eddy-current measurement, Oil & Gas Glossary

What Is Eddy-Current Measurement in Oil and Gas?

Eddy-current measurement in oil and gas is an electromagnetic inspection technique that passes a high-frequency alternating current through a transmitter coil to induce circulating (eddy) currents in adjacent conductive metal — most commonly steel casing, tubing, or drill pipe — and measures the resulting changes in the induced magnetic field at a receiver coil to detect wall thickness variations, corrosion pitting, cracks, perforations, and mechanical damage in the pipe wall without requiring physical contact or removal of the pipe from the wellbore.

Key Takeaways

Eddy-current tools detect changes in wall thickness by measuring the secondary magnetic field induced in the pipe metal — thinner walls from corrosion or erosion produce weaker eddy currents and a different secondary field pattern than intact wall sections.
The technique is sensitive to the near-surface of the metal facing the tool due to the skin effect — higher frequencies inspect shallower depths; lower frequencies penetrate deeper toward the outer wall.
Eddy-current measurement complements ultrasonic casing inspection by detecting surface-breaking cracks and pitting that may not produce a distinguishable ultrasonic reflection signal.
Multi-frequency eddy-current tools simultaneously acquire data at several frequencies to separate signals from different wall depths and discriminate true pipe defects from casing collar, perforations, and centraliser effects.
Eddy currents are not affected by non-conductive fluids inside the pipe but are affected by highly conductive saline brine that competes with the pipe for induced current paths.

How Eddy-Current Inspection Works in Wellbore Logging

An eddy-current logging tool contains one or more transmitter coils energised with an alternating current at frequencies ranging from a few hundred Hz to several hundred kHz. The alternating magnetic field from the transmitter induces eddy currents flowing in circular loops within the conductive steel pipe wall. These eddy currents generate their own secondary magnetic field that opposes the original transmitter field (Lenz's law). The receiver coil measures the combined primary and secondary field, and the amplitude and phase of the received signal relative to the transmitter signal are recorded. Any variation in the pipe's electrical conductivity, magnetic permeability, or wall thickness alters the eddy current pattern and produces a detectable change in the receiver signal. Corrosion pits, cracks, and wall thinning reduce the effective cross-sectional area of metal available to carry eddy currents, producing characteristic signal anomalies that can be identified and sized.

The skin depth of the eddy-current field in steel determines the depth of inspection below the pipe's inner surface: δ = 503 × sqrt(ρ / (f × μr)), where ρ is resistivity, f is frequency, and μr is relative magnetic permeability. For carbon steel at 1 kHz, the skin depth is approximately 0.5-1 mm. At 100 Hz, it increases to 2-4 mm, allowing inspection deeper into the wall. By operating at multiple frequencies simultaneously, the tool builds a profile of wall condition from the inner surface to the outer wall, enabling detection of both inner wall corrosion (visible to high frequencies) and outer wall damage requiring lower frequencies to penetrate through intact inner wall metal.

Eddy-Current Measurement Across International Jurisdictions

In Canada, eddy-current casing inspection is used in WCSB well integrity surveys for wells approaching abandonment, wells with suspected casing corrosion from sour service H2S exposure, and wells being considered for re-entry or change of use. AER Directive 020 (Well Abandonment) and Directive 079 (Surface Casing Vent Flow and Gas Migration) require documentation of casing integrity before certain well operations; eddy-current logs provide the inspection data used in integrity assessments submitted to the AER. Cold Lake and Lloydminster heavy oil wells with histories of CO2 and H2S co-production have accelerated internal casing corrosion that eddy-current surveys identify and quantify before workover decisions are made.

In the United States, eddy-current pipe inspection tools are used in the Gulf of Mexico for subsea wellhead inspection, as well as in onshore wells where casing integrity data is required for regulatory compliance under EPA Underground Injection Control (UIC) mechanical integrity testing (MIT) programmes. Class II disposal well operators must demonstrate mechanical integrity annually; eddy-current logs satisfy the internal inspection component of the MIT requirement. Permian Basin CO2 enhanced recovery injection wells face particular corrosion challenges from wet CO2; annual eddy-current surveys identify developing corrosion before casing failure. In Norway, Equinor uses eddy-current casing inspection tools in P&A programmes to verify casing condition before cement plug placement in wells where the production history suggests possible corrosion. In the Middle East, Saudi Aramco uses eddy-current pipe inspection in Arab Formation production wells with histories of water injection to assess casing condition and inform workover planning.

Fast Facts

Eddy-current measurement was first developed for non-destructive testing of aircraft aluminium components in the 1940s; its application to oil and gas wellbore casing inspection was commercialised in the 1970s. The fundamental advantage of eddy-current inspection over ultrasonic tools for casing evaluation is that it does not require liquid coupling between the tool and the pipe — it can inspect casing containing gas or air-filled sections where ultrasonic pulse-echo tools fail to achieve acoustic coupling. This makes eddy-current essential for gas-filled wellbore sections above the fluid level and in dry completions during P&A operations where the wellbore has been displaced to gas or nitrogen.

Eddy-Current Versus Ultrasonic Casing Evaluation

Eddy-current and ultrasonic methods are complementary pipe inspection technologies differing in their sensitivity to different defect types and their operating requirements. Ultrasonic pulse-echo methods transmit acoustic pulses through the pipe wall and measure two-way travel time to determine absolute wall thickness with accuracy of approximately 0.1-0.3 mm, providing a quantitative thickness map at every point of the tool's azimuthal coverage. Eddy-current methods measure the electromagnetic response and provide relative wall loss rather than absolute thickness, but can detect surface cracks and pitting that ultrasonic methods may miss. Ultrasonic tools require liquid coupling in the annular space; eddy-current tools work in gas or air. For a comprehensive well integrity assessment, both methods are often run together or in sequence to exploit the strengths of each.

Tip: When planning an eddy-current casing inspection in a well with a history of H2S exposure, run the tool at multiple frequencies and pay particular attention to low-frequency data that penetrates to the outer casing wall. Sulfide stress corrosion cracking (SSC) in H2S environments preferentially initiates at the outer casing surface where stress is highest under burst loading and at the inner surface adjacent to sour fluid contact. If the eddy-current survey shows anomalies at the low-frequency channels not visible in the high-frequency channels, outer wall damage is the likely interpretation. Request a follow-up ultrasonic inspection to quantify the remaining wall thickness before making workover decisions.

Eddy-current measurement is also referenced as:

Electromagnetic (EM) pipe inspection — the broader category including eddy-current, flux leakage, and other electromagnetic methods for pipe wall evaluation; used when the specific electromagnetic mechanism is not critical to the discussion
ECT (Eddy-Current Testing) — the non-destructive testing (NDT) industry abbreviation; used in engineering inspection standards and quality control contexts
Through-casing EM logging — used when the eddy-current tool is conveyed inside the production tubing and inspects the outer casing through the tubing wall; a more challenging application requiring lower frequencies and specialised tool designs to achieve useful penetration depth

Frequently Asked Questions

Can eddy-current tools inspect through two concentric casing strings?

Standard eddy-current tools designed for single-string inspection are generally not effective for inspecting through two concentric casing strings simultaneously because the outer casing signal is overwhelmed by the much stronger signal from the inner casing, and the outer casing is beyond the skin depth of the frequencies needed for inner casing inspection. Specialised low-frequency electromagnetic tools — such as pulsed eddy-current tools developed for through-tubing casing inspection — can achieve limited sensitivity to outer casing condition through an inner string. These through-tubing methods provide qualitative corrosion indicators for the outer string at reduced resolution. For definitive outer casing integrity assessment, the inner string must be removed, or the inspection tool specifically designed and validated for the multi-string configuration.

What is the difference between eddy-current and magnetic flux leakage for pipe inspection?

Eddy-current induces oscillating currents in the pipe metal using a high-frequency alternating field and measures the resulting secondary field — most sensitive to near-surface defects. Magnetic flux leakage (MFL) saturates the pipe wall magnetically using a strong permanent magnet and measures flux leakage at defect locations where reduced metal cross-section forces magnetic flux outside the pipe wall. MFL is less frequency-dependent and can detect both surface and buried defects more reliably for thicker-wall pipe above 10 mm. Eddy-current is preferred for thinner-wall tubing and surface crack detection. In pipeline inspection MFL (run by a "smart pig") is dominant; in downhole casing inspection, eddy-current is more common because the tool must be small enough to pass wellbore restrictions.

Why Eddy-Current Measurement Matters in Oil and Gas

Casing and tubing integrity is a fundamental safety and environmental requirement throughout a well's productive life and during abandonment. Corroded or mechanically damaged casing that fails during production can result in uncontrolled fluid migration between formations, surface blowouts, groundwater contamination, and costly remediation. Eddy-current logging provides a non-destructive, continuous depth profile of pipe condition that identifies deteriorating sections before they fail, enabling proactive workover interventions, targeted cement squeeze operations, or informed abandonment decisions. As regulatory requirements for well integrity documentation intensify globally — particularly for wells approaching abandonment, wells near groundwater sources, and wells subject to CO2 injection or storage service — eddy-current inspection is increasingly essential for demonstrating that the wellbore provides required containment across its intended service life.

Eddy-Current Measurement: Definition, Casing Inspection, and Pipe Integrity Evaluation