Historically, wells drilled in East Africa and mainly in Sudan have experienced drilling related problems due to the highly reactive and dispersive shales of the Aradeiba formation. Hole instability problems have included washed out hole sections combined with tight hole and poor coring efficiency. Frequent wiper trips were required and logging of the wells was not always successful.
Over time, several conventional inhibitive water-based drilling fluid types have been used, such as KCl/polymer, KCl/lime/polymer, KCl/PHPA, and KCl/glycol, but with only marginal improvements in hole stability and drilling performance. A sodium silicate/KCl system was designed to provide the necessary inhibitive qualities. The first field trial well in East Africa was drilled with a sodium silicate/KCl system and the following benefits were observed:
24% reduction in drilling days
Shale drill cuttings with exhibited excellent integrity
No evidence of cuttings hydration or dispersion
Reduced wiper trips
Caliper logs indicated in-gauge wellbore
Logging program completed as per program
Excellent coring efficiency
Low filtrate invasion
This paper describes the extensive technical teamwork between the service company and the operator that led to the development and successful application of a sodium silicate drilling fluid for use in the Aradeiba stressed shale sections.
The paper includes a description of the formulated silicate fluid and test data from the extensive laboratory testing. Enhancements in drilling and coring performance over other inhibitive water-based drilling fluid systems are documented and reviewed, in particular coring efficiency (hole stability in the shale zones, elimination of core jamming) and improved core data acquisition (obtaining fresh-state core samples with minimal mud filtrate invasion).
As a result of this fluid design and application, an additional thirty wells have been drilled with the sodium silicate system in the area, including the highest temperature well ever drilled in the world with a silicate drilling fluid.