In order to gain control over a well, rig personnel performed so called sandwich kill attempt. Weighted mud was pumped both from the drill string and annulus at the same time. Casing pressure was decreasing by bleeding off the gas. Eventually, it was impossible to decrease it more so they switched to new method.
Weighted mud was pumped through the drill strings in order to circulate the influx out. Soon after pumping started, drill pipe pressure reading was 0 psi. Choke had to be adjusted without any reference value for drill pipe pressure. It was decided to adjust the choke only when casing pressure increases. Eventually, rig run out of weighted mud due to big losses and bad preparation.
Next method that was used to gain control was annulus pressure control attempt. There was no reference value for operating the choke because drill pipe pressure was 0 psi. It was decided to maintain casing pressure constant or to decrease. Four LCM pills are injected in the well in order to deal with loss zone. Losses were reduced. The choke was opened slightly, big casing pressure drop occurred. There was new influx in drill string from the Emsian formation.
Last option was low choke attempt. It was decided to keep casing pressure around the last recorded value while circulating as fast as possible. It was noticed that there is no communication between drill pipes and annulus. Two separate systems are present in the wellbore which makes well control more difficult. There was possibility of plugged drill pipes and annulus packoffs. Pipe was worked out and mud pumped in annulus to break the packoffs in order to re-establish circulation. Drill pipe pressure increased to 3000 psi and rig personnel decided to abandon the well.
Cement plug was pumped through the drill string to plug them. At lowest possible point, drill pipe was perforated and cement was injected to close the annulus as well.
After unsuccessful well control of an underground blowout, there are several things that can be learned from the procedures that were performed (Aguilar, 2011):
· Formation integrity test (FIT) should be performed after the loss zone is repaired. It is also necessary to recalculate the kick tolerance and the maximum allowable surface pressure.
· If there is no credible formation pressure data, it is suggested to use the heaviest mud possible.
Very important is to have proper rig personnel training in kick detection, shut-in procedures and well control methods.
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