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Lag Time and How to Calculate Theoretical Lag Time

Lag time is traveling time interval required for pumping cuttings from each particular depth to surface. It can be expressed in terms of time (minutes) and pump strokes.


The lag time always changes when a well becomes deeper and/or pumping speed change. Two factors, affecting lag time calculation, are annulus volume of drilling fluid in and drilling mud flow rate.



With certain annular volume, the lag time, normally expressed in minutes, can be determined by dividing the annular volume (bbls) by the flow rate (bbl/min).



If there are changes in mud flow rate, the lag time figure will be changed as well. In order compensate for any changes, the lag time is transformed into pump strokes too; therefore, a change in speed of pump will not affect the lag time.



How to Calculate Theoretical Lag Time



There are 3 steps to do in order to calculate lag time as listed below;



1. Calculate pump output
2.
Calculate annular volume at certain depth of hole
3. Calculate the theoretical lag time


Example – Determine lag time from bottom to surface with the following information;



Bit depth = 9500’ MD


Pump rate = 300 GPM


Annular volume at 9500’ MD = 250 bbl


Pump details: Triplex pump, 97% efficiency, liner size 6” and stroke length 12”



Solution;



Triplex Pump Output Formula is listed below;



Triplex Pump Output in bbl/stk = efficiency x 0.000243 x (liner diameter in inch) 2 X (stroke length in inch)



Triplex Pump Output in bbl/stk = 0.97x 0.000243 x (6) 2 X (12)


Triplex pump output = 0.102 bbl/stroke



Pump rate = 300 GPM ÷ 42 = 7.14 bbl / minute



Lag time in minutes = 250 bbl ÷ 7.14 bbl / minute = 35 minutes
Lag time in strokes = 250 bbl ÷ 0.102 bbl/stroke = 2451 strokes

Views: 11480

Tags: drilling, lag, time

Comment by Anita J. Battreall on March 19, 2011 at 7:58am
Rickey,
Thank you! I appreciate the clarification. Gotcha on the OH.
Comment by aaron wilson on March 31, 2013 at 8:16am

hello,

          I am Aaron , working as data engineer ( mud logging). I need to calculate total depth by knowing lag strokes. Can you help me please?????

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