Negative Casing LL with DNO

Oselvar (A-1 H, A-2 H, A-3 HT2) – Production section

What happened:

DONG handover documentation:
When circulating prior to cementing initial pressure was 79 bar / 1500 lpm, this gradually declined to 11 bar. MW in returns was up to 1.84 sg for some time. After trouble shooting on surface with out finding any reasons for the low pressure it was observed that DP string was on strong vacuum. Circulation continued and pressure increased gradually and stabilized at 50 bar / 1500 lpm. MW out was as low as 1.65 sg indicating bartie sag inside casing during casing run. Mud low end rheology had been kept low due to swabbing concerns.

What was learned:

The balance between rheology to support barite versus swabbing when POOH is delicate. Recommendation is to be more aware that variations in circulating pressures could be caused by this as circulating pressure loss is so low that variations in MW can have serious effects on pump pressure.