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.
There are 3 steps to do in order to calculate lag time as listed below;
1.
1. Calculate pump output
2. 2. Calculate annular volume at certain depth of hole
3. Calculate the theoretical lag time
Read more: http://www.drillingahead.com/profiles/blogs/lag-time-and-how-to-
calculate#ixzz1qYXFAUnv
Pump Output Calculation for Duplex
Pump and Triplex Pump
by DrillingFormulas.Com on April 20, 2009
Rig pump output, normally in bbl per stroke, of mud pumps on the rig is important
figures that we really need to know because we will use pump out put figures to
calculates many things such as bottom up strokes, wash out depth, tracking drilling fluid,
etc. In this post, you will learn how to calculate pump out put for triplex pump and
duplex pump.
Triplex Pump Output Formula
Triplex Pump Output in bbl/stk = 0.000243 x (liner diameter in inch) 2 X (stroke length in
inch)
Example: Determine the pump output in bbl/stk at 100% and 97% efficiency
Linner size = 6 inch
Stroke length = 12 inch
Triplex pump output:
PO @ 100% = 0.000243 x 62 x 12
PO @ 100% = 0.104976 bbl/stk
Adjust the triplex pump output for 97% efficiency:
Decimal equivalent = 97 ÷ 100 = 0.97
PO @ 97% = 0.104976 bbl/stk x 0.97
PO @ 97% = 0.101827 bbl/stk
Duplex Pump Output Formula
Duplex Pump Output in bbl/stk = 0.000162 x S x [2(D)2 - d2]
Whrere:
D = liner diameter in inch
S = stroke length in inch
d = rod diameter in inch
Example: Determine the duplex pump output in bbl/stk at 100% and 85% efficiency
Liner diameter = 6 inch
Stroke length = 12 inch
Rod diameter = 2.0 in.
Duplex pump efficiency = 100 %.
PO @ 100% = 0.000162 x 12 x [2 (6) 2 -22 ]
PO @ 100% = 0.13219 bbl/stk
Adjust pump output for 85% efficiency:
PO @ 85% = 0.132192 bbl/stk x 0.85
PO @ 85% = 0.11236 bbl/stk
Calculate Annular Capacity
by DrillingFormulas.Com on May 5, 2009
Annular capacity is one of basic values that you really need to understand. This post
demonstrates you how to calculate annular capacity between casing or hole and drill pipe,
tubing, or casing. There are several formulas as shown below to calculate annular
capacity depending on unit of annular capacity required.
Note: Dh is bigger ID and Dp is smaller OD. The examples below will show the Dh as
hole size and Dp is drill pipe OD
a) Calculate annular capacity in bbl/ft
Annular capacity in bbl/ft = (Dh2 – Dp2) ÷1029.4
Example: Hole size (Dh) = 6-1/8 in.
Drill pipe OD (Dp) = 3.5 in.
Annular capacity in bbl/ft = (6.1252 – 3.5 2) ÷1029.4
Annular capacity = 0.0245 bbl/ft
b) Calculate annular capacity in ft/bbl
Annular capacity in ft/bbl = 1029.4 ÷ (Dh2 – Dp2)
Example: Hole size (Dh) = 6-1/8 in.
Drill pipe OD (Dp) = 3.5 in.
Annular capacity in ft/bbl = 1029.4 ÷ (6.125 2 – 3.5 2)
Annular capacity = 40.743 ft/bbl
c) Calculate annular capacity in gal/ft
Annular capacity in gal/ft = (Dh2 – Dp2) ÷ 24.51
Example: Hole size (Dh) = 6-1/8 in.
Drill pipe OD (Dp) = 3.5 in.
Annular capacity in gal/ft = (6.125 2 – 3.52) ÷24.51
Annular capacity = 1.031 gal/ft
d) Calculate annular capacity in ft/gal
Annular capacity, ft/gal = 24.51 ÷ (Dh2 – Dp2)
Example: Hole size (Dh) = 6-1/8 in.
Drill pipe OD (Dp) = 3.5 in.
Annular capacity in ft/gal = 24.51 ÷ (6.125 2 – 3.5 2)
Annular capacity in ft/gal = 0.97 ft/gal
Annular volume can be determined by this following formula;
Annular volume in bbl = annular capacity (bbl/ft) x length of annulus (ft)
Note: annular volume can be expressed in several unit depending on unit that you use in
the calculation.
Example:
Annular capacity = 0.0245 bbl/ft
Length of annulus = 1000 ft
Annular volume = 1000 x 0.0245 = 24.5 bbl.
** Please remember that if you have several annular profile, you must calculate volume
based on each annular profile in order to get total annular volume.
Example – Determine lag time from bottom to surface with the following information :
Bit depth : 9500 ft MD
Pump Rate : 300 GPM
Annular volume at 9500 ft MD : 250 bbl
Pump detail : 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 = 0.97 x 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 stroke = 250 bbl / 0.102 bbl/stroke = 2451 strokes
