Sweet Corrosion | Materials And Corrosion Control

Damage Description

Critical factors

·         Flow regime and flow rate with temperature are one of the dominant factors controlling sweet corrosion. The passivity of the corrosion products is affected by flow velocity, flow character, the partial pressure of CO₂, temperature, water cut, hydrocarbon type, microstructure and metal chemistry.

·         Increasing partial pressures of CO₂ result in higher rates of corrosion.

·         Corrosion occurs only in the liquid water phase, often at locations where CO₂ condenses from the vapor phase.

·         Corrosion rate increases up to 180-200°F then decreases rapidly as passivity of F₂CO₃ increases.  The peak temperature is a function of alloy chemistry (especially, Cr), solution chemistry, etc.

·         Increasing the level of chromium in steels offers improvement in resistance.

·         Increasing flow velocity increases corrosion rate.

Material of Construction

Affected Units or equipment

§  Production: Wellhead components, tubing hangers, hanger subs, R & X nipples, flow couplings, production tubing, casing, production liner, tail pipe, PBR seal and packer assemblies, sand screens, and other downhole accessories

§  Drilling: drill pipe, heavy weight drill pipe, drill collars, drill jars, cross-over subs, tool joints, drill bits, bit subs, Kellys,

§  Workover: coiled tubing, wirelines, fishing tools (over-shots, tubing/casing spears, milling tools, reverse circulating junk catchers, fishing magnets, fishing jars, wash over pipe). CT performs a wide variety of tasks including acid stimulation, wellbore cleanouts, milling operations, gas lifting, fishing an cementing work.

·         Surface equipment: flowlines, trunklines, manifolds

Appearance or Morphology of Damage

·         Uniform wall thinning at temperatures where iron carbonate film does not have significant passivity

·         Localized isolated or lined pitting corrosion of carbon steel

·         Trench-type attack occurs in a flowing condition due to preferential water presence

·         Mesa-type attack occurs due to localized turbulence or the presence of liquid hydrocarbons

·         Localized cold work would promote localized attack

·         Difference in chemistry or microstructure could promote localized attack

Corrosion Control Methodology

·        Corrosion inhibitors can reduce corrosion for surface facilities. For downhole equipment, a chemical injection line should be installed during completion. Limited application of corrosion inhibitors for downhole treatment because of the lack of treatment facilities.

·        Chrome-containing steels such as 1% (4130 or 4140) or 3%Cr perform better than carbon steels for use in surface facilities.
9Cr-1Mo, 13Cr are highly effective to cope with sweet corrosion

·        Internal coating must be abrasion-resistant and corrosion-resistant. Currently, CSD does not allow any organic coating yet.

Corrosion Monitoring & Inspection Techniques

·         Caliper logging is the most reliable method but needs discretion because caper logging (or any typed of well interventions) could damage protective iron carbonate films. This in turn would shorten the incubation time of localized weight loss sweet corrosion, accelerating sweet corrosion

For surface:

·         VT, UT and RT inspection techniques should focus on general and local loss in thickness where water wetting is anticipated.

·         Preferential corrosion of weld seams may require angle probe UT or RT.

·         Corrosion may occur along the bottom surface of the pipe if there is a separate water phase, at the top surface of the pipe if condensation in wet gas systems is anticipated, and in the turbulent flow areas at elbow and tees.

·         Monitor water analyses (pH, Fe, etc.) to determine changes in operating conditions.

·         Many prediction models are available but needs to be used with discretion.

Inspection Frequency

·         OSI

KPIs

·         For surface: corrosion rate less than 5 mpy

Competencies and Training

§  e-COE 101 Corrosion Basics

§  e-COE 701 Corrosion & Corrosion Prevention

§  PEW 407 Corrosion Technology

§  COE 104 Chemical Treatment for Producing Operations

Reference Resources (Standards/GIs/BPs)

·         NACE “H₂S Corrosion in Oil and Gas Production – A compilation of Classic Papers (1981)

·         API “Corrosion of oil and gas-well equipment,1990

·         NACE “Corrosion Control in Petroleum Production” TPC Publication 5,1999