Cathodic protection works by delivering current to the metal surface. We verify it's working by measuring potential at the metal surface. But a well casing is a steel cylinder buried hundreds; sometimes thousands; of feet underground, encased in cement, surrounded by multiple soil strata, and sealed at the wellhead. You cannot walk a close-interval survey down it. You cannot lower a reference electrode to the midpoint. You cannot verify that the current you're applying at the surface is actually reaching and protecting the deepest section of the casing.

And yet that casing needs to be protected. If it fails, the consequences aren't a pipeline repair, they're a compromised well, a potential environmental incident, and a very expensive problem.

The E-Log I test exists to solve exactly this problem. It doesn't measure protection directly. Instead, it measures the casing's electrochemical response to applied current, and that response; when plotted as a curve; tells you with remarkable clarity whether you've crossed the threshold into effective cathodic protection. The entire structure. Top to bottom.

The concept takes some upfront effort to understand, but once it clicks, you'll see why it's been the standard for well casing CP verification for decades.

Before getting into the test itself, let's be clear about what you're up against.

When you protect a buried pipeline with cathodic protection, you can verify it with a close-interval potential survey (CIPS). Walking the line, taking measurements every few feet, building a picture of protection along the entire length. Not always perfect, but manageable.

A well casing doesn't give you that option. It runs vertically, through changing soil layers, surrounded by cement grout that may or may not be continuous or conductive. The bottom of the casing might be hundreds of feet deeper than the surface equipment. You can measure the potential at the wellhead. You have no practical way to measure what's happening at 400 feet of depth.

Generic current density tables give you a starting estimate. But those tables don't know your specific soil strata, your cementing quality, your local groundwater conditions, or the geometry of your particular well. E-Log I testing gives you a site-specific answer based on the actual electrochemical behavior of your actual casing in your actual environment.

The "E" in E-Log I stands for the structure-to-soil potential (E), measured in millivolts relative to a reference electrode (CSE). The "Log I" is the base-10 logarithm of the applied current in amperes.

The test works like this: you apply current to the well casing in incremental steps, measure the polarized potential at each step (using instant-off measurements to eliminate IR drop), plot those data points with potential on the vertical axis and log current on the horizontal axis, and read the shape of the curve that results.

The curve has a predictable shape.

And that shape is the whole point.

At low applied currents, near the casing's native (resting) potential, the plot curves. This curvature reflects a mixed electrochemical state: the applied cathodic current is being partially offset by anodic reactions still occurring on the casing surface. Somewhere on that structure, metal is still dissolving. The protective current is present, but it isn't enough to suppress all the anodic areas. The casing is not fully protected.

As you increase the applied current, something changes. The anodic reactions (corrosion) progressively get driven out. At some point, the last anodic sites are eliminated, and only the cathodic reaction remains on the casing surface. When this happens, the curve straightens into a well-defined linear segment. This is the Tafel segment, and its appearance on the plot is the signal you're waiting for.

The Tafel segment follows a predictable mathematical relationship (the Tafel equation: η = a + b log i), and that linearity tells you something specific: the cathodic reaction now dominates everywhere on the casing surface. There are no more anodic areas. Corrosion has been effectively suppressed across the entire structure, including the sections you cannot reach with a reference electrode.

The transition point between the curved section and the Tafel segment; often called the "knee" or "break-away point"; is the answer to the question you came to ask. The current at that transition is the minimum current required to achieve cathodic protection of the well casing.

"This is the practical power of the test: you can't verify protection at 400 feet of depth by any direct measurement. But the polarization curve can tell you the current threshold at which the entire casing has crossed into protection. The electrochemistry doesn't lie about what's happening underground."

Understanding the concept is step one. Running the test correctly is step two. Here's how a properly conducted E-Log I test comes together.

Electrical isolation is non-negotiable. The well casing must be electrically disconnected from all other metallic structures: flowlines, wellheads, casing strings from adjacent wells, surface piping. Any unintended connection provides a current path that skews your results. Confirm isolation.

Allow the casing to depolarize. If the well has been under CP, switch it off and wait at least 48 hours before testing. You need to measure from a true native potential baseline. Residual polarization from a prior CP system will make your curve unreadable.

Check for stray current interference. Nearby CP systems, electrified railways, or HVDC power lines can impose currents that shift potentials independently of what you're applying. Survey the area and note any interference sources before you start. If interference is severe, schedule the test during periods of minimal activity.

You'll need a temporary anode bed installed 300 to 500 feet from the well, on the opposite side from where you'll place your reference electrodes. Steel rods, galvanic anodes, or existing CP beds - whatever works. Connect the positive terminal of a portable rectifier (DC current source) to the anode bed and the negative terminal to the wellhead. This makes the casing the cathode. Current flows from anode bed through the soil to the casing.

Install a current interrupter in the circuit, set to approximately 3-4 seconds ON and 1 second OFF. This cycling lets you capture instant-off potential measurements. Important for eliminating IR drop from your readings.

Place your reference electrodes (CSE) at three positions from the well: 10 feet, 100 feet, and 500 feet, on the side opposite the anode bed. The 500-foot electrode is your primary measurement point. At that distance, you should be reading "remote earth" (a potential measurement largely free of the IR field created by your applied current). The 10-foot and 100-foot readings help you understand what's happening closer in and catch any anomalies.

Record native (static) potentials at all three reference electrode positions before energizing anything. This baseline is your starting point — typically somewhere between -0.300 V and -0.750 V vs. CSE for an unprotected steel casing.

Then apply current in incremental steps. Start low — 0.1 to 0.5 A — and step up in consistent increments (0.5 A is common). At each step:

Continue stepping up current until you've clearly passed through the knee and the plot shows a defined linear Tafel segment. For typical well casings, this often occurs somewhere in the 3–12 A range, but site conditions vary considerably. Your rectifier should be sized with 20% headroom above your anticipated maximum.

Plot your instant-off potential (vertical axis) against the logarithm of applied current (horizontal axis) after the test. The shape tells the story:

Take that current value and calculate current density: divide by the external surface area of the casing (A = π × d × L, where d is outer diameter and L is casing depth, both in centimeters). Typical results for well casings range from 1 to 20 µA/cm², but your specific number may be anywhere in that range depending on soil resistivity, cementing quality, and environmental conditions. That site-specific figure is exactly why you ran the test instead of using a table.

Document everything: native potentials, all current and potential data points, curve plot, identified knee current, calculated current density, and environmental conditions. This record is your compliance baseline and your reference for future testing.

The curve doesn't show a clear knee. This usually means one of two things: the test didn't go to high enough current (the Tafel segment hadn't fully developed), or there's interference corrupting the data. Extend the test to a higher current range, or investigate stray current sources before repeating.

Potentials fluctuate at each step instead of stabilizing. Interference from nearby CP systems or AC induction is the most common cause. You can also see this on wells with very low native polarization resistance. They stabilize slowly. Extend your dwell time per step to 10 minutes and check interference sources.

The 10-foot and 500-foot reference readings don't agree. A large discrepancy suggests IR drop is affecting your close-in readings. Rely on the 500-foot (remote) reference for your primary data. If even the remote reference is showing IR effects, move it farther away.

Current won't hold steady. Check all connections. Particularly at the wellhead and the anode bed. A loose connection under current can create voltage noise that makes every reading suspect.

Established in 2011, Roberts Corrosion Services, LLC delivers comprehensive, turn-key cathodic protection and corrosion control solutions nationwide. Our end-to-end expertise encompasses design and inspection, installation and repair, surveys and remedial work. We provide drilling services for deep anode installations and a full laboratory for analysis of samples and corrosion coupons, as well as custom CP Rectifier manufacturing.

While our initial focus was on the Appalachian Basin area, we complete field work all over the US. We are a licensed contractor in many states and can complete a wide range of services.

Our biggest strength is in our flexibility for our clients. Solutions and Results.

Let us know how we can help.

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