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 bitBending 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
-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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