Friction factors

Softdrill NL   

 

Drill string drag and torque forces are a product of the normal force (Fn) and the friction factor (FF):

 

 

Friction factors in Torque & Drag are entered as a dimensionless number and should be considered as a collection of factors that influence the friction along the wellbore. Friction factors can therefore vary from well to well or even from hole section to hole section. For example: formation/casing smoothness, mud system, (hidden) dogleg severities, etc. will all influence the friction factor.

 

Because (hidden) dogleg severities and borehole tortuosity are normally "included" in the friction factor, care should be taken when determining the friction factor for a hole section when applying tortuosity to wellplan data.  If tortuosity is applied to the wellplan data, it is realistic to use lower friction factors.

 

Typical friction factors that are used are shown in the table below but this may vary. 

 

 

Water based mud

Oil based mud

Synthetic OBM

 

Min

Max

Min

Max

Min

Max

Cased hole

0.18

0.25

0.15

0.20

0.15

0.20

Open hole

0.30

0.35

0.22

0.25

0.22

0.25

 

For best results it is recommended to back-calculate the friction factors from actual field results.  For best results, use benchmark values for hookload and torque and back-calculate the friction factor.

 

A typical scenario would be to take hookload and torque readings at the last casing shoe and at a depth in the open hole below.  Using the values obtained at the casing shoe, back-calculate the friction factor for the cased hole. With this friction factor applied to the cased hole section(s), change the bit depth to the open hole depth and back-calculate the friction factor for the open hole from the hookload and torque values obtained there.

 


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