The continual increase in exploration drilling in southern Africa has
translated into a number of remote deepwater campaigns, the most recent ones
being in Namibia. One particular three-well campaign was exceptionally
challenging as there was no near offset-well data available. The challenges
were especially acute in the riserless tophole section.
The well designs called for top of cement (TOC) at seabed for the
surface casing. This was of the utmost importance for adequate structural and
axial support for the blowout preventers (BOP) and subsequent casing strings.
The very low fracture gradient near the seabed was the main challenge as the
formations would not support the hydrostatic pressure of the cement column. On
the first well, total losses were encountered prior to and during the entire
cementing operation. As a result, no cement returns were observed at seabed,
contrary to what was expected from hydraulic simulations and volume
calculations and required to meet the job objective.
To achieve objectives required for the success of the subsequent two
wells, all aspects of drilling and cementing operations were reviewed based on
the findings of the first well. Mud weight and casing setting depth were
critically challenged, with other parameters adjusted. Cement formulation and
density were optimized to reduce hydrostatic and hydrodynamic pressures and to
increase the chance of success. The cement slurry was changed to a bimodal
lightweight system with better fluid- and set-cement properties. Lost
circulation fiber technology was also incorporated in the spacer preflush and
in the cement slurry to mitigate any losses during placement.
Alignment of service company and operator objectives and optimization of
drilling and cementing parameters were critical for the successful cementation
of these challenging tophole sections. Continuous improvements resulted in the
second well being effectively cemented to seabed, even though intermittent
losses were observed. After further optimization, the third well was cemented
to seabed with full returns. Reaching the target TOC eliminated the need for a
top-up job, saving valuable rig-time.
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