Rejuvenating a Late-Life Offshore Field Through Hydraulic Fracturing: The Successful Case History from North Alwyn

Abstract The Alwyn North Field is located 465 km north-east of Aberdeen. The Alwyn North Triassic Reservoir has consistently presented challenges due to its disconnected, low-permeability sand bodies, with nearly 60% of the reservoir possessing permeability below 1mD. Being the deepest and most complex of the Alwyn reservoirs, the Triassic's low permeability can result in wells being constrained by their productivity. Consequently, hydraulic fracturing has been seen, both historically and recently, as a way to maximize its recovery. A total of six wells were hydraulically fractured between 1998 and 2003 before a significant gas release, attributed to the clean-up of a frac well, halted further fracturing attempts. The gas release, combined with strong production from other fields on Alwyn, and a depressed gas price meant that hydraulic fracturing was not performed at Alwyn again until 2024. The strategic decision to concentrate efforts on N57 and N58Z was influenced by the unique challenges presented by these wells. N57, once a strong producer, declined significantly in 2021 after a re-perforation intervention caused clay particles to obstruct the reservoir and effectively kill the most productive zone. N58Z was drilled in 2022 in a new panel which found lower permeability than prognosed and subsequently underperformed. Both wells then became ideal candidates for hydraulic fracturing. The fracturing designs refined the fracture half-length in N57 to address 194m of open perforation intervals with variable permeability and pressure layers, and limit N58's fracture half-length to avoid intersecting the nearby mother-bore well or water zones. The flowback design featured a cyclone for initial proppant separation, a dual filtration unit, a multi-phase flow meter (MPFM) for real-time monitoring, and two solid detection sensors: acoustic and intrusive. The fracturing campaign timeline was accelerated and based on the availability of fully booked stimulation vessels schedule across two service providers. An 8-day window in September, though not ideal for North Sea operations, was secured. The operation was integrated into a busy platform schedule and required close coordination with the offshore team and asset planners to allow simultaneous works of drilling completion, a platform pit-stop, and a flare tip change out. Key aspects included optimising the timeline, using space on the BOP deck, pipe deck, and rig floor, and scheduling personnel on board. The campaign was executed successfully without any incidents or accidents, resulting in a significant increase in production. The contributions of N57 and N58Z added significant reserves and raised Alwyn's areas output by one third whilst providing valuable insights for future fracturing operations. This case history of remedial offshore fracture stimulation addresses in detail the challenges, engineering, execution and resulting production success of this campaign.