Building Assemblies

As previously stated, the building assembly uses a stabilizer acting as a fulcrum to apply side forces to the bit. The magnitude of that force is a function of the distance from the bit to the tangency point. An increase in bit weight and/or decrease in drill collar stiffness will increase the side force at the bit increasing the rate of build.

The strongest building assembly consists of one stabilizer placed 3 to 6 feet above the bit face with drill collars and drill pipe above the stabilizer. This assembly will build under the majority of conditions. Of course, the rate of build will be controlled by formation tendencies, bit and stabilizer types, lithology, bit weights, drill collar stiffness, Drillstring RPM's, penetration rate, and hole geometry.




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Building Assemblies

Another strong to moderate building assembly consists of a bottomhole stabilizer placed 3 to 6 feet from the bit face, 60 feet of drill collars, stabilizer, collars, and drill pipe. This is the most common assembly used to build angle. The second stabilizer tends to dampen the building tendency. This assembly can be used when the previous assembly builds at an excessive rate. Other building assemblies can be seen above.

Dropping Assemblies

A dropping assembly is sometimes referred to as a pendulum assembly. In this assembly, a stabilizer is placed at 30, 45, or 60 feet from the bit. The stabilizer produces a plumb bob or pendulum effect; hence the name pendulum assembly. The purpose of the stabilizer is to prevent the collar from touching the wall of the hole causing a tangency point between the bit and stabilizer.

An increase in the length of the bottomhole assembly (the length below the tangency point) results in an increase in the weight. Since the force is determined by that weight, the force is also increased exceeding the force due to bending. The net result is a side force on the bit causing the hole to drop angle.

Additional bit weight will decrease the dropping tendency of this assembly because it increases the force due to bending. Should enough bit weight be applied to the assembly to cause the collars to contact the borehole wall (between the stabilizer and the bit), the assembly will act as a slick assembly. Only the section of the assembly below the tangency point affects the bit side force.

If an increase in dropping tendency is required, larger diameter or denser collars should be used below the stabilizer. This increases the weight of the assembly, which results in an increase in dropping tendency. As an example, suppose a dropping assembly with 7" (178mm) drill collars was being used in a 12 %" (311mm) hole. By substituting 9" (229mm) collars for the 7" collars, an increase in dropping tendency can be achieved.

Dropping assemblies will have a higher rate of drop as hole inclination increases.


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Dropping Assemblies

Holding Assemblies

Holding the inclination in a hole is much more difficult than building or dropping angle. Under ideal conditions, most assemblies either have a building or dropping tendency. Most straight hole sections of a directional well will have alternating build and drop tendencies. When holding inclination, these build and drop sections should be minimized and spread out over a large interval. The most common assemblies are indicated below indicating their strength at holding inclination.


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Holding Assemblies

When selecting a hold assembly, research the well records in the area to find out which assembly works best for the types of formations being drilled. If no formation is available, use a medium strength assembly and adjust it as necessary.

These build and drop assemblies are still used on directional wells but generally limited to slant hole drilling. The hold assemblies are very commonly used on deep vertical wells to minimize the amount of directional drilling required.


Tandem Stabilizers

It's fairly common to run a string stabilizer directly above the near-bit stabilizer. This is normally for directional control purposes. An alternative is to run a near-bit with a longer gauge area for greater wall contact. High rotary torque may result in either case. It is dangerous to run tandem stabilizers directly after a more limber BHA due to the reaming required and potential sticking.

April 2002


Chapter 4

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