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
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