Bottom Hole Assembly Notes for undrstanding

By Dr. Santosh Gupte

BOTTOM HOLE ASSEMBLIES

DEFINITION OF BHA

A bottom hole assembly (known as BHA) is a component of a drill string. A BHA resides

in the drill string above the drill bit and below the drill pipe. The primary component of

the BHA is the drill collar. The following figure shows the possible components of a

BHA and their typical location within a BHA.

PURPOSE OF BHA

1. Protect the drill pipe in the drill string from excessive bending and torsional loads, 2.

Control direction and inclination in directional holes,

3. Drill more vertical holes,

4. Drill straighter holes,

5. Reduce severities of doglegs, key seats, and ledges,

6. Assure that casing can be run into a hole,

7. Increase drill bit performance,

8. Reduce rough drilling, (rig and drill string vibrations), 9. as a tool in fishing, testing,

and work over operations, 10. Not to place weight on the drill bit

TYPES OF BHA'S

The "SLICK" BHA is composed only of drill collars. It is least expensive and perhaps

carries the least risk in regard to fishing and recovery.

The "PENDULUM" BHA is designed to drill holes more vertically and to drop

inclination in inclined holes. Lubinski and Woods published tables and charts to locate

the lowest most stabilizers in the BHA. Most BHA theories which were intended for

vertical holes apply to holes which are inclined 20 degrees or less.

The "PACKED" BHA is designed to drill straight holes and to reduce the severities of

doglegs, key seats, and ledges. It provides the highest assurance that casing can be run

into a hole. The theory which supports the packed BHA was developed by Roch. A

packed BHA can be expensive and perhaps carries the highest risk in regard to fishing

and recovery.

The "DIRECTIONAL" BHA is designed either to turn the hole to a chosen inclination

and direction or to maintain a course selected for the hole. The directional BHA is based

on the principles of levers and fulcrums.

The "FISHING, TESTING, and WORKOVER" BHA is designed to assist the many

tools found in these areas.

Many theories and practices apply equally to all four types of BHA's while others are

very restricted to one of the types. The performance of BHA's are affected most by the

rugosity (hole enlargements) of the hole. Other factors are

1. Formation dips 2. Torque while drilling 3. Drag during trips

4. Stabilizer blade wears 5. Drill collar OD wear 6. Mud filter cake

COMPONENTS OF BHA

Drill Bit:

Technically, the drill bit is not a component of the BHA; however, it does generate and

sends axial and torsional loads to the BHA. Its basic function is to cut rock at the bottom

of the hole.

Junk Sub:

A narrow trough which exists in the junk sub is designed to collect small pieces of metal;

such as, balls and rollers from bearings and broken bit teeth. y Shock Sub:

The purpose of shock subs is to dampen the vibration produced by the drill bit and the

drill string. It is reasonable to surmise that shock subs prolong the life of drill bits and

drill .strings and in some cases the rig. Shock sub are not as stiff (resistance to axial

bending) as drill collars and, for this fact, often have limited application in straight hole

drilling. In addition, large drill collars may be more effective in reducing bottom hole

vibrations.

Roller Reamers:

A reamer serves two functions: (1) it cuts the wall of the hole to gauge and (2) it centers

the BHA in the hole at its location. Reamers with 3 and 6 rollers are most popular. The

rollers are on bearings and they contain tungsten carbide inserts which are similar to

those placed in drill bits. Roller reamers are used in hard rock where blade reamers cause

excessive drill string torque.

Stabilizers:

Stabilizers center the BHA in the hole at their location. In soft rock, a rotating blade type

can effectively ream a hole to gauge. There are two basic types: rotating and non-rotating.

The blades of the rotating type may be spiraled around the body of the stabilizer or

aligned vertically and attached by welding, with a sleeve, or by pins. The rubber blades of

the non-rotating type are aligned with the vertical axis of the body of the stabilizer. The

rubber blades are popular because they are easily washed-over and do dig into the wall of

the hole causing enlargements. Short stabilizers can pivot in the hole and allow bending

moments to be transmitted to adjacent BHA components. They act as fulcrums. Long

stabilizers or stacked short ones do not pass bending moments and align themselves in the

hole. A BHA which does not transmit bending moments is called a "locked-up" BHA.

Square Drill Collars:

Square drill collars accomplish four goals: (1) they provide continuous centralization

over their length, (2) they maximize bending resistance (stiffness), (3) they maximize

torsional damping, and (4) they minimize axial vibrations. On the down side, they are

expensive to buy and to maintain. They usua1ly create high rotary torque. They grind

drill bit cuttings and cavings to fines which radically increases mud consumption. They

are difficult to fish. Note that an 11.2 inches round collar has the same stiffness as a

12.125 inches square collar.

Spiral Drill Collar:

Spiral drill collars reduce the risk of differential pressure sticking of the BHA. About 4%

of the weight of the drill collar is lost because of the machining of the spirals. Thus, the

linear weight per foot of an 8 by 3 inches spiral collar is 141 ppf rather than 147 ppf.

Non-magnetic Drill Collars:

The primary purpose of non-magnetic drill collars is to reduce the interference of the

magnetic fields associated with those sections of the BHA which are both above the

below the magnetic compass contained in the survey tool with the earth's magnetic field.

The non-magnetic collars reduce this type of interference by moving the BHA sections

away from the survey compass.

There are four critical factors in selecting non-magnetic collars: (1) their total length, (2)

the location of the survey compass with the non-magnetic collars, (3) the type of material

of which the collars are composed, and (4) distinguishing "hot spots".

"Hot spots" are zones of high magnetic field strengths within the material of the collars.

These zones are detected with a magnetic permeability probe. The measurements are

recorded on a strip chart versus distance into the collar. Hot spots can affect compasses

by as much as 4 degrees (2 degrees is common). A "cold" collar will affect a compass by

less than 1/4 degree over its entire length.

Non-magnetic collars may be manufactured from many types of material. The selection is

based primarily on the corrosion resistance of the material. The most common non-

magnetic material is stainless steel while monel, which is 60% nickel and 30% copper, is

seldom used.

A practical method of ascertaining the best location for the survey compass and the

requisite non-magnetic collar length is to run a BHA into the drill hole and pull a

compass through the non-magnetic collars. The portion of the nonmagnetic collars which

do not show the effects of the fields of the BHA may be removed from the BHA.

Medium and Large Round Collars:

The purposes of large round collars are to provide stiffness next to the drill bit and add

weight to the BHA.

The medium collars add weight to the BHA and reduce ever present flexure stresses

between large collars and drill pipe or other tools of less rigidity than the large collars.

Both may be used for jarring weight.

Heavy Weight Drill pipe:

Heavy weight drill pipe is small drill collars with drill pipe tool joints. They serve the

same purposes as medium weight collars. They may be operated in a buckled mode while

drilling or a part of a fishing BHA.

Key seat Wiper:

The purpose of key seat wipers is to ream the "key" section out of the wall of the hole.

Key seat wipers are run in the BHA or the drill pipe. The outside diameter of the cutting

structure of the key seat wipers are commonly selected as 1/8 to 1/2 inch larger than the

collars or tool joints in which they are installed.

Key seat wipers are either single or double acting; that is, the blade section which slides

on a mandrel reams either in the up or down position if single acting but not both.

The double acting type act if the blade section is up or down, but not if the section is

floating between up and down because most rigs cannot raise and rotate the drill string

simultaneously, the single acting wiper which reams in the down position is required.

Bumper Sub:

Bumper subs are used to counter the heave of floating drilling vessels by permitting the

extension and contraction of the drill string. In this use the bumper subs are placed

between the BHA and the drill pipe. They are also used in fishing operations in a manner

similar to that of jars.

Safety Joint:

Safety Joints are rarely used in a drilling BHA. Their purpose is to provide a means of

easily releasing a BHA.