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Well integrity offshore
There are different definitions of what is Well Integrity. The most widely accepted definition is given by NORSOK D-010: “Application of technical, operational and organizational solutions to reduce risk of uncontrolled release of formation fluids throughout the life cycle of a well.”
Other accepted definition is given by ISO TS 16530-2 “Containment and the prevention of the escape of fluids (i.e. liquids or gases) to subterranean formations or surface.”
Well Integrity is a multidisciplinary approach. Therefore, well integrity engineers need to interact constantly with different disciplines to assess the status of well barriers and well barrier envelopes at all times.
Offshore wells are considered dry tree wells that have much similarity to onshore wells, apart from the fact the consequence of loss of containment is extremely large in respect to environment and people. This environment is therefore much more stringent regulated by industry standards like API14 and country regulating authorities like BSEE and European offshore directives.
These require more complex shutdown systems and subsurface safety valves with minimum requirements and performance criteria. The country regulators often demand a higher level of conformance to these performance standards with regular reporting and inspections.
There is also more focus on standards for this category of wells and records or data base analysis that provide information on barrier element performance like Sintef, these have led to the next generation subsurface safety valves to be developed with separate hydraulic pistons and stronger flapper and flow tube configurations. A good reference is the Barosa Santa Barbara SSSVstudy by RPSEA for Design Investigation of Subsurface Safety Valves for XHPHT Document: 07121-1603.FINAL, 3 (PDF).
Eddy current sensors
The additional risk of offshore wells is in the external corrosion area. The space between outer load bearing casing and surface conductor is often filled with seawater this often causes corrosion at the interface between the seawater level and air contact that can lead to complete failure of the well and result in extensive repairs. The risk of this can be managed by applying an interface on top of the seawater level with for example rapeseed oil and biocide. This however has to be topped up and maintained on a regular basis and its effectiveness confirmed with corrosion logging by inserting eddy current sensors inside the conductor, a technology that is available from Sonovation Rheinland called PEC (Pulsed Eddy Current).
Eddy current logging
Alternative is the use of Eddy current logging. This is a method whereby multiple casing are read from inside the well to the outside. This method is successfully applied by Vanguard EDMS; they have a number of history cases. The results are however not as accurate as the PEC method but is a good alternative. Other logging tools are on the market and in development that are becoming competitive to this technology.
The rapeseed solution does not work under elevated temperatures as bacteria will degrade the rapeseed oil or any natural product very fast. An alternative is use of polymers and glass beads but that would restrict access to verify if you have corrosion from the outside.. With elevated temperatures the corrosion rates double between North Sea and Far East water temperatures. In the Persian Gulf a history case was recorded that had 70 degrees Celsius measured inside the space between conductor and casing. This well corroded the entire casing in a couple of years. The NACE Babonian corrosion handbook for seawater is a good reference for establishing corrosion rates.
In addition you have metocean movement or sea swell effect that moves the platform jacket and its conductors. Inside the conductors the wells that may affect the rate of corrosion due to fatigue issues. This is a subject not clearly understood as some wells on the same jacket will corrode faster than others with no obvious reason.
In the event the casing is corroded, grouting of the space between conductor and casing with a top fill job using cement is a solution applied, but often losses through conductor shoe will prevent this unless a floating spacer is placed or a light weight cement. The risk of grouting is that you glue the well and conductor together. This means with every thermal cycle the well normally moves up and down a little and over time he conductor may start to hang on to the casing. This is the case when the conductor has insufficient spare load capacity based on soil strength stick. Then the conductor will start to transfer its load to the well outer load bearing casing and may induce collapse of the casing below the conductor shoe of the casing. Before embarking on grouting wells inside conductor, one should do a proper engineering assessment of the load capacities.
Casing clamps and shims
In case of reduced load capacity due to corrosion the well load may be transferred to the conductor by using a casing clamp and shims that rest on the conductor. Again an engineered calculation should be conducted. If all fails, i.e. conductor cannot handle load and well casing is nearing collapse, load transition is possible by supporting it from the platform structure. This again needs to be supported by an engineered load case assessment.
External corrosion of conductor occurs only in cases where there is no coating applied or conductor is not protected by the cathodic anodes of the platform jacket.
- D-010:2013. Well integrity in drilling and well operations. 2013. Lysaker, Norway: NORSOK. https://www.standard.no/en/sectors/energi-og-klima/petroleum/norsok-standard-categories/d-drilling/d-0104/
- Corrosion Handbook 5, 5. Weinheim: Wiley-VCH, 2005.http://www.worldcat.org/oclc/314669887
Noteworthy papers in OnePetro
Dethlefs, J. and Chastain, B. 2012. Assessing Well-Integrity Risk: A Qualitative Model. SPE Drill & Compl 27 (02): 294-302. SPE-142854-PA. http://dx.doi.org/10.2118/142854-PA.
King, G.E. and Valencia, R.L. 2014. Environmental Risk and Well Integrity of Plugged and Abandoned Wells. Presented at the SPE Annual Technical Conference and Exhibition, Amsterdam, 27-29 October. SPE-170949-MS. http://dx.doi.org/10.2118/170949-MS.
Singh, S.K., Subekti, H., Al-Asmakh, M., et al. 2012. An Integrated Approach To Well Integrity Evaluation Via Reliability Assessment Of Well Integrity Tools And Methods: Results From Dukhan Field, Qatar. Presented at the SPE International Production and Operations Conference & Exhibition, Doha, Qatar, 14-16 May. SPE-156052-MS. http://dx.doi.org/10.2118/156052-MS.
Vignes, B., and Aadnøy, B.S. 2010. Well-Integrity Issues Offshore Norway. SPE Prod & Oper 25 (2): 145-150. SPE-112535-PA. http://dx.doi.org/10.2118/112535-PA.
Watson, V. 2010. A Quantitative Risk Assessment Approach. SPE International Conference on Health, Safety and Environment in Oil and Gas Exploration and Production, Rio de Janeiro, 12-14 April. SPE-127213-MS. http://dx.doi.org/10.2118/127213-MS.
Wilson, V. A. 2014. HSE and Well Integrity: Friends or Foes? SPE International Conference on Health, Safety, and Environment, Long Beach, California, USA, 17-19 March. SPE-168407-MS. http://dx.doi.org/10.2118/168407-MS.
SPE papers grouped by conferences: https://www.onepetro.org/conferences/spe
Hopmans, Paul. 2013. Journey of Well Integrity. https://webevents.spe.org/products/journey-of-well-integrity
Dethlefs, Jerry. Near Surface External Casing Corrosion; Cause, Remediation and Mitigation. http://www.spe.org/dl/docs/2011/Dethlefs.pdf
SPE. Well Integrity Technical Section. http://connect.spe.org/WellIntegrity/home.
Federico Juarez - Well Integrity Engineer
MJ Loveland - Well Integrity Supervisor ConocoPhillips