Offset wells in this region for the past 20 years were used as water disposal wells, having 40,000 bpd water production. Past decade observed remarkable decline in production leading to the development of the current well as a replacement while abandoning the previous well. The well was ranked as medium critical considered from its long water disposal period as no integrity test were performed due to the well location. The lithology of anhydrite alternation with limestone, dolomite and thin shale layers caused a risk of losses and differential sticking. In addition, this was a H2S bearing (100 - 119ppm) salt-water flow formation.

With a rotating control head in place with the flow line valve closed providing a closed loop system, the return is dumped to the sea through the extended diverter lines, ensuring that the H2S emissions are diverted away from the rig floor and from the manned complexes.

Total losses and sour water flow made closed loop flow drilling an engineering solution for its ability to ensure no H2S migration to the rig floor and continuity of the drilling operation with returns diverted away from the rig at all times. The introduction of side entry flow line on the rotating control device (RCD) allowed utilization of an additional fill up line planned to bullhead back to the formation preventing excessive gas migration.

Extensive planning of rig interface with rotating control head with side inlet connection from the standpipe manifold to manage time and space constraints in addition to losses management providing well continuity.

The well drilled successfully with a rotating control device- RCD at surface and returns diverted safely. The closed system with rotating control device - RCD and a well head pressure monitoring gauge provided an additional security of analyzing well conditions, though risk of having gas influx was initially identified as a medium hazard being a top hole section with a higher chances of losses while drilling.

As circulation and conditioning was done traces of gas and H2S were observed with an increase in pressure observed at the RCD. Bullheading from the side inlet of the RCD from standpipe was utilized to balance the well eliminating the risk of high exposure of H2S gas at surface. Having the only pressure monitoring system in place with the RCD the overhead pressure could be identified to raise the mud weight and to balance the well. This operation was successful and resulted in zero gas at surface with casing and cementing operations on the well conducted safely without any quality, health or safety issues.

Understanding the risk of less information about the formation led to the approach of utilizing a low pressure rotating head system to drill safely into a H2S risk zone. This paper identifies how a previously used system, could have an innovated approach to drill safely in a total loss and H2S prone formations.

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