METHODS OF DEFLECTING A WELLBORE con't 2

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

1.        FP > FB, the hole inclination will drop
2.        FP < FB, the hole inclination will increase
3.       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

1. -Assembly A proved to be the most successful even though it maintained inclination only 60 percent of the time
2. -Assembly B maintained inclination less than 50 percent of the time
3. -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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METHODS OF DEFLECTING A WELLBORE

METHODS OF DEFLECTING A WELLBORE con't

 

METHODS OF DEFLECTING A WELLBORE con't 3

METHODS OF DEFLECTING A WELLBORE con't 4