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Bottom Hole Assembly BHA For Rotary Directional Drilling

A conventional rotary drilling assembly is normally used when drilling a vertical well, or the vertical or tangent sections of a deviated well. When using a Rss Rotary steerable assembly or Non-Rotating Steerable Downhole Mud Motor Assembly in a deviated well it is of course possible to drill the tangent sections of the well with the the steering action. But In this article we introduce how to use the Rotary Directional Drilling Bottom Hole Assembly BHA to Build ( Fulcrum Assembly), Drop (Pendulum BHA) or Hold (Packed BHA).

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The BHA of the conventional assembly can also be designed in such a way as to result in an increase or decrease in the inclination of the wellbore but it is very difficult to predict the rate at which the angle will increase or decrease.

The tendency of a Conventional Bottom Hole Assembly BHA Components & Design to result in an increase or decrease in hole angle is a function of the flexibility of the BHA & WOB (Check how to run WOB Calculations). Since all the Drill String Design Components are flexible to some degree the BHA will bend when weight is applied to the bit( Check Full guide For Types of drilling Bits). This will introduce a tilt angle at the bit. It is very difficult to predict the impact of the above variables on the rate at which the angle will increase or decrease.

Three types of directional drilling BHA assemblies:

There are three basic types of assemblies used in directional drilling. They are:

  • 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 may 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

Fulcrum Assembly As A Building Directional Drilling Bottom Hole Assembly BHA

The principles behind a Fulcrum Assembly is to place a reamer near the bit (check also Drilling Bits Types) and apply a high WOB. When WOB is applied, the Drill collars above the reamer will tend to bend against the low side of hole, making the reamer act as a fulcrum forcing the bit upwards. The rate of build up depends on WOB, size of collars, position of reamer and stabilization above the reamer.

This building fulcrum assembly uses a stabilizer (Check Drilling Stabilizer Design & Types) acting as a fulcrum in the directional drilling bottom hole assembly BHA 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 fulcrum assembly consists of one stabilizer placed 3 to 6 feet above the bit face with collars and drill pipe above the stabilizer (a second stabilizer can be placed 90 feet above the bit).  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 (at lower inclinations), drill collar stiffness, drill string rpm’s, penetration rate, and hole geometry.

Another strong to moderate building fulcrum assembly consists of a bottom hole stabilizer placed 3 to 6 feet from the bit face, 60 feet of collars, stabilizer, collars, and any Drill Pipe Description & Specs.   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 in Figure 1.

Fulcrum As A Building Directional Drilling Bottom Hole Assembly BHA
Figure 1 Building Bottom hole Assembly In Directional Drilling

The basic Directional Drilling Bottom Hole Assembly BHA For Building Actiony is:

Bit – sub- reamer – Monel DC – DC – stab – DC – stab – 90’DC – stab

To increase the build in any Directional BHA ( Fulcrum Assembly):

  • Add more WOB
  • Use smaller size monel (increase buckling effect)
  • Reduce RPM and pump rates in soft formations

Packed Hole Assembly As a Holding BHA For Directional Drilling

This type of configuration is a very stiff assembly, consisting of drill collars and stabilizers positioned to reduce bending and keep the bit on course. This type of assembly is often used in the tangential section of a directional hole.

We can say that, 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 sections.  When holding inclination, these build and drop sections should be minimized and spread out over a large interval to reduce dogleg severity.

Amoco had statistically analyzed various holding assemblies comparing their performance. Figure 2 shows three of the most common holding assemblies.  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, and assembly “C” even less.

When selecting a holding assembly, research the well records in the area to find out which assembly works best for the types of formations being drilled.  If no information is available, use assembly “A” and adjust it as necessary.

The Mechanism Of Directional Holding BHA

The idea behind a holding assembly is to minimize the side force at the bit.   By placing the stabilizers closer together, the amount of bending between is the stabilizers is substantially reduced.   There are also three points of stabilization; one above the bit and then two more points up the hole.  Holding assemblies will be rigid with low bit side force, and the bit side force will be relatively unaffected by bit weight.

Rotary BHA’s can also be made with an adjustable stabilizer.  The adjustable stabilizer is a stabilizer where the gage can be adjusted while the stabilizer is downhole.  They are usually adjusted by cycling pump pressure and weight (mechanical and hydraulic).   The adjustable gage stabilizer is usually placed in a rotary assembly as shown in Figure 7-18.   With the adjustable gage stabilizer on top, the assembly can be made to hold, build or drop inclination. With the assembly on the left side of Figure 7-18, the assembly will have a build tendency if the top stabilizer is significantly under gage, will have a drop tendency if the top stabilizer is full gage and will have a hold tendency if the top stabilizer is moderately under gage.  With the assembly on the right, the middle stabilizer is the adjustable gage stabilizer.  If the stabilizer is under gage, it will be a building assembly.   If the stabilizer is full gage, it will be a holding assembly.

The adjustable gage stabilizer makes the rotary assembly partially steerable.  The gage on the stabilizer will have a predictable affect on inclination but it does not have a predictable affect on direction.  However, it is very useful in wells where the hole drag will minimize the effectiveness of  the  steerable motor  assembly.    It  is  also  considerably less  expensive than  the  rotary steerable assembly.

In practice it is very difficult to find a tangent assembly which will maintain tangent angle and direction. Short drill collars are sometimes used, and also reamers or stabilizers run in tandem.

holding bottom hole assembly for directional drilling

Pendulum Assembly or Dropping BHA

The principles behind a Pendulum Assembly is that the unsupported weight of drill collars will force the bit against the low side of the hole. The resulting decrease or drop off in angle depends on WOB, RPM, stabilization and the distance between the bit and the first reamer.

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 effective length of the pendulum bottom hole assembly (the length below the tangency point) results in an increase in the weight.  Since the force FP is determined by that weight, the

force FP is also increased exceeding the force FB due to bending.  The net result is a side force on the bit causing the hole to drop angle.

Additions of bit weight will slightly decrease the dropping tendency of this assembly because it increases the force due to bending FB.  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 similar to 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 inch OD collars was being used in a 12¼ inch hole.  By substituting 9 inch OD collars for the 7 inch OD collars, an increase in dropping tendency can be achieved.  Figure 3 shows the calculated pendulum force for a 60 foot pendulum versus inclination with various size collars.

Pendulum Assembly or Dropping BHA
Figure 3

Dropping assemblies will have a higher rate of drop as hole inclination increases.  The force FP (which causes the dropping tendency) is calculated using the following Equation 1:

Fp  = 0.5 × W × Sin(I ) —- Equation 1


FP=Side force at the bit caused by the weight of the unsupported section of the
W=bottom hole assembly, lbs.   Buoyant weight of the unsupported section of the bottom hole assembly, lbs.
  I =  Hole inclination, degrees.

An increase in hole angle will result in an increase in FP  resulting in an increase in dropping tendency.  There is no pendulum force at zero degrees inclination. The sine of zero is zero and the pendulum force will be zero.   Pendulum assemblies are ineffective at low inclinations. Additional dropping assemblies can be seen in Figure 4.

Pendulum Assembly or Dropping BHA

The basic drop off assembly is:

Bit – Monel DC – reamer – DC – stab – DC – stab – 90′ DC – stab

To increase the tendency to drop angle :

  • Apply less WOB (lower penetration rate)
  • Apply more RPM and pump pressure in soft formations where jetting and Reaming down is possible
  • Use bigger size Monel DC below the reamer, small DCs above.

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