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 ..

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.

NOTE : 30 FEET = 1 SINGLE OF DRILL COLLAR

HOLDING ASSEMBLY OR PACKED ASSEMBLY

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.

DROPPING ASSEMBLY OR PENDULUM ASSEMBLY

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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14. Pendulum Effect of Drill Collar Towards Hole Deviation

Here I am going to discuss about the Pendulum Effect, its mechanism and the forces acting, thus causing the drill bit along with active string to tend towards the lower side of wellbore.

Pendulum effect is the effect which tends to bring the drill bit along with the active string to the lower side of hole.

Consider the figure below:

Here,

W₁ = Weight of bit along axis of drill collar

W₂ = Component of W₁ along axis of hole

W₃ = Component of W₁ along normal to hole axis

W₄ = Vertical force of active drill collars assembly

W₅ = Component of W₄ acting along axis of assembly

W₆ = Component normal to W₅ but opposite to W₃ 

The effect of gravity on mass of drill collars above the bit is vertically downward (W₄).

A part of this force is transmitted to the bit along the axis of assembly W₅.

The complimentary component of this part W₆ acts towards perpendicular to the axis of the assembly. It is supported by formation at the wall contact point at the bit. The influence of this lateral force at bit is always to reduce inclination and it's magnitude, automatically increases with inclination. If desired we can enhance the effect by increasing the active drillstring length the natural hole straightening tendency can usually be employed to avoid excessive inclination in vertical holes.

Formation dips also influences the hole deviation. For laminated formation when dips are less than 45 degrees the bit tries to drill perpendicular to bedding plane, and if it is in excess of 45 degrees bit tends to drill parallel to bedding plane. Likewise formation attitudes also have effect on directional tendencies. If proposed direction is due to up dip it follows the natural bit tendencies and drift angle can be readily built If proposed direction is due to up dip, it follows the natural bit tendencies and drift angle can be readily built. If the proposed direction is in left of up dip the bit will try to turn to right and if proposed direction is in right of up dip the bit will deviate to left. 

Bit drilling up dip where formations are dipping at angles less than 45 Degrees

Bit drilling up dip where formations are dipping at angles greater than 60 degrees

13. The Drill Collar Moment Theory

The drill collar moment theory states that when a bit drills from a soft formation to an inclined hard formation, it will support a great portion of bit weight.

This causes a moment at the bit that tends to buckle the collar and deflect them from centre line of the hole. It may also shorten the active assembly length and can reduce any existing pendulum effect. The resultant tendency is up dip.
 

Now, it can be argued that when drilling from harder to softer formation, forces are reversed and constitute a down dip deflection.

Buckling and corresponding deviation forces increase with smaller size collars. greater annular clearance and high weight on bit. As more bit weight is applied, buckling increases.

The length of assembly between bit and first point of drill collar to wall contact, tends to shorten. This section is called active drill string length. Usually not more than 150 ft (45m) of assembly affects it's deviation characteristics and with build up assemblies using high bit weight probably less than 25 ft (7.5 m) is involved.

The active drill string length is usually determined by the position of the first full gauge item in assembly above the bit (reamer or stabilizer). When the distance of bit to first stabilizer or reamer is short (5 to 12 ft), buckling will be induced in next drill collar above. The full gauge tool becomes a fulcrum or pivot point. The shortened effective length increases the angular difference between axis of hole and axis of assembly, and promotes hole deviation.

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