Following the successful completion of Phase 1 of this project for Occidental Petroleum, the Inspection Phase, Nautec was then contracted by Dolphin Energy Ltd in Qatar to provide preventative well integrity services and remedial works on a selection of their conductors located in the North field, Ras Laffan offshore Qatar, in order to secure problem free operation for the remainder of their design life and beyond. 

The Scope 

  • Cleaning of Annulus between 18 5/8” structural casing and 26” conductor casing for clearing of steel flakes and debris using Ultra High Pressure Water Jet.
  • Final Inspection on Annulus using Downhole Camera after cleaning.
  • Grouting and filling of annulus between 18 5/8” structural casing and 26” conductor casing with NaX® Q110.

The Challenges

  • The annulus temperature was approximately 70° C.
  • The ambient temperature was approximately 40° C.

NaX® Premix System’ binder technology is derived from state-of-the-art research into applied nanotechnology in cement hydration reactions, synergised with several compatible modern chemical additives, each designed to improve or enhance specific properties. This results in superior binder properties such as higher packing density from improved quality of nanostructures in the hydrated cementitious matrix and increased bond adhesion, both internally between the cementitious matrix and aggregate, as well as externally between the grout material and substrate.

The Services

  • The team used an Ultra High Pressure Water jet unit to dislodge and flush steel flakes and debris away and then pump them out of the annular space. This was completed to ensure that the grout materials have a good bonding with the conductor/surface casing walls and it also makes the inspection process more effective. The high pressure jet washer has a special nozzle which can spray both downwards and upwards at the same time. The nozzle was lowered down into the annulus and shifted around in order to clean a complete surface area of the annulus. Simultaneously, large amounts of sea water was pumped into the annulus in order to flush out the agitated mud and debris.
  • The operation continued until the jet washer met an impenetrable surface and sufficient depth was achieved. Cleaning and flushing was continued until clean water returned at the top of the annulus.
  • Following the cleaning process, a final inspection was carried out using Nautec’s specialist downhole inspection camera. Observations were made to ascertain the following:
  • Condition of the both the conductor and surface casing – to visually check the integrity of the conductor.
  • Existence of holes in the surface casing if present – to avoid loss and non-return to surface of grout mix.
  • Condition, size and depth of the annulus between the conductor and surface casing to establish the size of grout hose to be used during the grouting operation as well as estimate the amount of grout material to be used and prepared for each site.
  • Presence of water/mud/debris or centralisers that may restrict access.
  • Height of the top of cement.
  • In addition to the above, inspection using the down-hole camera was carried out during the grouting operation to monitor the rate of return-to-surface of the grout material.
  • The annulus between the 18 5/8 and 26” conductor was filled with dense ultra-high strength NaX® Q110 grout, with the purpose of sealing any holes or cracks in the casing; arrest existing corrosion; and to prevent further corrosion and to transfer loads from the casing string to the conductor.
  • The NaX® grout material was pumped into the well annulus through various lengths of 2-3” flexible hose and monitored and the quantity of grout pumped was recorded into the dedicated QC documentation.
  • Verification of grout strength was carried out by conducting crush testing on the cube specimens cast during grouting from grout mix samples taken for each well. The testing of the grout samples was carried out by an independent 3rd party SINGLAS-accredited test laboratory Setsco Services Pte Ltd in Singapore.

Client Benefit

  • Restoring the structural capacity of well strings by injecting UHPC materials into the casing and conductor annuli is possibly the most cost effective and technically efficient option available in the market today. The injection of UHPC materials is a swift operation with comparatively little offshore work involved and through the use of UHPC materials, a very significant increase in load bearing capacity of the well string is often achieved. The high strength and non-shrink characteristics of the NaX® material facilitates substantial load transfer between casing strings and the conductor, significantly reducing the stress concentration levels at weak sections of the casing and improving the overall structural integrity of the wells.
  • Sustained, effective corrosion protection solutions are offered based on the use of UHPC materials. Due to the low water and chloride permeability of the UHPC material, on-going corrosion of casings and conductors is effectively arrested through the use of UHPC materials. A 15 – 20 mm layer of UHPC effectively arrests existing corrosion and protects the steel surface from further corrosion, thus extending the service life of the well.

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