11.6.1 Introduction
For upper completion and sizing of production tubing, both 7” and a 5 ½” tubing sizes are under evaluation. The main construction issues related to upper completion design are:
• Open hole zone isolation (cement, swell packers, etc.)
• Downhole Chemical injection
• Casing design wear tolerances
• Cementing of production casing vs. well integrity
• Production packer type
• Well intervention interface
• Well control during installation
• Equipment reliability and HPHT run record
• Formation compaction / well deformations
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R-018832 In addition to technical well construction issues above, simulations are being analysed by subsurface to determine the production profiles for the two relevant tubing sizes.
The upper completion design is independent of well trajectory. Maximum inclination at production packer depth is 60°.
11.6.2 Production tubing
The tubing design is conducted mainly based on:
Suitable grade and material selection to handle downhole HPHT conditions
Tubing size to optimize gas production
Stress analysis during life of well
The proposed tubing configuration and connection for Zidane is:
Tubing Connection
OD (in) Weight (lb/ft) Grade Name Grade
5.5 26 Super 13%Cr-110 VAM TOP HC Super 13%Cr-110
VAM TOP HC (High Compression) is a metal-to-metal connection which offers gas tight sealing under severe combined loads. This connection is recommended based on good experience form Norwegian HPHT wells. Further reference is made to chapter 11.8.
11.6.3 Production packer
The following design criteria’s are important for correct selection of production packer for Zidane:
Performance envelope beyond production tubing
Hydraulic set to create an initial stretch in the tubing, reducing effects from compressive production loads and buckling.
ISO 14310 V0 qualified
Qualified for use in un-cemented casing
Robust with regards to premature setting while running in hole
Retrievable
According to HPHT requirements, the production casing cement quality needs to be verified prior to running top completion. The packer should, as far as possible, be set in a cemented interval at an impermeable formation. Further, a casing wear log (multifinger calliper) must be run prior to completion in order to verify that the casing integrity is acceptable due to potential casing wear caused by reservoir drilling.
The minimum setting depth of the production packer is given by gas pressure ≤ minimum formation horizontal stress at packer setting depth. For Zidane West, this gives an
approximate minimum production packer setting depth equal to 3500 m TVD.
A hydraulic set production packer is used as base case for Zidane. The production packer is hydraulically set against a plug and prong or bridge plug below the packer.
No expansion joint will be integrated above the production packer to avoid additional leak paths above the primary barrier.
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The Kristin completions suffered three premature settings of the permanent production packer during running in hole, mainly due to small clearances combined with dirty wells (ref /14/). Removal of permanent packers involves cutting tubing, milling packer and several clean-out runs.
A new HPHT retrievable packer has been recently developed and successfully used in the Morvin completions. This packer incorporates a cut-to-release sub below the packer. Hence, the packer can be released by WL conveyed cutting. Further, this packer has a smaller diameter and offers a higher stress performance envelope.
11.6.4 Flow control equipment
A no-go nipple profile will be installed just below the production packer. The blanking plug that seats on it is used to hold the pressure from above during the setting of production packer. It can also be used to pressurize the tubing for leak detection and production string integrity.
A self-orienting muleshoe without seals will be used to integrate upper and middle
completion. A positive sting-in into the middle completion PBR ensures easy wireline access during interventions. A perforated pup joint is included above the muleshoe to allow a
circulating path for displacing the well to completion fluid.
11.6.5 Downhole chemical injection (DCI)
Salt, scale and asphaltene precipitations are not expected on Zidane. Hence, downhole treatment is not part of the base case design. There are no field proven HPHT DCI downhole systems qualified in the market today.
11.6.6 Downhole pressure and temperature monitoring
The Zidane completions will all be planned with electrical downhole gauges for pressure and temperature monitoring. No control line penetration is recommended through the production packer as this will result in a weaker non-HPHT style production packer. Hence, the gauges will be placed as close as possible above the production packer.
The reliability of downhole gauges under HPHT conditions is generally poor as gauges tend to fail after relatively short time after production. Based on recent field record, roughly 50%
of the downhole gauges can be expected to fail two years after installation. Both electrical and fibre optic gauges have been used on the continental shelf, with similar behaviour.
Zidane will be limited to electrical gauges due to the subsea XMT interface.
11.6.7 Down hole safety valve (DHSV)
A single, 5½” tubing retrievable surface controlled subsea safety valve is planned for Zidane.
A 10 k design would cover the Zidane pressure requirements.
A single valve DHSV recommended over a dual valve as keeping completion design as simple as possible and minimizing potential leak paths. The most probable cause of a DHSV failure is hydrate or scaling precipitation.
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R-018832 Using two independent DHSVs does not result in spacing out dual valve in different scaling environments. Maximum fail safe DHSV setting depth is limited to approximately 600 mTVD to avoid premature opening of DHSV in case of a control line leak. Safety valves have shown good reliability from Norwegian HPHT fields Morvin, Kristin, Kvitebjørn and Huldra fields, all installed with single safety valves.
The following input is recommended for correct selection of DHSV for Zidane:
Performance envelope higher than the production tubing
Single control line
Non-equalising
WRSCSSV insert valve contingency
Setting depth minimum 50 m below seabed
Water based control line fluid
Control line pressure approximately 850 bar (Zidane West)
As the 5 ½” DHSV will have an outer diameter > 9.0”, the production casing will have to include a top section of 10 ¾” casing from wellhead to below DHSV setting depth, to accommodate room for the downhole gauge control lines.
If some conductors are to be installed after start-up of production, DHSV setting depth should be moved down to below conductor shoe depth in order to avoid need for plugging production wells with bridge plugs while drilling top hole.
A water based control line fluid, Transaqua HT2 has been used for the subsea HPHT on Kirstin and Morvin. During production, a maximum surface control line pressure of 850 bars is expected on Zidane West, based on a shut-in wellhead pressure of 630 bar.
The downhole safety valves on Zidane will have to be installed at a depth where hydrates can potentially be formed. Equalizing and opening of the DHSV should be done with a MeOH or MEG/ freshwater mix which prevent potential hydrate formation.
Expected maximum DHSV control line test pressures are described in Table 11-7.
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