Lessons Learned: Gas Release from Storage Tank
Circulation of gas through a Mud Gas Separator, beyond its capacity, to both active and storage mud tanks, led to gas release from the storage tank.
Leading up to the gas release from the storage tank, during the abandonment of a gas well, the 14’’ casing was cut and then, with the annular BOP closed on the work string, the annulus was displaced with treated fresh water.
Returns were directed through the choke manifold, Mud Gas Separator (MGS), shaker, degasser to the active pit before being transferred to an external storage tank. The liquid returns were highly gas cut and exceeded both MGS and degasser capacity to effectively remove all the gas. This resulted in gas cut liquid being transferred to the storage tank and free gas being released outside of the drilling module triggering alarms and mustering, before the circulation was stopped and the well was shut in.
What happened?
Plan:
Circulate out old mud in C-annulus with fresh water after cutting 14″ casing.
Incident:
Cut 14″ casing at 1212 m.
Closed BOP annular and started to displace C – annulus through choke manifold, MGS, shaker, degasser to active pits with 1,0 sg treated fresh water. Pumped with 500 lpm / 5 bar, increased pump rate in steps up to 1200 lpm / 16 bar.
Observed increased gas level, reduced pump rate back to 500 lpm / 12 bar. Gas level in mud increased to 78 % in shaker box, Gas level in shaker area increased to 5 % LEL at the same time and decreased back to 3 % LEL. When active return tank was transferred to almost empty storage tank, gas was released from the storage tank (outside Drilling module).
General alarm activated due to high gas readings in tank top area – personnel mustered according to station bill.
Stopped MP’s and closed in well on choke.
What Went Wrong?:
Gas from the mud was not sufficiently separated in the mud/gas separator. The returns were transferred to empty storage pit in Mud module, where more gas was released, triggering alarms.
Lack of understanding with regard to the limited capacity of the MGS and degasser system.
Corrective Actions and Recommendations
Lessons Learned:
Mud was circulated through gas separator systems with too high flowrate so that the gas was difficult to remove from mud.
Lacking risk assessment for transferring gas cut mud to storage pit in Mud module.
Original plan called for circulating C-annulus after pulling 14” seal assembly; the decision made by execution team to circulate through C-annulus valves prior to removing seal assembly, reduced severity of the incident.
Recommended actions:
Revise operational procedure for displacing annulus to account for MGS capacity and ensure that limitations are understood.
When high gas readings are expected, evaluate to take returns from annulus to production and flair system.
Content source/credit: IOGP (International Association of Gas & Oil Producers), “Circulation of gas through a Mud Gas Separator, beyond its capacity, to both active and storage mud tanks, leading to gas release from the storage tank,” Well Control Incident Lesson Sharing 26-2.
