Tuesday, July 12, 2011

15. Rotary Assemblies - Building Assembly, Holding Assembly and Dropping Assembly

Rotary Assemblies, as the name suggests, the assemblies which are driven by rotary table at the surface. Actually these are the bottom hole assemblies (BHA's) designed to drill a well directionally (build or hold or drop) without the use of downhole motors.

The designing of the various rotary assemblies is based on the principle that as we shorten the active length of drill collar, it gives rise to fulcrum effect (which helps in building), when we increase the active length of drill collar it gives pendulum effect (which helps in dropping, condition: optimum WOB) and the combination of the two principle gives rise to holding.

Various Rotary Assemblies have been designed till date by placing stabilizers at various positions in a BHA and observing the result thus produced.

Here I am to going to discuss few such Assemblies which are useful in building, holding and dropping the inclination of a wellbore.

A question arises here is that when we have downhole motors easily available, then whats the need of using such old techniques !
So, I should tell you that the Operator companies proving Direction Drilling Service charges for the their tool on either day rate basis or hourly basis. Since the rental charges for downhole motors along with the survey tools (MWD/ LWD) becomes a costly affair, rotary assemblies are prefered after the well is kicked off or after acheiving a desired trajectory.
Its all upto you to decide whether at a prevailing situation the Rotary Assembly would be more cost effective or a downhole motor !
Figure below shows some of the common rotary assemblies currently being used :

Building, Holding and Dropping Assemblies
Lets kick off our discussion with Building Assembly ..


As said above, building assembly works on fulcrum principle.

It is used after the well is kicked off using a deflection tool to eliminate the use of expensive downhole mud motors. 

Here we use a near bit stabilizer which acts as a pivot or fulcrum for the lever. The drill collar portion above the near bit stabilizer to the first point of contact with the lower side of the wellbore acts as lever.

As we apply WOB, the bit is pressed towards the high side of the hole due to fulcrum effect and thus increases the inclination.

Now, here is something interesting : As we increase the WOB, there is bending of drill collar and the first contact point of drill string above the stabilizer with wellbore slides downwards. This results in decrease in lever length or "a decrease in active drill collar length". As I have discussed above, as the active drill collar length reduces, it increases the fulcrum effect and thus we get higher building rate.

Thus, an additional string stabilizer can be used to reduce the length of lever and hence increasing the build rate.

The addition of stabilizers other then the near bit stabilizer (NBS) modifies the build rate to match the required well trajectory.

Assemblies A and B respond well in soft or medium formations.

The inclusion of an undergauge stabilizer in assembly C will build slightly less angle. By bringing the second stabilizer closer to the near-bit stabilizer, the building tendency is increased.

In hard abrasive rocks, the problems of bit wear are significant.

To maintain gauge hole, the near-bit and second stabilizer should be replaced by roller reamers.

The build rate should be kept below 2° per 100 ft to reduce the risk of dog-legs.

The amount of WOB applied to these assemblies will also affect their building characteristics.

Too much WOB will cause rapid build-up of angle.

Typical building assemblies (A,B,C) are shown in the figure above.



The holding or packed assembly uses packed hole stabilization principle to maintain the inclination and direction.
Once the inclination has been built to the required angle, the tangential section of the well is drilled using a holding assembly OR packed assembly. The object here is to reduce the tendency of the BHA to build or drop angle. Actually this is dificcult to acheive, since formation effects and gravity may alter the hole angle.

The packed BHA relies on the principle that two points will contact and follow a sharp curve, while three points will follow a straight line as shown below :

                               Two Point contact          Three Point Contact                                    

To eliminate building and dropping tendencies, stabilizers should be placed at close intervals, using pony collars if necessary.

Assembly D in figure above has been used successfully in soft formations.

The undergauge stabilizer in assembly E builds slightly to counter gravity.

In harder formations the near-bit stabilizer is replaced by a reamer.

Generally only three stabilizers should be used, unless differential sticking is expected.

Changes in WOB  will not affect the directional behaviour of this type of assembly, and so optimum WOB can be applied to achieve maximum penetration rates.

Note : A packed hole assembly with several stabilizers should not be run immediately after a downhole motor run.


As I said earlier, Dropping Assemblies are based on pendulum principle. The pendulum technique is used to drop angle especially on high angle wells where it is usually very easy to drop angle.The pendulum technique relies on the principle that the force of gravity can be used to deflect the hole back to vertical. 

In directional wells, only an S shape profile requires a planned drop in angle. The other application of a dropping assembly is when the inclination has been increased beyond the intended trajectory and must be reduced to bring the well back on course.

It is best to drop angle in a section of softer formation, since the response to a pendulum type assembly in hard rock is very slow.

Figure above gives some typical dropping assemblies (F and G).  These are more effective in high-angled holes. If hole angle does not reduce, the WOB can be reduced, although this will also reduce the penetration rate.



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