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Ideal Motor Size
04 May 2016
Dear All,
We have generally been using 962 [i.e. 9-5/8" ] Motors in 12-1/4" Hole,but recently I've been told to use 8-1/4" motors.
What are your suggestions regarding the motor size to be used in 12-1/4" Hole? with the merits and demerits of each.
When inquired, from SLB experience they say, they have a lot of twist off failures due to BSR and hence they prefer 9-5/8" Motors.
8 answer(s)
Drilling Engineer
Pakistan Petroleum Ltd
Total Posts: 19
Join Date: 02/08/15
Thank you all for your suggestions,
I guess the 9-5/8" Motors are the preferable option then, because of greater power, since we currently do not have any limitation with SPP.

Drilling Supt
Nostrum Oil & Gas
Total Posts: 45
Join Date: 14/11/10
One reason to use 8'' PDM was not mentioned yet:
in case you have weak pumps and beeing limited with SPP the 8'' motor needs less minimum flowrate to run. I have planned a lot of directional BHAs in Eastern Europe with 8'' instead of 9 1/2'' because of limitation in SPP.  Some other applications  used the 8'' motor because operators requested it to enable fishing with overshot in 12 1/4'' hole. 
In case you have to fish for a motor most likely the rotating bending moment was too high and you have to fish for a lot parts of the of bearing assembly.  So most operators risk assessment resulted in using the 9 1/2'' motor.  

Sometimes these changes are requested by service companies because of availability of tools.

If it comes to performance you should discuss an RSS (maybe with motor). 
Drill Site Manager
Chevron Corporation
Total Posts: 1
Join Date: 10/02/12


The advantages of the 9 5/8" motor are in its greater power and capability to take higher flow rates. This improves your ROP and hole cleaning. If you are slide-drilling it will be less prone to stalling out, improving directional effectiveness & efficiency of each slide. There are other advantages that others have raised too. The downside may be reduced DLS capability for any given bent housing angle, but in most standard directional applications this should not matter.

On SLB's concern about twist-offs etc, I think the BSR is a "red herring". Where I think they are coming from (having historically done DD with them for many years) is that the smaller motors are weaker in the mid-section motor body connections, where the housings are relatively thin-walled to accommodate the stator, universal joint, bearings etc. These connections are weak points and will break preferentially over others in the BHA. Being bigger od & id, the 9 5/8" motor construction is proportionally stronger at these points.

Since the early 90's I have drilled a lot of 12 1/4" sections and almost all have used 9 5/8" mud motors (North Sea). We quickly moved away from using 8 1/4" tools in the early years and never looked back.



Project Quality
Total Posts: 103
Join Date: 06/05/09


Reducing stress on a tool is always, always, preferable to the alternative.  And, all other factors being equal, the 9-5/8" motor has a much higher resistance to stress than an 8-1/4" motor.

Unless those other factors, such as build rate, turbulent flow v laminar (around the motor), to name just two, make the 8-1/4" motor the better choice, stick with the 9-5/8".

In reference to the comment by Slb on the BSR, that is an incorrect understanding of BSRs.  BSR is only applicable to rotary shoulder connections ... so, if the motor is being used in a 'standard' application, BSR applies to the top and bottom box connections only.

Without digressing from the issue, in reality, BSR applies to the top box only on a motor as BSR assumes bending ... the bit is a pivot point so there is no bending on the motor's bit box.

If there are failures in the bit and top box connections, then there is another problem entirely ... the drive shaft, rotor-to-cv joint, cv-joint-to-drive shaft, stator-to-ako, and ako-to-bearing housing are all much 'weaker' than those two connections.  A 'back of the envelope' comparison of the CSAs of the various motor connections will provide a relatively accurate pointer as to which has the highest and lowest resistance to stress.

Hope that helps.


Drilling Consultant
PT Drilling Services
Total Posts: 62
Join Date: 15/09/14
As Lain mentioned above, the advantage of 9-5/8" motor is the power and disadvantage is tool size and fishing complication. 
One thing I would like to stress is that the OD around motor sleeve 11.??". Sorry I can't remember the exact OD as I did the measurement years ago (<1996).
We used to drill very soft formation at high ROP. Thus had frequent pack-off. It was due to hole cleaning and finally we suspected the OD around the sleeve being one of the culprit for the pack-off. After switching to 8-1/4" motor, we had less pack-off problem. Of course controlling ROP is one option, but there is other factors such as directional control and rig time. 
On the other hand, if you are drilling harder and low ROP formation, then 9-5/8" motor will be a better choice. 
I hope it helps in you decision making. 
Lead ERD Advisor
Merlin ERD Limited
Total Posts: 4
Join Date: 04/05/16
there are a number of pro's and con's with regards to utilisng 9 5/8" motors over 8" motors

  1. Torque - more torque generated by 9 5/8" motor
  2. Flow - higher flow capability, if the rig can deliver it - I'd rather not have to run a nozzle on the rotor to get high flow rates if required
  3. Assett utilisation - you can use the 9 5/8" motor in larger hole sizes - if you go with 8" motors and are doing directional work in previous hole sections you will need a bigger set of motors

  1. Decreased annular space can lead to higher propensity to packing off if you have well bore stability / hole cleaning issues.
  2. Fishability - you may have a problem fishing the motor in 12 1.4" hole, although NOV now have some high strength, thin walled overshots which may be able to latch onto the outside of the stator can
  3. Availability - how many 8 1/4" motors are actually available. If 9 5/8" motors have been the prefered option you may find the supply of 8 1/4" motors is limited - although in the current environment this should not be a problem.
Regards, Iain

Well Integrity Lead
Total Posts: 5
Join Date: 07/01/16
Fishability. Is there room to get an overshot over it? Performance with achievable flow rate! Stiffness if doing short radius work?
GeoGlide Well Positioning
Total Posts: 1
Join Date: 08/12/15

Dear Umair,

A 9 5/8" / 9 1/2" motor will give you more torque / power for drilling performance as long as you have enough pump to power it. In some circumstances there may be a concern about hole conditions raising the risk of stuck pipe with a larger OD motor but this is generally not an issue. If your rig lack sufficient pump capacity in a deeper 12 1/4" hole section to get the flow rate for a 9 5/8" motor to perform then an 8" may be preferable.

Rgds, Brod


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