Merits of Suspending the First Platform Well as a Cuttings Injector

Abstract Under Norwegian regulations, available disposal options for drill cuttings are discharge of cuttings from costly synthetic fluids, re-injection, or land disposal of oil base fluid cuttings. On a recent North Sea platform development, the first well drilled was temporarily suspended at the 13–3/8" casing and completed for drilling waste injection. This well will be deepened late in the program as a production well. The net cost of temporarily using this well for disposal will be about $500,000. The benefits of an injection well weresignificant cost savings over other options,oily waste from the first well drilled to the reservoir did not have to be sent to shore,having an injection well available earlier allowed proper commissioning and solution of problems inherent in new platform startup,concerns about hanger erosion, annulus plugging, and casing collapse of producers were minimized,recovering the well for production is more economical than permanently dedicating the well for disposal. Oily wastewater and drainwater have been the dominant components of the injected waste, and would have incurred a significant land disposal cost. This wastewater disposal cost savings enhanced the attractiveness of the injection alternative generally. Introduction Jotun B is a recent ExxonMobil Norway platform development in Block 25/8 of the Norwegian North Sea. Drilling began in April 1999. The decision was made to use an organic phase fluid, either mineral oil- or synthetic oil- based, to avoid wellbore stability problems in the 12–1/4" and 8–1/2" sections. These sections penetrate Tertiary shales that are weak and water sensitive. After examination of options possible within the framework of Norwegian Regulations including using costly synthetic muds (ester or olefins) and discharging associated cuttings, using mineral oil muds and disposing of cuttings onshore, using mineral oil muds with cuttings reinjection (CRI) was the option chosen based on cost and operational considerations. At the time of platform and conceptual well design in 1997, Exxon had no offshore drilling waste injection projects. The decision to use injection at Jotun was based on favorable experience among other operators in the North Sea1,2,3,4, and an economic analysis of other disposal options. The use of centrifugal pumps with hardfaced impellers was the method chosen to grind and circulate the cuttings slurries. The casing program for Jotun will not accommodate injection into an upper annulus while drilling a deeper hole section with oil mud. This would have required that oil mud cuttings and contaminated water from the first well would have had to have been sent to shore if injection were restricted to annuli. In order to avoid sending any cuttings or well-related waste to shore, it was decided to suspend the first well on the platform, B-17, at the 13–3/8" casing and complete it with tubing for annulus injection. This way, there would be a disposal well available before oil mud and its associated wastes were introduced to the platform. A well was selected that would not be drilled to the reservoir until the end of the program, so that it could accept waste as long as possible. Several annuli were also prepared for injection, to receive the cuttings from the dedicated well when it was deepened, and as a contingency against plugging of the dedicated well. By early May 2000, eight Jotun wells were drilled and completed. Drilling waste injection had taken place since June 4, 1999 with a 99+% waste acceptance efficiency and no incidents of injection well plugging. Most of the drilling waste, over 18,600 m3 (volume before slurrying the cuttings), had been injected into the dedicated B-17 well. The total injected volume was near 8,800 m3 of slurry and 14,000 m3 of wastewater, completion, and displacement fluid. Only one annulus had been put into service, because the trajectories of two wells were in close proximity to the dedicated injector. After these two wells were drilled through the injection horizon without evidence of trapped pressure, the dedicated well was put back into service and used in alternation with the annulus injector.