Rigless Well Abandonment – Frequently Asked Questions

Decommissioning Project

  1. What is the benefit of using a vessel based approach versus a rig based approach for well abandonment? –
    A vessel is able to move quickly and easily between work site locations, conducting operations using dynamic positioning, which provides significant time savings when compared to a semi-sub or a jack-up or drilling rig which require positioning and moorings. A vessel based approach also provides significant cost savings when compared to the hire of a rig.
  2. What is the maximum water depth you can operate in? –
    The equipment spread is designed to operate at a water depth of circa 1,600ft.
  3. How do you ensure you only perforate one casing string? –
    Comprehensive shot testing including repeatability tests has been carried out at our specialist perforation subcontractors facilities and these tests are witnessed using test coupons of the identical material, size and spacing to that of the casing material. The perforating charges are designed to only perforate the required casing and not to severely damage the outer string. Should our team encounter an unusual grade or casing size then additional testing could be performed to ensure perforating success. Note: The perforation hole size is also measured to achieve at least a 0.25 inch hole to minimise friction losses during circulation.
  4. What happens if you encounter pressure in the well? –
    The well should be categorised by means of an independent review process performed by an examiner which will assess the suitability of abandonment without a drilling BOP package. The SWAT tool has a SWP of 5000psi however it would never be used on a well where pressure behind the casing was expected. Our team has previously seen pressure after perforation of a casing string however this will typically be due to factors such as the degradation of OBM rather than reservoir pressure. Additional pressure may also be due to the gases emitted during the firing of the perforation guns in a small volume annulus but, in both scenarios, the pressure spike can be bled off into a vented deck storage tank. In the extremely unlikely event of reservoir pressure being present behind the casing cement the pressure would be bullheaded back into the reservoir and the well secured prior to bringing in a MODU with full BOP capability. Note: Hydrocarbons are never brought back to the vessel.
  5. How to ensure hydrocarbons are safely contained? –
    Any hydrocarbons encountered are contained by the existing annular cement bond, suspension cement plugs and mechanical barriers. The SWAT tool is used to place and position an additional shallow environmental barrier to isolate normally pressured water bearing formations where the existing shallow plug does not conform to current legislative guidelines.
    It can also be used to circulate out OBM from a sealed annulus at the lower perforating depth and to position a cement plug in the bore and annuls to prevent OBM from below the lower perforation from leaching into the sea once the wellhead and casings have been severed and recovered.
  6. How long does it take to abandon a well? –
    Intervention durations are approximately as provided below:
    Category 2.1 well is +/- 36hrs approx.
    Category 2.2 well is +/- 60hrs approx.
    Severance operations for either of the categorised wells shown above is +/- 24hrs including well head recovery time.
    These figures are indicative as there may be well specific issues that require more time to be added, that said, they are based on a catalogue of over 100 wells successfully abandoned by our team and, as such, represent accurate estimates gained from real project experience.
  7. Are there any wells you cannot abandon? –
    The tool has been designed only for suspended subsea wells which have internal casings from 9 5⁄8” to 13 3⁄8″. SWAT is not suitable for mudline wells as the system lands out on the 9 5⁄8″ hanger. However, for mudline wells, an alternative twin low pressure packer system can be utilised. The Twin Low Pressure Packer system is similar in principle to the SWAT system and has been utilised to abandon over 20 UKCS wells since 1996 for clients such as GDF SUEZ E&P UK.
  8. How does the SWAT system latch on to a well? –
    The tool incorporates a ring which is spaced out to land off on the 9 5⁄8 ” casing hanger. Two integral packers are inflated to effect a pressure seal and the packers anchor the tool against the casing wall.
  9. How is the circulation/cementation controlled? –
    Once the casing has been perforated fluid is pumped from surface through a circulation line in the umbilical into the casing bore. This displaces fluid within the wellbore down and out of the lower perforations.
    This in turn displaces annular fluid up the well annulus and through the upper perforations through the tool body and back through a return line in the umbilical. The tool is entirely controlled by an umbilical to surface and a control cabin.
  10. What happens in the event of bad weather during SWAT operations? –
    The weather forecast is reviewed prior to starting any well abandonment work to ensure that there is a sufficient weather window to perform abandonment operations. If the weather conditions worsen during the abandonment then additional slack would be added to the umbilical and operations would be suspended until the weather has improved.
    The tool incorporates an emergency disconnect mechanism which can be activated as a last resort.
    If activated the tool is failsafe closed and all of the subsea valves automatically close. The packer inflate lines also close thereby maintaining pressure integrity whilst the upper subsea disconnect releases from the tool allowing the vessel to move off location. Note: This function would only be used in an emergency and not as a routine operation.
  11. How did the development of the tool come about? –
    The SWAT system was developed in 1996 following an enquiry from BP to investigate whether a system could be designed that could successfully enter and abandon suspended wells from a vessel, with no need for the provision of a platform or rig.
  12. Does the system require diver intervention? –
    No. Since the year 2000 the SWAT system has been completely diverless. Design improvements have meant that control is fully automated relying on hydraulic actuators and the control umbilical to surface.
  13. What specification of vessel is required to support the SWAT system? –
    • Ideally a DP2 vessel
    • Moon pool (required for deployment of SWAT system)
    • 100te AHC crane (60te for only severance operations)
    • 800m2 available space on the main deck
    • OBM/seawater under deck storage
    • Below deck bulk cement storage saves deck space.
    • Accommodation for 28 project crew
    • Helideck is preferable for marine crew changes.
    • Min of 1x WROV
  14. What happens to the recovered structures and oil based muds/fluids? –
    The team arranges transfer of the returned wellhead casing stubs and structures such as guide bases and wellhead protection frames via an approved, registered waste carrier. Other waste products are disposed of via specialist agents. Items are tracked from seabed to the agent’s site and suitably documented with waste transfer notes. In both cases the contractors conform to ISO9001 and ISO14001 standards.
  15. What is the deepest/shallowest you can set cement plugs? –
    To date, the deepest cement plug set is 2400 ft BML. With the introduction of a SWAT extension module, developed in response to industry demand in 2015, barriers can now be set at any depth required. The new technology enables us, to set deeper environmental and intermediate barriers, increasing the number of wells that can be abandoned using rigless tools. The shallowest plug that has been set by our team was 150ft BML. An allowance for a 100ft cement plug and a precautionary 50ft allowance for possible cement slumping must be left for safety and to allow access for the severance equipment when well head removal operations commence.
  16. How does the SWAT extension module enable barriers to be set deeper than before? –
    The SWAT extension module utilises the existing SWAT tool to enable cement to be positioned deeper within the well. A wiper plug is positioned before and after the cement column, which ensures that the wellbore is cleaned ahead of the cement. The lower plug forms a base for the column and that slurry is uncontaminated when it enters the annulus. The cement is then displaced to the required depth. Previously, the SWAT tool was limited to environmental barriers up to 600 ft BML.

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