EXPLANATION:
A gas kick was received after drilling through a high-pressure zone (10.8 ppg) with improperly low mud weight. Due to its very low density (2 ppg) compared to the mud’s density (9.62 ppg) the gas-kick column migrates up the annulus despite the fact that there is no pumping (no circulation).

While it migrates it also expands (gets taller) and the shut-in pressures increase in response. An underground blowout (tensile fracture initiation) is induced directly below the casing shoe at 0:57 once the shut-in casing pressure (SICP) on the choke exceeds the maximum allowable annular surface pressure (MAASP).


CALCULATIONS:
MAASP = p_frac - 0.052ρ_mud(Dcsg) = 1,577 - 0.052(9.62)(2,297) = 429 psi


NOTES:
p_frac = fracture (tensile) initiation pressure (psi)
ρ_mud = mud density (ppg)
Dcsg = casing-shoe depth (ft)