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While drilling in the offshore Hasbah field, a polymer mud with conventional lost circulation material was used to drill the 16-in. hole section on the HSBH-49. The initial mud weight was 9.6 lb/gal and increased to 10.2 lb/gal while drilling to 8,760-ft. (Lower Fadhili formation). The circulation rate was 900-925 gal/min with an average rate of penetration (ROP) of 60 ft/hr. The transition zone to the Dhruma shale (at 9,160-ft.) consists of interbedded layers of dolomite and limestone, with possible fractures. While drilling through this section, lost circulation was encountered and the rig was unable to regain circulation. The decision was made to drill ahead with the mud cap, with no returns to the surface. Stuck pipe was encountered at 9,031-ft. Fishing operations were unsuccessful and the BHA could not be retrieved. The decision was made to sidetrack the well.
The polymer mud formulated for the sidetrack and included an addition of 5 lb/bbl of FLC 2000 wellbore stabilizer. The mud weight was maintained at 9.6 lb/gal and drilling rate was controlled at 30-40 ft/hr with a circulation rate of about 800 gal/min. The mud system was later converted to an enhanced water-based mud (WBM) system with the addition of shale inhibitors added from +/- 9,000-ft. before entering Dhruma shale. The change in drilling parameters and mud formulation resulted in no drilling fluid related issues while drilling to 9,268-ft. (16-in. hole) as well as the 83⁄8-in. pilot hole to a depth of 9,873-ft.
FLC 2000 helped to mitigate induced fractures on the new sidetrack. Drilling of the transition zone from the base of Lower Fadhili Reservoir to Dhruma shale continued while maintaining 35-40 lb/bbl of conventional LCM/bridging material and 5 lb/bbl of FLC2000. The use of 80 API screens on the shakers avoided excessive stripping of bridging material from the drilling fluid. Mud weight held steady at 9.6 lb/gal and the controlled drilling rate allowing the wellbore to be properly cleaned and minimized risk of solids loading in the annulus while controlling ECD. The 83⁄8-in. pilot hole was drilled and after logging operations opened to 16-in. hole section without incident. Casing was ran to total depth (TD) and cemented with no problems.