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.