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

Tags: drilling, lag, time

Comment by RICKEY PAUL BAILEY on April 6, 2010 at 3:50pm: VERY GOOD INFORMATION, RACHAIN, AND WELL PUT. BUT DON'T FORGET O.H. WASHOUT AND GAS MIGRATION.

RICK

Comment by mohamed radwan on April 13, 2010 at 2:48pm: Good information, thanks..

Comment by Rachain Jetjongjit on April 13, 2010 at 9:54pm: RICKEY PAUL BAILEY, thanks very much for your contribution. I agree with you that I should add the assumption about wash out and gas migration into calculation as well.

Comment by Nick Humphreys on April 16, 2010 at 2:21am: And cased hole, when you do intermediate casing programs.

Comment by scott bienvenu on May 24, 2010 at 3:49am: i have a question...How do u determine washout percentage? Do u take ur hole volume in barrels and divide that into your washout barrels to get your washout percent? or do u add ur washout barrels to your open hole volume and then divide the volume into your barrels of washout to get your percent of washout? or is it neither one. as of right now we are getting like a 1/3 of the open hole volume in washout. Im assuming that would then be 30% washout? I would really appreciate any insight into the answer to my prob. THks :)

Comment by scott bienvenu on May 24, 2010 at 3:53am: this is open hole washout by the way. i forgot to mention it

Comment by Rachain Jetjongjit on May 24, 2010 at 10:34am: Practically, you can pump sweep or carbine so you will get the actual lag time. When you compare both theoretical value and actual value you will know how much % wash out and you can use it for future reference.

Comment by scott bienvenu on May 25, 2010 at 12:03am: Thnks Rachain but well i guess my question i was too vague with it..ok here. the lag at lets say 8000ft is 6000 strokes...the lag from that depth came up at 6500 stks. 500 stks late and pump outputs is .0997 so thats 49.85 bbls of washout. Now here is my question, how do u calculate that into percent washout%?? Thats really what i want to know... My hole volume is 800 bbls and my annular vol is 600 bbls. So how do i calculate washout%? i know how to do everything with washout except convert it to %. Do u take ur 800bbls and divide that into your 49.85 bbls of washout? Do you add your washout bbls to ur 800bbls and then divide that into your 49.85bbls or do u divide ur annular volume into ur washout bbls to get your washout %...ive looked everywhere on the internet and NO ONE seems to know how to figure that out...or can give me the answer to that question...

Comment by Anita J. Battreall on March 18, 2011 at 3:09am: I was taught to figure WO in the example below. You would take the strokes and divide for example: 6500stks/6000stks= 1.08. 1 representing true lag and .08= 8%. Would really like to know how accurate this formula is?

Comment by RICKEY PAUL BAILEY on March 18, 2011 at 5:14am: Anita,
Good question, easy answer. Remember that your washout factor is only relevant in Open Hole. In your example, if 6500 was actual, and 6000 was calculated: 6,000/6,500= .923 (9%). To be accurate, suubtract strokes at the shoe from total lag strokes, then divide calculated strokes by actual strokes=w.o. in ope
n hole. Bueno?

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Lag Time and How to Calculate Theoretical Lag Time

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