Gas Migration Calculator
Migration rate from a shut-in pressure buildup, and the volumetric-method bleed volume that holds bottomhole pressure steady while gas migrates.
Reviewed for accuracy by Jabin Travis, IADC WellSharp-certified driller.
The Formulas Used
Once you have a migration rate, checking whether a planned bullhead pump rate would actually outpace it is the bullheading calculator's last section. For the shut-in pressures and kill weight this feeds into more broadly, see the kill sheet calculator.
Frequently Asked Questions
What actually causes gas migration on a shut-in well?
A gas kick doesn't stay still once the well is shut in — gas is buoyant and rises through the mud column even with no pumps running, and because gas expands as it rises into lower pressure, both shut-in drillpipe and casing pressure climb over time even though nothing is being pumped. The rate that pressure climbs at is what this calculator converts into a migration rate.
Why does the migration rate use mud gradient and not mud weight directly?
The formula is a pressure-per-hour reading divided by a pressure-per-foot reading, which cancels to feet per hour — the actual speed gas is climbing at. Mud weight in ppg has to be converted to a gradient in psi/ft first (weight × 0.052) so the units on both sides of the division actually match.
What is the volumetric method, and why does it need a bleed volume at all?
The volumetric method holds bottomhole pressure roughly constant while a shut-in well's casing pressure climbs from migrating gas, by bleeding a small, calculated volume of mud off the annulus in controlled steps rather than shutting in and doing nothing. Bleeding exactly the right volume for a given pressure step keeps the well barely overbalanced instead of building pressure toward equipment limits — bleed too little and pressure keeps climbing, bleed too much and the well goes underbalanced.
Is a fast migration rate always bad?
It is a warning sign, not an automatic emergency — some formations and gas types migrate faster than others as a matter of course. What matters operationally is whether the crew's monitoring interval and bleed procedure can keep pace with the rate actually being seen. A slow, well-understood migration rate on a known formation is routine; a fast or accelerating rate with no clear procedure keeping up with it is the situation worth escalating.