An Offline Approach

Stuart Slater, Unity, UK, explains the merits of employing more agile technology solutions for shallow intervention operations.

ive years ago, the UK’s North Sea Transition Authority (NSTA) set an ambitious target for the UK continental shelf (UKCS) oil and gas decommissioning industry to achieve a 35% cost saving by the end of 2022. Each year, progress has been tracked, and the latest report published in July 20211 showed that a saving of 23% had been achieved. The NSTA has called for faster decommissioning of inactive wells, with increasing collaboration between and amongst licensees and the supply chain, for example, to combine wells into decommissioning campaigns to achieve cost efficiencies. Over 8000 wells have been drilled in the UKCS to date. The current active well stock comprises 2625 wellbores with 1736 producing and 871 inactive/suspended.2, 3 With cost and speed high on the industry’s agenda, new technology and processes introduced by innovative supply chain vendors are helping operators to deliver these targets.

One area where significant cost and efficiency improvements can be realised offshore is in rigless or offline, rather than rig-based, well intervention. This is particularly applicable in the later and simpler stages of near-surface well decommissioning when all

Figures 1 & 2. A lightweight and compact SIS is deployed for surface plug setting operations in a cramped and busy wellbay on a North Sea platform.

permanent barriers have been installed and heavyweight well control is not necessary. This includes the setting of environmental/surface barriers and the removal of near-surface and surface equipment including the casing strings, conductor, and wellhead. By dividing the full P&A sequence into phases and applying fit-for-purpose solutions for each phase and each well, the operator has the opportunity to deliver significant cost savings.

North Sea P&A challenge

This approach has been implemented to great effect at a fully integrated fixed offshore platform in the Northern North Sea, equipped with manned production, drilling and utilities facilities. Operating since the late 1970s, the platform is now undergoing phased decommissioning, with well P&A work currently active.

The operator’s P&A programme had earmarked a number of wells on this platform which were suitable for an offline approach to set shallow plugs in preparation for the removal of the wellhead equipment, however, several challenges needed to be overcome. The well intervention work would need to be conducted while simultaneous rig-based drilling and slickline operations were ongoing at other wells on the same platform, resulting in limited deck space and POB allowance. Height access was also restricted due to the wells being located under the drilling rig. The final challenge was an extremely tight schedule to fit around other planned operations, so the plug setting solution would need to be simple to deploy and compact, yet also manoeuvrable and fast.

Compact technology

To combat these challenges, the operator chose a technology from Unity, which was launched in 2019. The surface intervention system (SIS) can be used for a variety of near-surface operations such as setting shallow plugs and tubing hanger plugs, Xmas Tree removal, well inspection, milling or well-bore clean out, with interchangeable tools on a sectional rod-based intervention system. Powerful hydraulic cylinders apply a push force of 70 000 lbs and a pull force of 40 000 lbs, whilst an ATEX rated motor can be added for a 300 rpm/1200 ft/lbs rotational function, delivering an agile solution to rival conventional intervention methods.

The current model is rated for operations up to 10 000 psi with an integral pressure control package. This uses upper and lower seal cartridges, each with three separate uni-directional seals which straddle the rods to ensure a minimum of two seal contacts at all times. The rods are retained with a rod catcher mechanism which must be operated by two separate personnel before the rod can be released, providing enhanced prevention controls for human error. The seal stack housing also incorporates a grease injection port to allow additional well control measures to be applied if required and allow safe retrieval from the well.

Additional safety aspects include the SIS shear sub which is the tool’s primary disconnect device. In the event of any equipment becoming stuck, the shear sub can quickly be activated, and SIS rods pulled out of hole, leaving behind a standard fishing neck profile.

This year, Unity has developed a data gathering version of the SIS, which utilises the same assembly but deploys ported rods to allow through-rod communication. This further investment in product engineering will allow for enhanced

pressure control and fluid circulation, improved direct injection, and could lead to live electrical communication, potentially including a live camera feed to give clients instant access to valve conditions or frac sleeve wear.

Enhanced well control

To complement the SIS, Unity has recently developed another technology in the form of a compact shear-seal (CSS) valve. This upgrades the well control offering with a fast-acting isolation function, but ensures the lightweight, compact offering is still retained. The new CSS utilises a hydraulically actuated dual ram but is 50% lighter and 30% smaller than the next closest comparable product, being designed for tight spaces and small rig hatches. Additional models are currently in development to offer solutions for higher pressure or more aggressive fluid environments, which could benefit from the compact design, or potential bespoke cutting requirements in already large stack up designs. The SIS system measures in at 660 mm x 3455 mm and is lightweight at 1060 kg. Adding the CSS increases the weight by 952 kgs. The compact size and reduced weight help improve handling and reduce concerns over structural integrity on late-life assets.

Well intervention often requires multiple vendors with considerable manpower, heavy well control packages and a large wellsite footprint, resulting in significant cost and risk for the operator, especially offshore. While this equipment is necessary for deeper well intervention, the SIS and CSS provide a high performing, compact, and cost-effective solution for shallow operations.

Offline deployment

For this multi-well, surface plug-setting project in the North Sea, the SIS was deployed by Unity as part of a fully managed, independent service. Work on the first two wells was successfully completed in March and May 2022, with a third deployment scheduled for 3Q22. In this third deployment the CSS will be included as part of the well control package, serving to complete the first field application of this new technology.

Unity’s rig-up, for both wells, took place in an extremely confined and busy wellbay below deck. Following detailed pre-job planning, the SIS and the pressure control equipment were quickly mobilised and installed to the Xmas Tree using a small A-frame gantry crane.

For both projects, SIS rig-up and testing were complete in under five hours, then a 4.5 in. bridge plug was successfully set in the well at around 90 ft MD-BRT. The plug was set first time, within 30 minutes of rig-up. Rig-down was complete in around six hours and just one overnight stay was required for two technicians.

Figure 3. The Surface Intervention System (SIS) is complemented by a compact shear seal (CSS) valve and small pressure control and operating units.

Operator benefits

Both completed plug-setting projects delivered significant savings to the operator, with a cost and personnel package reduction of 66% with a 60% faster turnaround compared to traditional solutions. This is particularly important as the industry looks for ways to reduce the unavoidable decommissioning cost burden on many operators. SIS plug setting work is now planned for a further 14 wells on this platform to support the operator’s wider decommissioning programme. The multi-disciplinary support service from Unity may also include pre and post SIS intervention operations, such as suspending the well or tree removal, thereby delivering further project efficiencies. The technology offers operators the benefit of an onsite surface intervention system that can be deployed instantly to rectify surface safety concerns or production inhibiting issues, returning wells to a safe or productive status. It has delivered similar savings in other projects around the world, including North Africa and the Middle East, as well as in other European applications. The technology can be mobilised quickly with only 48 hours redress required between deployments. With targets to reduce near-surface integrity issues in producing wells and the cost of well decommissioning programmes still high on the agenda of the NSTA and other regional authorities, this is one example of how supply chain innovation and pragmatism in developing new technology can meet this need.

References

1. North Sea Transition Authority (NSTA): UKCS Decommissioning Cost Estimate 2021 - 2021 - Publications - News & <br/>publications (nstauthority.co.uk) 2. North Sea Transition Authority (NSTA): Wells Insight Report reveals activity slowdown in the UK North Sea and untapped potential in well maintenance - 2021 - News - News & <br/>publications (nstauthority. co.uk) 3. UKCS Suspended Well Stock (arcgis.com)