Lag time is the estimated time for drilling fluid to travel from the bit face to the surface through the annulus. It can support planning discussions for sample correlation and surface-return expectations, but it does not approve mud-logging depth picks, gas-detection timing, trip conditioning, or well-control operations.
This guide covers local annular volume calculations, pump output determination, converting between strokes and time, and the source gaps that must be resolved before operational use.
Annular Volume Calculation
The annular volume between the drill string and the wellbore is calculated section by section, because the hole diameter and pipe OD change at different depths (open hole vs. cased hole, drill pipe vs. collars):
Vann = (Dh² − Dp²) / 1029.4 × L
- Vann = annular volume (barrels)
- Dh = hole or casing ID (inches)
- Dp = pipe OD (inches)
- 1029.4 = conversion constant (in² to bbl/ft)
- L = length of the section (feet)
For a local bottoms-up screen, compute the annular volume for each section (open hole around collars, open hole around drill pipe, cased hole around drill pipe) and sum them. Verify tool joints, BHA details, riser and surface-system volumes, washout, and marker data before using the number in field workflows.
Section 1: 8.5" hole, 6.5" collars, 500 ft
V = (8.5² − 6.5²) / 1029.4 × 500
V = (72.25 − 42.25) / 1029.4 × 500
V = 30.0 / 1029.4 × 500 = 14.57 bbl
Section 2: 8.5" hole, 5" DP, 7,500 ft
V = (72.25 − 25.0) / 1029.4 × 7,500
V = 47.25 / 1029.4 × 7,500 = 344.2 bbl
Total annular volume = 358.8 bbl
Lag Time & Bottoms-Up Calculator
Calculate bottoms-up lag time and strokes from well geometry and pump data. Track drilling fluid returns for mud logging, gas detection, and wellbore monitoring.
Pump Output and Strokes to Surface
Pump output per stroke depends on the pump type (triplex or duplex), liner size, and stroke length. For a triplex pump (three single-acting cylinders):
Qstroke = 0.000243 × Dliner² × S × η
- Qstroke = output per stroke (barrels)
- Dliner = liner diameter (inches)
- S = stroke length (inches)
- η = volumetric efficiency (typically 0.95–0.98)
The 0.000243 constant already accounts for all three cylinders (3 × π/4 ÷ 9,702 in³/bbl), so do not multiply by 3 again.
Strokes to surface = Total annular volume / Output per stroke
Lag time = Strokes to surface / Pump speed (strokes per minute)
For example, if total annular volume is 358.8 bbl, pump output is 0.117 bbl/stroke, and pump speed is 60 SPM: Strokes = 358.8 / 0.117 = 3,067 strokes. Lag time = 3,067 / 60 = 51.1 minutes. Treat that as a planning estimate until checked against rig observations and surface-delay assumptions.
5" liners, 12" stroke: 0.0693 bbl/stroke
6" liners, 12" stroke: 0.0997 bbl/stroke
6.5" liners, 12" stroke: 0.1170 bbl/stroke
7" liners, 12" stroke: 0.1357 bbl/stroke
Always verify pump output with a stroke counter
and pit volume measurement before using the
screen in field planning.
Planning Uses and Source Boundaries
Geological sample correlation: Lag time can help plan how a shaker sample might relate to drilled depth, but final correlation needs current marker data, gas-detector delay, surface-system volume, rig data quality, and qualified mud-logging review.
Surface-return monitoring: Annular-volume math can frame expected return timing, but it does not approve gas-detection thresholds, response timing, or well-control action. Those requirements belong in the operator, contractor, regulatory, and company well-control program.
Bottoms-up circulation: The local stroke count can support planning discussions, but trip conditioning, flow checks, and before-drilling requirements must come from the approved program and rig procedure.
Well-control documents: Kill sheets, MAASP, safe drilling margin, APD compliance, and influx tracking require dedicated well-control calculations and qualified review. This guide does not create those records.