Various organic (formic, acetic) or inorganic (HCl, HF) acids for well stimulation or wellbore cleaning cause both general and localized corrosion.
Acid Corrosion (Acidization) | Materials And Corrosion Control
Damage Mechanism |
Acid Corrosion (Acidization) |
Damage Description |
· Various organic (formic, acetic) or inorganic (HCl, HF) acids for well stimulation or wellbore cleaning cause both general and localized corrosion. These acids are very aggressive to most common materials of construction across a wide range of concentrations. Matrix acidization involves the placement of acid within the wellbore at rates and pressures designed to attack an impediment to production without fracturing or damaging the reservoir (typically, HF is used for sandstone/silica-based problems, and HCl or acetic acid or formic acid is used for limestone/carbonate-based problems). Most matrix stimulation operations target up to a ten foot radius in the reservoir surrounding the wellbore. Chemical inhibitors are useful for the full spectrum of well operations that can be injected into the well with acidization or other such stimulation activities to reduce fines generation and organic deposition, or introduced in surface flowlines on a regular basis to prevent scale precipitation and build-up. Operational techniques are needed such as bringing on production or injection slowly after stimulation activities (to prevent damaging flow surges which could mobilize once immobile fines within the pores, plug perforations, or cause sand control problems downhole or at the surface), and routine maintenance and surveillance (like cleaning out process filter traps which can easily clog lines and cause the transfer of damaging suspended particles, or monitoring production decline to identify potential deviances before the problem is exacerbated). |
Affected materials |
· All common materials of construction used in surface or sub-surface facilities. |
Critical factors |
· Acid strength, exposure time, temperature and alloy composition.
· Un-spent acid flow-back would cause severe metal loss to production tubing and surface facilities. · The severity of corrosion increases with increasing temperature. · Acid inhibitors are water-dispersible and their effectiveness is time-dependent. Proper mixing between acid inhibitors and acids is critical. |
Affected Units or Equipment |
· Production: Wellhead components, tubing hangers, hanger subs, R & X nipples, production tubing, casing, production liner, tail pipe, PBR, packer assemblies, and other downhole accessories · Workover: coiled tubing during well stimulation. |
Appearance or Morphology of Damage |
· Carbon steel and low alloy steels suffer general uniform thinning and localized corrosion attack.
· Acid-attacked areas by acidization or in-situ pickling jobs may suffer rupture during production. |
Prevention/Mitigation |
· Choosing and mixing acid inhibitor properly just before bullhead treatment or coiled-tubing injection.
· Implement a proper mothball procedure to coiled tubing after acidization or in-situ acid pickling · Screen out corroded pipe |
Corrosion Monitoring & Inspection Techniques |
· Visual or UT for equipment which were exposed to acid treatments |
Inspection Frequency |
· After each job |
KPIs |
· Less than 5 mpy of metal loss |
Competencies and Training |
· Corrosion Courses
o e-COE 101 Corrosion Basics o e-COE 701 Corrosion & Corrosion Prevention o PEW 407 Corrosion Technology o COE 104 Chemical Treatment for Producing Operations |
Reference Resources (Standards/GIs/BPs) |
· NACE Publication on CO2 Corrosion · API “Corrosion of Oil and Gas-Well Equipment,1990 · NACE “Corrosion Control in Petroleum Production” TPC Publication 5, 1999 · Metals Handbook, “Corrosion,” Volume 13, ASM International, Materials Park, OH. |
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