In 2017, 2019 and 2021, an Operator drilled 3 ultra-deepwater exploration wells in offshore West Africa. The water depth for well A is around 2900m, well B is around 2800m while well C is circa 2100m. The objective of these wells is to evaluate the potential hydrocarbon accumulation in the subsurface area. Besides the usual challenges being wildcat wells and located far offshore, the wells drilled through pre-salt formations. Hole enlargement due to salt dissolution is also a concern apart from salt movement to close the hole owing to its plastic nature. Furthermore, the lack of offset wells for determining wellbore behavior also caused another challenge. Experiences from the nearby wells showed many signs of wellbore instability such as tight spots and losses. The common causes of wellbore instability are unbalanced stress dependent on stress change and rock strength, and shale-fluid interaction due to hydration of clays. A geomechanical model was constructed and detailed salt creep modelling was developed. Well A was drilled until the final target depth with minimal wellbore instability and encountered no losses at all while drilling. Well B which was drilled later, also drilled to the final target depth with very minimal wellbore instability. Well C was drilled with longer salt exposure around 490m. The mud weight selected for all wells also successfully minimized salt creeping. These are strongly evident while drilling and running in casing, no tight spots were encountered. Both wells successfully drilled through salt formation with very minimal salt dissolution. Through the application of geomechanics and systematic drilling fluids design reduce potential non-productive time (NPT) related to salt plastic movement that could be faced by Operator eventually saving Operator's drilling hours.

The objective of this paper is to present the planning, simulations, laboratory testing and operational results for the initial deepwater deployment of a new cementing technique which utilizes a "heat sweep" of warm seawater circulated inside the casing after cement placement to accelerate early compressive strength development. This technique is made possible through a novel stabbed-in inner string cementing technology which also reduces operational risk for the cement job and saves rig time by eliminating conventional cement shoe tracks. The pre-project planning included comprehensive thermal simulations for placement of the "heat sweep", the 22″ surface casing cement job's temperature profile over time and the corresponding effect on compressive strength development. Additional laboratory testing of the "rig-blend" cement to be used in the well was also completed with and without the effect of the "heat sweep" to finalize the wait-on-cement (WOC) criteria for the 22″ cement job. Finally, a set of detailed operational steps were formalized in the drilling program. The 1000 m (3281 ft) 22″ surface casing cement job at 1532 m (5026 ft) water depth was successful, and several best practices and lessons learned were recorded for the deployment of the new technologies. Highlights included preparing the "heat sweep" utilizing rig systems to the initial placement temperature of 75°C (167°F), cementing through the stabbed-in inner string system, placement of the "heat sweep" inside the casing, and recovering a downhole cement sample and temperature logger from the bottom-hole assembly (BHA). The downhole temperature logger recorded that a maximum 37.07°C (98.72°F) was delivered to the casing shoe, which was roughly double the maximum recorded environmental temperature and an exponential increase above the minimum environmental temperature, near freezing, at the mudline. The "heat sweep" generated approximately 9 times more compressive strength in the cement over 8 hours (1150.55 psi) when compared to the base case without the "heat sweep" effect (129.81 psi). This increase in compressive strength development was equivalent to a 4-hour WOC reduction to develop 100 psi in the tail slurry or a 16-hour reduction in WOC to develop 500 psi in the lead slurry near mudline. Additionally, the 22″ casing pressure tested to 2000 psi, and the shoe was drilled out in less than 20 minutes, saving 6 1/2 hours of rig time when compared to the Operator's most recent subsea well. The formation integrity test (FIT) achieved a slightly higher pressure than expected, and the subsequent 17 ½" section was drilled in a single fast run. The subject novel cementing technologies have the potential to reduce costs and drive efficiency for deepwater drilling operations. This case study presents the first deepwater application for utilizing heated seawater to help rapidly build compressive strength in a cement job after placement through a novel stabbed-in inner string cementing system.