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Bit Torque estimation
10 December 2015

Folks,

What is the best way to estimate the bit torque for a 8.5" - 5 bladed PDC-drilling soft formation?...Trying to model some T&D with realistic numbers.

Thanks


13 answer(s)
shortbits
co-owner
Short Bit & Tool Co.
Total Posts: 17
Join Date: 11/05/13
Try one last time
Documents uploaded by user:
Math Model.compressed.pdf
shortbits
co-owner
Short Bit & Tool Co.
Total Posts: 17
Join Date: 11/05/13
If you were to develop one for a specific 8 1/2" 5 blade bit with 16mm cutters it would look something like this sketch. You can see that quite a few variables have to be included in your model and several of them will change almost every revolution. We lumped all the unknowns (hydraulic effects, layered formations, etc.) into what we called Formation Resistance. Not all formations fail in the same way so a direct correlation to shear strength is difficult. The area, width of cut, depth of cut, cutter back rake, weight transfer vector angles for each cutter, torque arm lengths, distance travelled/rev, linear cutter speeds all have to do with what torque is developed.
Documents uploaded by user:
8.5 Math Model.jpg
shortbits
co-owner
Short Bit & Tool Co.
Total Posts: 17
Join Date: 11/05/13
In the 50's and 60's my Father, the late Bill Short, Sr. invented the Math Model for Diamond Bits at the original Drilling & Service, Inc (Dallas, TX). It was bought by Hycalog in 1967 and my Dad & I further developed the Math Model at Williams Diamond Bits (Greenville, TX). It was bought by Hycalog in 1974. He and I further developed the concept in practical applications at Hycalog until his death in 1983. I enclose the basic tables we made for natural diamonds. In 1978-1980 I worked as design engineer at ACC (Dallas) and I applied the math model to PDC bits.
vzbaratskiy
NOV Europe DSE Manager
National Oilwell Varco
Total Posts: 2
Join Date: 04/07/13

All great comments below. I'd like to add that bit designers compare efficiency of cutting structure between designs and one of the curve they use is WOB/TQ response for any given formation compressive strength (usually 5,7,10 kpsi). This allows to understand how responsive the bits are to WOB change, at what WOB the secondary components engage and etc. Though the absolute values might be off, as the model does not account for vibrations and other variances, the % difference between curves is much closer to reality.

If you had a bit run of certain design and have an accurate TQ curve you associated with the run you can try to calibrate your "new" bit TQ anticipated using this chat.

I have attached an example for your reference.


Thanks

Documents uploaded by user:
TQ_WOB bits comparison (by design).pdf
vc79495
DE
SPREADAssociates
Total Posts: 11
Join Date: 07/12/15

thanks a lot...Very useful  Rgds

VC

JaapvdS
BDM
Compass Energy
Total Posts: 10
Join Date: 09/11/15
The 6 kNm (4.5 k ft.lbs) Peter-Joern supplies as a maximum is a useful number. In soft formations you won't be able to put high WOB on and so your number will probably be between 2.5 - 4 kft.lbs 
I don't think going into detailed mechanical calculations and formulas is going to improve your outcome significantly.


smenand
Research Engineer
DrillScan
Total Posts: 11
Join Date: 17/08/10
Hello,

Agree with other posts. Depending on the degree of accuracy required, I can recommend this basic formula (as already suggested) as shown in the file attached. The coefficient k depends on the bit characteristics: cutter size, bit profile, back rake, side rake, etc.... This formula will give you fair results especially for Torque & Drag calculations.

If you want better accuracy, we have to go through a full 3D Rock-bit interaction model. We have developed such a model, so in case you want more details , please contact me directly.

Regards

Stephane Menand
DrillScan US Inc
Houston


smenand
Research Engineer
DrillScan
Total Posts: 11
Join Date: 17/08/10
Hello,

Agree with other posts. Depending on the degree of accuracy required, I can recommend this basic formula (as already suggested) as shown in the file attached. The coefficient k depends on the bit characteristics: cutter size, bit profile, back rake, side rake, etc.... This formula will give you fair results especially for Torque & Drag calculations.

If you want better accuracy, we have to go through a full 3D Rock-bit interaction model. We have developed such a model, so in case you want more details , please contact me directly.

Regards

Stephane Menand
DrillScan US Inc
Houston


iain.hutchison
Director Engineering
Merlin ERD Limited
Total Posts: 40
Join Date: 20/02/09
Great guidance in latest IADC Manual (Rev 15), chapter 14: Downhike practices. 

In simple wells (vertical, shallow etc), it's a simple on/off btm. Uses range of 1.5 to 6 Kftlbs), dependant on bit & "aggression". Confirm during Ops 


For or complex wells, eg ERD or complex horizontal, parameters become interdependent and will influence ability to reach founder point. See reference above. 

p-j.palten
Drilling Supt
Nostrum Oil & Gas
Total Posts: 44
Join Date: 14/11/10
usually I use some worst case Bit TQ in the beginning (i.e. maximum operating TQ delivered by the mud motor - app 6kNm for a 6 3/4'' Motor) to be on the safe side for the surface TQ calculation (MUT of DP). Same I apply for the FF which has an influence on the surface TQ as well (starting values OBM 0.16/0.32, WBM 0.2/0.4, WBM with Calciumcarbonat 0.3/0.5). Apart from that I aplly worst case tortuosity and noise on the planned survey (depends on RSS or motor use - motor 0.8deg/30m inclination and 1.6deg/30m walk, RSS 0.4deg/30m inclination and 0.8deg/30m walk).    
As soon as we start drilling  I check FF at the shoe by taking TQ reading at surface and adjust  the FF for the case hole. On bottom I check off andf on bottom TQ to see how much TQ the bit takes and to adjust FF for open hole.  Next well I use the offset data. . 
  
yousaf70
Sr. Advisor, Well Engineering specialist Deepwater, ERD, HPHT & Special Projects
Chevron Corporation
Total Posts: 4
Join Date: 07/11/15
As when weight is applied torque will increase to a point where more weight generally results in a flattening out and maximum of torque. 

Optimal PDC bit blade, cutter size, design, / BHA/ Drive selection can result in lower weights torque, greater MSE and optimal ROP. The poorly designed and engineered  bit will result In more weight, higher torque that will often ramp up and down in a sandstone,  which caused the decrease MSE and lower ROP in the sands.

The offset mud logs of similar bit  runs in Compatible Stratigraphy would be the best place to get a minimum to  maximum range  for to have the better objectives. 
Companyrep
Drilling Specialist/Well Engineer/Training Consultant
SPREADAssociates
Total Posts: 339
Join Date: 10/01/05
Resulting torque on a PDC in sandstone will depend on
- wob
- size of cutters
- depth of cut
- rake, slew etc of cutters 
- other features of bit, gauge cutters, back up cutters etc, 
- drive type rotary, motor, type of motor ( slow med high) 
- strike and dip of formation 
- sandstone matrix constituents ( what else is in the sand) can have added torque consequences 

and why often it is rarely like for like so an engineer should be fine tuning as best as they can for all of these factors. 

as weight is applied torque will increase to a point where more weight generally results in a flattening out and max of torque. 

Optimal PDC bit blade, cutter size, design, / Bha/ drive selection can result in lower weights torque, greater MSE and optimal ROP. If poorly designed and engineered will result In more weight, higher torque that will often ramp up and down in a sandstone, decrease MSE and lower Rop in the sands.

offset mud logs of similar bit  runs in compatible stratigraphy would be the best place to get a min max range to use for objectives you have in mind. 



mhayes
Director
Stanfield-Hayes Consulting
Total Posts: 31
Join Date: 25/03/11

I tend to use the Bit Specific Coefficient of Sliding Friction relationship to make this estimate from offset data:

T = Mu (Db * WOB)/36

(See Pessier & Fear, "Quantifying Common Drilling Problems With Mechanical Specific Energy and a Bit-Specific Coefficient of Sliding Friction, SPE 24584). Values from Mu are available from some vendors and you should have a fair idea of the WOB you are planning to apply (at the bit)

An alternative method I have used is to derive from the method described in Falconer, Burgess and Sheppard "Separating Bit and Lithology Effects from Drilling Mechanical Data", SPE 24584. With the us of dimensionless torque it is possible then to move on to looking at different hole size comparisons.


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