Rotary BHA

The rotary BHA consists of a bit, drill collars, stabilizers, reamers, subs and other special tools run below the drill pipe
Motors were used to put the wellbore on course and rotary BHA’s were used to drill the majority of the well
Even though rotary assemblies are used only occasionally, we will still look at them
Steerable motor assemblies in the rotating mode are still rotary BHA’s subject to the same influences as the rotary BHA
A slick assembly is simply a bit and drill collars

The deviation tendency is caused by the bending of the drill collars
The point at which the collars touch the wall of the hole is the tangency point
The resultant force applied to the formation is not in the direction of the hole axis but in the direction of the drill collar axis

The resultant force can be broken up into its components FB and FP

FB is the side force caused by the bending of the collars or building force
FP is the force due to gravity or pendulum force
Ideally, if

FP > FB, the hole inclination will drop
FP < FB, the hole inclination will increase
FP = FB, the hole inclination will remain constant

The building force can be increased by increasing bit weight, which drives the tangency point down
The building force is also affected by the stiffness of the collars
Stiffer collars will bend less
As the diameter of the collar increases, the stiffness of the collar increases

The pendulum force can be increased by reducing bit weight and using larger diameter collars
ØSome inclination is required to have a pendulum force
ØFor a slick assembly, the building and pendulum force will balance at relatively low inclinations; therefore, they are not expected to build much inclination

So why does the inclination exceed 1 to 2o in some areas while it does not in other areas?
If the bed dip is relatively flat, we seldom have any deviation problems
When bed dip is encountered, we can experience deviation problems in harder rock

Deviation problems are associated with formation dip
The anisotropy of the formation causes deviation

If the formation deviation tendency can be defined as a force FF, the resultant force at the bit would be
-FB + FP + FF
The wellbore will continue to build angle until the sum of the forces is equal to zero
Unfortunately it is difficult to define FF and it changes with depth

Rule of thumb

If the bed dip is less than 45 degrees, the bit will have a tendency to deviate perpendicular to the bed dip (up dip)
If bed dip is above 65 degrees, the bit will have a tendency to deviate along the bed dip
Between 45 and 65 degrees, the bit can do either
In directional drilling, it is the difference between the bit angle and the formation dip
The formation may want the bit to drop inclination when the wellbore is at an inclination greater than bed dip
The two forces associated with a rotary assembly are the building force (FB) and the dropping force (FP)
If we want to make a building assembly, the building force must be maximized
Stabilizers are used as fulcrums in order to increase the side force at the bit

Building assembly

A building assembly is constructed by placing a stabilizer near the bit
Bending of the drill collars above the stabilizer causes the building force at the bit to increase substantially

At low inclinations, the drill collars are bent by increasing bit weight

At higher inclinations, gravity will bend the collars and the build tendency is less affected by bit weight
In order to make a dropping assembly, the pendulum force is maximized by placing a stabilizer at least 30 to 90 feet above the bit

If the collars touch the wall of the hole between the stabilizer and the bit, the tangency point becomes the place where the collars first touch the wall of the hole
If the stabilizer is too far above the bit, the collars will touch the wall and the pendulum force will be reduced

-A holding assembly is constructed by placing stabilizers closer together so that the collars are more rigid
-Bit side force is minimized

Holding inclination is the most difficult with a rotary assembly

Analysis by Amoco indicated that

-Assembly A proved to be the most successful even though it maintained inclination only 60 percent of the time
-Assembly B maintained inclination less than 50 percent of the time
-Assembly C held inclination less than 50% of the time

To better understand the forces on a rotary BHA, lets look at a single stabilizer assembly with the stabilizer positioned at various distances from the bit

Rotary BHA’s were not very efficient as it was difficult to predict the actual performance of any rotary BHA
Frequent trips were required to change stabilizer placement and size

The build, hold or drop tendency of the rotary BHA could be adjusted but the walk tendency could not be changed substantially

A pure rotary assembly is used only occasionally today
Derivatives of the rotary assembly are used frequently
Steerable motor assembly
Rotary steerable assembly

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