Minimizing Salt Creeping Through Geomechanics Application and Systematic Drilling Fluids Design in Ultra Deepwater Wells - Operator's First Experience in West Africa

Abstract

Abstract 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.