Rotary Bottomhole Assemblies

There are three basic types of rotary bottomhole assemblies used in directional drilling: Building Assemblies, Dropping Assemblies, and Holding Assemblies

A building assembly is intended to increase hole inclination, a dropping assembly is intended to decrease hole inclination, and a holding assembly is intended to maintain hole inclination. It should be noted that a building assembly might not always build angle. Formation tendencies may cause the assembly to drop or hold angle. The building assembly is intended to build angle. The same is true for the dropping and holding assemblies.

Before the invention of measurement while drilling (MWD) tools and steerable motors, rotary bottomhole assemblies (BHA) were used to deflect wellbore. A bottomhole assembly is the arrangement of the bit, stabilizer, reamers, drill collars, subs and special tools used at the bottom of the drill string. Anything that is run in the hole to drill, ream or circulate is a bottomhole assembly. The simplest assembly is a bit, collars and drill pipe and is often termed a slick assembly. The use of this assembly in directional drilling is very limited and usually confined to the vertical section of the hole where deviation is not a problem.

In order to understand why an assembly will deviate a hole, let's consider the slick assembly, which is the simplest and easiest to understand. The deviation tendency in this assembly is a result of the flexibility of the drill collars and the forces acting on the assembly causing the collars to bend. Even though drill collars seem to be very rigid, they will bend enough to cause deviation.

The point at which the collars contact the low side of the hole is called the tangency point. The distance L from the bit to the tangency point is dependent upon collar size, hole size, applied bit weight, hole inclination, and hole curvature. Generally, the distance L is less than 50m (150 feet).

Above the tangency point of the slick assembly, the remainder of the drill string generally has no effect on deviation. As weight is applied to the bit, the tangency point will move closer to the bit.

Because of the bending of the drill collars, the resultant force applied to the formation is not in the direction of the hole axis but is in the direction of the drill collar axis. As bit weight is applied, the tangency point moves toward the bit increasing the angle. It can readily be seen that an increase in bit weight leads to an increase in deviation tendency.

Unfortunately, the direction of the resultant force is not the only force involved. The resultant force can be broken up into its components. The primary force would be the drilling force in line with the axis of the borehole. The bit side force is caused by the bending of the collars and is perpendicular to the axis of the borehole. The force due to gravity (acting on the unsupported section of drill collars) is in the opposite direction and counteracts the side force. The net deviation force is then equal to the summation of the bit side force and the force due to gravity. If the force due to gravity is greater than the bit side force the angle will drop.

Changing the weight on bit can control the deviation tendency. Increasing the bit weight will lower the tangency point increasing the angle. Since resultant force is proportional to the sine of angle, an increase in bit weight increases the bit side force and ultimately the deviation tendency. Of course, a decrease in bit weight will decrease the deviation tendency.

Tangency Point Drilling
Effect of Increased Bit Weight

Another factor affecting deviation tendency is the stiffness of the drill collars. Stiffer collars will bend less, which increases the height to the tangency point. If the tangency point moves up the hole, then the deviation tendency will be reduced. Therefore, small diameter drill collars will enhance the deviation tendency.

The addition of a stabilizer above the bit significantly affects the deviation tendency of a bottomhole assembly. The stabilizer acts as a fulcrum around which the unsupported section of the bottomhole assembly reacts. The addition of the moment arm between the bit and stabilizer increases the bit side force. In fact, the single stabilizer assembly is a very strong building assembly.

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