Tripping Pressure Effects
How much hydrostatic pressure is lost per foot pulling dry or wet pipe, and how far the mud level actually drops tripping out without topping up.
The Formulas Used
Where This Fits
These three calculations are the mechanics behind the same swab risk the trip margin calculator and the wet and dry pipe pull margin calculator both plan around — trip margin sets how much extra mud weight to carry, pull margin says how far pipe can safely come out, and this page works out the actual pressure and mud-level consequence of a specific length pulled dry or wet. On a real trip, mud level drop is what tells the crew exactly how much to top up the annulus after every stand or connection to keep hydrostatic pressure from sagging as pipe comes out.
Frequently Asked Questions
Why is the pressure drop per foot different for dry pipe vs. wet pipe?
Pulling dry pipe (empty, no fluid inside) only removes the pipe's own steel volume from the hole, so the mud level drop is driven by steel displacement against the full open-hole or casing capacity. Pulling wet pipe (still full of fluid) also removes the fluid inside the pipe's bore, so the drop is driven by steel displacement plus the pipe's internal capacity, against just the annular capacity rather than the full hole capacity — a meaningfully different, usually larger, drop per foot.
Why keep the mud topped up while tripping out?
As pipe comes out, the mud level in the hole drops, and a dropping mud level means dropping hydrostatic pressure at the bottom of the well. If enough pipe is pulled without topping up the annulus, hydrostatic pressure can fall below formation pressure and the well starts flowing — filling the hole with mud as it's tripped keeps hydrostatic pressure roughly constant instead.