Lag time is the time required for drilling fluid to travel from the bit face to the surface through the annulus. It determines how long after drilling a formation before cuttings from that formation appear at the shale shaker, and how long a gas kick takes to reach the surface during circulation. Accurate lag time calculations are essential for geological sample correlation, kick detection, and well control operations.
This guide covers annular volume calculations, pump output determination, converting between strokes and time, and practical applications of lag time in drilling operations.
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 complete bottoms-up calculation, compute the annular volume for each section (open hole around collars, open hole around drill pipe, cased hole around drill pipe) and sum them. The total annular volume is the volume of mud that must be circulated to bring cuttings from the bit to the surface.
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 × 3 × η
- Qstroke = output per stroke (barrels)
- Dliner = liner diameter (inches)
- S = stroke length (inches)
- η = volumetric efficiency (typically 0.95–0.98)
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. This means cuttings drilled right now will arrive at the shakers in about 51 minutes.
5" liners, 12" stroke: 0.0826 bbl/stroke
6" liners, 12" stroke: 0.1189 bbl/stroke
6.5" liners, 12" stroke: 0.1395 bbl/stroke
7" liners, 12" stroke: 0.1618 bbl/stroke
Always verify pump output with a stroke counter
and pit volume measurement before relying on
calculated values for well control operations.
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.
Practical Applications of Lag Time
Geological sample correlation: When the geologist needs to determine the formation being drilled at a specific depth, the cuttings sample taken at the shakers must be correlated with the depth drilled one lag time earlier. If the lag time is 50 minutes and the ROP is 60 ft/hr, the shaker sample represents the formation drilled 50 feet above the current bit depth. Accurate lag time is essential for picking formation tops, casing points, and coring depths.
Gas kick detection: When a gas influx enters the wellbore at the bit, it must travel the entire annulus to reach the surface gas detectors. The lag time tells you how long after drilling into a gas-bearing zone the gas will appear at surface. In deep wells with lag times of 60–90 minutes, a gas kick detected at surface means the gas entered the wellbore over an hour ago and may have migrated significantly.
Bottoms-up circulation: After a trip (pulling and running pipe), a complete bottoms-up circulation ensures that any gas or formation fluid that entered the wellbore during the trip is circulated out before drilling resumes. The bottoms-up time equals the lag time. Many operators require a full bottoms-up before drilling ahead to verify well stability.
Kill operations: During a well kill, knowing the annular volume and lag time is critical for tracking where the kill mud and the kick influx are located in the annulus. The kill sheet includes strokes for kill mud to reach the bit (inside the drill string) and strokes for the kick to reach the surface (annular volume).
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.