A polymer mud incorporating conventional lost circulation material (LCM) was used to drill the 16-in. hole section of a well in the offshore Hasbah field. The initial 9.6 lb/gal (72 pcf) mud density was increased to 10.1 lb/gal (76 pcf) to drill the fractured Lower Fadhili carbonate zone at 8,760 ft (2,670 m), with circulation and average drilling rates of 900-925 gpm and 60 ft/hr (18m/hr), respectively. However, total returns were lost at 9,160 ft (2,792m) while drilling the transition zone to the Dhruma shale, comprising interbedded layers of dolomite and limestone with possible fractures. Unable to regain circulation, the decision was made to drill ahead using a mud cap and no returns to surface. Stuck pipe at 9,031 ft (2,753m) eventually forced unsuccessful fishing operations to retrieve the BHA from the hole, thus prompting the first of two sidetrack attempts. After two weeks of battling lost circulation and two sidetracks with BHAs left in hole, the operator devised an alternate strategy for the third sidetrack.
The polymer mud system used for the latest sidetrack was uniquely formulated with 5 ppb of the FLC 2000® wellbore shielding additive, which deposits an impenetrable barrier to minimize fluid and pore pressure invasion into matrix pores and microfractures, thereby stabilizing troublesome zones. The mud density was maintained at 9.6 lb/gal, but the drilling rate was controlled at 30 to 40 ft/hr (9-12 m/hr) with a circulation rate of around 800 gpm. Prior to entering the Dhruma shale, the drilling fluid was displaced with a water-based mud, with shale inhibitors added from +/- 9,000 ft (2743m), along with continual additions of FLC 2000.
The combination of the FLC 2000 ultra-low invasion additive in an enhanced mud formulation and changes in drilling parameters resulted in zero fluid-related issues while drilling both the 16-in. hole to 9,268 ft (2826m)and the 8 3/8-in. pilot hole to a depth of 9,873 ft (3009m). FLC 2000 is credited with helping to mitigate induced fractures on the third sidetrack and prevent the whole losses previously encountered. Drilling through the transition zone from the base of Lower Fadhili reservoir to the Dhruma shale continued uneventfully while maintaining 35-40 ppb of conventional LCM/bridging material and 5 ppb of FLC 2000. Moreover, the use of API 80-mesh shaker screens avoided excessive stripping of bridging material from the drilling fluid, which held steady at 9.6 lb/gal. The controlled drilling rate, meanwhile, facilitated wellbore cleaning and minimized the risk of solids loading in the annulus while controlling the equivalent circulating density (ECD). The 8 3/8-in pilot hole was drilled and, following logging operations, opened to the 16 in hole trouble free. Casing likewise was run to total depth and cemented with no problems.