Downhole lag time is the difference between the time it takes drilling mud to go from the surface to the bottom of the drill stem and the length of time it takes a marker, introduced into the drilling mud, to go to the bottom of the drill stem and return to the surface. Changes in the downhole lag time can alert the driller or toolpusher to overpressure conditions in the hole due to sediment undercompaction, with the goal of preventing a blowout during drilling.
Instructions
1. When the drilling mud is introduced to the borehole, start timing. When the mud hits the bottom end of the drill stem, stop timing. This is the down time.
2. Introduce a chemical marker into the drilling mud and start the stopwatch.
3. When the chemical marker returns to the surface, stop the stopwatch. This is the marker time.
4. Subtract the down time from the marker time. The result is the lag time.
5. Subtract the down time from the total time it takes for a marker introduced into the drilling mud at the surface to travel to the bottom of the borehole and back to the surface.
Tags: bottom drill, bottom drill stem, down time, drill stem, into drilling, time takes
