Simple Machine) technique are
primarily used to increase hole deviation, as exemplified by the
build-up section of a directional well. The Near Bit stabilizer
behaves like a fulcrum point for this effect

10. Vaarious BHA

11. Packed Hole Assembly
The packed hole assembly relies on the
principle that two points will contact and
follow a sharp curve, while three points
will follow a straight line.

12. Mild crooked hole packed BHA
• The three-point stabilization is provided at
• Zone 1: immediately above the bit
• Zone 2: immediately above a Pony Collar
• Zone 3: on top of a standard drill collar
• Vibration dampener (if required) should be
placed at Zone 2 for maximum effectiveness
• A vibration dampener is used to absorb bit
bounce and prevent vibrations from being
transmitted to the surface.

13. Medium crooked hole packed BHA
• A second stabilizer is included at
Zone 1 in order to provide increased
bit stabilization against the
deviation effects of the formation.
Zones 2 and 3 are similar to those
of Type (1) packed BHA.

14. Severe crooked hole packed BHA
• In this type of packed BHA three
stabilizers are included at Zone
1. Zones 2 and 3 are similar to
those of Type (1) packed BHA.