A cuttings sample at the shaker is worthless until you know which depth it came from. Lag time is the bridge — the strokes and minutes it takes a particle to travel from the bit up the annulus to the flowline.
What lag time buys you
Get lag right and everything downstream lines up: the cuttings on the screen match the formation on the log, a gas show ties to the depth that produced it, and your solids-generation numbers are anchored to the interval being drilled. Get it wrong and you are describing the wrong rock.
Two pieces: annular volume and pump output
Bottoms-up is annular volume divided by what the pump moves per stroke. Annular capacity per foot is the hole and pipe geometry; multiply by depth for the volume. Pump output per stroke comes from the liner bore, stroke length and volumetric efficiency. Divide one by the other for strokes, then divide by pump speed for minutes.
The equation
Annular capacity and volume:
Annular capacity (bbl/ft) = (Dh² − Dp²) ÷ 1029.4
Triplex pump output, and the result:
Output (bbl/stk) = 0.000243 × Dliner² × Lstroke × eff
Bottoms-up strokes = Annular volume ÷ Output · Lag (min) = strokes ÷ SPM
Diameters and stroke in inches; capacity constant 1029.4 for bbl/ft.
Re-run lag every time depth, mud volume or pump output changes — a worn liner quietly drops volumetric efficiency and stretches your true lag beyond the number on the board, and your samples start lagging the log.
Common questions
Is lag time the same as bottoms-up time?
In common field use, yes — bottoms-up is the full annular travel from bit to surface. Strictly, lag can be quoted for any point in the annulus, but for shaker sampling it is the bit-to-flowline value that counts.
Why does pump efficiency matter so much?
Because output per stroke feeds directly into the stroke count. A drop from 95% to 85% volumetric efficiency lengthens the real lag by more than 10%, so worn liners and valves push your samples out of step with depth if you don't re-measure output.
