drilling · hydraulics
Lag Time / Bottoms Up
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Inputs
bbl
gpm
Description
Computes lag time — the time for cuttings, gas, or other material entrained at the bit to travel up the annulus and reach surface — from annular volume and flow rate. Also known as bottoms-up time; these are the same quantity under two common field names, not two different calculations, consolidating both backlog candidates into one calculator.
Variables
| Symbol | Unit | Description |
|---|---|---|
| t | min | Lag Time / Bottoms Up |
| Vann | bbl | Total annular volume from the bit (or the depth of interest) to surface, from Annular/Hole Capacity. |
| Q | gpm | Circulating flow rate, from Pump Output or a measured pump rate. |
Assumptions
- Flow rate is held constant over the full lag-time interval — a pump-rate change partway through invalidates a single static lag-time figure
- Annular volume is accurately known (e.g. from Annular/Hole Capacity, accounting for actual hole size, not just bit size, in an already-drilled interval)
- No significant fluid loss or gain (lost circulation, kick) during the interval, which would change the effective annular volume
Limitations
- A single average figure — does not account for washouts or hole enlargement changing effective annular volume with depth
- Assumes plug flow up the annulus; real cuttings transport includes slip velocity effects not captured by this bulk-fluid transit-time calculation
- Must be recalculated whenever pump rate changes materially during the interval being tracked
Use Cases
- → Mud logging sample depth assignment: Determine how long ago surface-observed cuttings or gas actually originated at the bit, to correctly assign a measured depth to logged samples.
- → Kick/gas-show response timing: Estimate when a gas show detected at surface actually entered the wellbore, to help pinpoint the influx zone.
- → Circulating/conditioning time planning: Plan how long a full bottoms-up circulation will take before displacing mud, running a wiper trip, or conditioning mud properties.
Related Calculations
Region Notes
Global
Always use actual (caliper or offset-well-informed) hole volume rather than bit-size volume alone once a section is drilled — washouts can materially shorten true lag time versus a gauge-hole assumption.
References
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