A horizontal well has been drilled in the depleted Siedenburg sour gas field with an H2S content of 6.7%. Concession holders of this field, located in N-W Germany, are BEB and Mobil.

The field has been producing since its discovery in 1964, from 2 reservoirs, the Middle Bunter sandstone and the Main Dolomite, both now depleted to a pore pressure gradient of 0.5 and 0.3 bar/10 m respectively.

A horizontal well in the Main Dolomite, if feasible, was considered as the best solution to increase recovery from the reservoir, by reducing the water influx and creating greater access to the reservoir by increasing millidarcy-feet.

After the reservoir engineering aspects of this project indicated a potential 5-8 fold production increase with a 400 m long horizontal completion interval, the drilling and completion aspects were analyzed for technical feasibility.

Restrictions in surface location and target description (2 intermediate targets) required build rates of appr. 4°/30 m in the 8-3/8" hole section and a medium radius profile of about 13.5°/30 m in the 5- 7/8" section. Both drilling phases had to be accomplished in geologically difficult formations with tight target tolerances.

The proposed entry point was planned to be as close as possible to the old Z-2 well in an area of the reservoir where best facies was expected. The walk tendency experienced in the offset well Z-2 was used for planning the final profile and therefore no directional work was required in the top hole section to line up the well for the final target direction.

Finally it was required to drill horizontally with a hydrostatic overbalance as high as 240 bar to maintain safe operating conditions in a severe H2S environment. The hydrostatic overbalance of that magnitude required precautions to be taken with both mud properties and BHA design to avoid differential sticking. The completion design included corrosion resistant alloy (CRA) for the unprotected pipe below the 7" liner top as well as for the 4-1/2" slotted liner covering the horizontal section.

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