First Dual-Plug Cementing in a 4.5in Liner in Sub-Saharan Africa

Abstract

Cementing of a casing string or liner is always considered a crucial phase of the well construction process. The difference with the other phases, is that once cement has been mixed and pumped in the well, there is a limited amount of time for operators to take decisions for troubleshooting or surpassing any obstacles encountered, before cement is no longer pumpable, as at such point, decisions become exponentially costlier, and can even result in the loss of the well, or sidetrack.

Steps to mitigate these risks involve extensive pre-planning to identify all possible risk scenarios, and prepare mitigation, elimination and correction methods. Another step is to count with appropriate equipment, backup equipment, and contingency procedures ready to be implemented if required. Decisions such as how much more volume should be pumped if plug bump is not seen, or if volume count should be reset when dart latch is seen, should be reviewed in advance, as a decision-making process cannot take place when the event is seen, as the timeframe to take these decisions are too narrow.

Cementing a liner in a deepwater scenario increases the risks during the operation. Inaccuracies for volume counting during displacement are increased, and in situations when there is a small liner, and the displacement volume is greatly larger than the slurry volume, increases inaccuracies even further. Free-fall of cement is also increased, as a heavier slurry is being displaced throughout a longer interval, until it U-turns at the shoe and is finally placed on the annular space. This also increases the risk of not seeing the dart latch with the plug, not being able to reset the volume count to a more precise volume.

The cementing phase of the 4.5" liner for the Deepwater Total Moho-Nord Albian Project, presented these risks due to the short 4.5" liner interval. The liner interval can be as short as 450 mts of liner length, equivalent to 10 m3 of cement slurry) in comparison with the TOL depth (average 4,100 m MD, equivalent to over 50 m3 of displacement). The risks were increased as the project relies on plug bump to set a series of annular inflatable packers to help seal the formation from water influxes. This meant that if the theoretical volume was pumped and the plug did not bump at the time, displacement had to stop, and the packers could not be inflated. Rotation, which often helps cement placement, was also not an option in this project, as T&D analysis proved that there was a high risk to exceed the torque limit of the liner connections.

All these risks combined, made the cementing operation of the 4.5" liner of the Albian project, one of the most complex and crucial parts of the well operation.