casing-potential profile

A casing potential profile (also called a casing potential survey or pipe-to-soil potential log) is a wireline measurement of the electrochemical potential between the steel casing string and a reference electrode drawn through the wellbore or positioned at surface, recorded as a continuous depth log that maps the cathodic protection status, corrosion activity zones, and electrical current flow along the entire casing string from surface to the cemented depth, providing the well integrity engineer with a direct indicator of where the casing is cathodically protected, where it is experiencing active galvanic corrosion, and where stray current or telluric current interference is degrading the cathodic protection system. The measurement is made by lowering a reference electrode (a copper-copper sulfate half-cell or silver-silver chloride electrode, with the copper-copper sulfate standard preferred for terrestrial WCSB wells) through the wellbore on a wireline cable while a second reference electrode is placed at surface adjacent to the wellhead, and the potential difference between the subsurface electrode and the steel casing at each depth is recorded continuously as the tool is pulled from bottom to surface, generating a potential-versus-depth profile that is compared against the standard cathodic protection criterion of minus 850 millivolts or more negative relative to the copper-copper sulfate reference electrode (the API RP 651 and NACE SP0169 cathodic protection adequacy criterion for buried steel). In Western Canada Sedimentary Basin well integrity programs, casing potential profiles are run on wells with cathodic protection systems (impressed current or sacrificial anode systems installed at the wellhead to protect the surface casing from external corrosion) and on wells in areas of high soil corrosivity, stray current from pipeline cathodic protection rectifiers, or telluric current interference from natural geomagnetic activity, all of which are significant concerns in the WCSB plains region where high-salinity soils, active oil and gas pipeline CP rectifiers, and seasonal telluric current fluctuations create complex electrochemical environments around shallow wellbore casing. The profile interpretation identifies three critical zones: protected zones where the casing potential is more negative than minus 850 mV (indicating adequate cathodic protection and minimal corrosion activity); active corrosion zones where the potential is between minus 500 and minus 850 mV (indicating insufficient cathodic protection and elevated corrosion rate); and anodic interference zones where the potential is more positive than minus 500 mV or shows anomalous positive excursions (indicating stray current pick-up where the casing acts as an anode and accelerated corrosion is occurring at the point of current discharge). In WCSB mature producing areas such as the Pembina Cardium and Redwater fields where wells may be 40 to 60 years old and cathodic protection systems have degraded or were never installed, casing potential profiles provide the primary evidence for external corrosion risk assessment that informs the decision to install or upgrade cathodic protection equipment before the casing perforates from external attack. Understanding casing potential profile measurement principles, the copper-copper sulfate reference electrode standard, the minus 850 mV cathodic protection criterion, the interpretation of protected versus active corrosion zones, and the use of profile data to optimize cathodic protection system design gives well integrity engineers, corrosion specialists, and production engineers the electrochemical foundation needed to assess and manage external casing corrosion risk throughout the producing life of WCSB wells.

• Copper-copper sulfate reference electrode and the minus 850 mV criterion: The copper-copper sulfate half-cell (CSE) is the standard reference electrode for WCSB terrestrial casing potential surveys, providing a stable reference potential of plus 316 mV relative to the standard hydrogen electrode. The NACE SP0169 and API RP 651 cathodic protection adequacy criterion requires the pipe-to-soil potential to be minus 850 mV CSE or more negative (more cathodic) to demonstrate adequate cathodic protection; readings between minus 500 and minus 850 mV CSE indicate a partially protected but still corrosion-active condition, and readings more positive than minus 500 mV CSE indicate an unprotected actively corroding casing section requiring immediate CP remediation.
• Stray current interference in WCSB well fields: WCSB plains areas with dense pipeline and gathering system networks have numerous cathodic protection rectifiers (impressed current systems) that inject DC current into their pipelines, and this current partially migrates through the soil and is picked up by wellbore casing strings as stray current. Where the stray current enters the casing (cathodic zone), the casing is temporarily over-protected; where it leaves the casing and returns to the source pipeline (anodic discharge zone), corrosion is accelerated. Casing potential profiles identify these anodic discharge zones as anomalous positive potential excursions, and the remediation is installation of decouplers or bond resistors to interrupt the stray current path.
• Survey timing and telluric current interference: Telluric currents (natural earth currents driven by solar geomagnetic activity) fluctuate diurnally and seasonally in the WCSB plains, with amplitudes of 10 to 100 mV that can mask or distort casing potential readings if the survey is run during high geomagnetic activity. WCSB casing potential surveys are best run during periods of low geomagnetic index (Kp index below 2) and with simultaneous surface potential monitoring to allow correction of telluric interference from the casing potential log; surveys run during geomagnetic storms (Kp above 5) may show apparent corrosion zones that are artifacts of telluric interference rather than true unprotected casing sections.
• Integration with multi-finger caliper for corrosion mapping: Casing potential profiles identify where corrosion is occurring electrochemically (current-based evidence of active metal dissolution), while multi-finger caliper logs provide morphological evidence of corrosion that has already occurred (wall thinning and pitting measurements). Running both surveys on the same WCSB well provides a complete corrosion assessment: the caliper reveals historical damage already in the casing wall, and the potential profile identifies zones of ongoing corrosion activity that will cause future wall thinning if cathodic protection is not improved.
• Cathodic protection system optimization from profile data: A WCSB casing potential profile showing inadequate protection (potential less negative than minus 850 mV CSE) over a significant depth interval indicates that the impressed current rectifier output or sacrificial anode capacity is insufficient to drive adequate protective current to the exposed casing section. The remediation depends on the cause: for insufficient rectifier output, the operating current density is increased; for inadequate anode bed configuration, additional anode beds are installed; for coating holidays (defects) that allow excessive current demand, the casing is isolated at the wellhead with a flange isolator kit to redirect available CP current to the uncoated sections requiring protection.

Casing Potential Profile Investigation at a WCSB Pembina Cardium Well

A 38-year-old Pembina Cardium producer in west-central Alberta was scheduled for routine casing integrity assessment after a neighboring well in the same battery was found to have a 3-metre external corrosion perforation at 220 m depth in the surface casing. A casing potential profile was run on the subject well using a copper-copper sulfate reference electrode tool on slickline from surface to 400 m. The profile showed potentials of minus 920 to minus 1,050 mV CSE from surface to 80 m (fully protected by the impressed current system installed on the wellhead), transitioning to minus 620 to minus 750 mV CSE from 80 to 280 m (inadequate protection, active corrosion zone), and minus 850 to minus 920 mV CSE from 280 to 400 m (protected by the cemented casing annulus providing electrolytic isolation). A positive potential excursion of plus 50 mV at 195 m indicated a stray current discharge point coinciding with the depth of the failed adjacent well's perforation. The operator upgraded the impressed current rectifier from 3 A to 12 A output and installed a bond resistor on the adjacent pipeline CP system to interrupt the stray current path. A repeat casing potential survey 90 days later confirmed potentials of minus 890 to minus 960 mV CSE across the full 80 to 280 m previously unprotected interval, with no further positive anomalies detected.

Related Terms

Cathodic protection is the electrochemical corrosion prevention system whose effectiveness the casing potential profile directly measures, with impressed current or sacrificial anode systems driving the casing potential to more negative values than minus 850 mV CSE to suppress the anodic dissolution of iron that constitutes external casing corrosion in WCSB soil environments. Corrosion of steel casing in WCSB formations is an electrochemical process driven by potential differences between the casing metal and surrounding electrolyte (formation water or saline soil moisture), with galvanic corrosion, stray current corrosion, and microbiologically influenced corrosion all detectable as anomalous zones on a casing potential profile log. Casing inspection log (multi-finger caliper, electromagnetic inspection, or ultrasonic imaging) complements the casing potential profile by providing direct measurement of wall thinning and pitting morphology that documents the cumulative damage from corrosion activity identified electrochemically by the potential survey. Stray current from pipeline cathodic protection rectifiers and high-voltage DC transmission lines creates anodic discharge zones on WCSB wellbore casing strings that are identified as positive potential excursions on the casing potential profile log and remediated by installing bond resistors, decouplers, or isolation flanges to interrupt the stray current path. Well integrity programs in WCSB mature producing fields include periodic casing potential profile surveys as part of the external corrosion risk assessment program under AER Directive 008, with profile data used to prioritize cathodic protection upgrades and casing inspection surveys at wells showing inadequate cathodic protection coverage.