Well integrity assessment using a non-destructive method to ascertain the structural condition of casing in situ. Corrosion, damage, and wear can cause the casing to fail under stress, such as drops in pumping levels to aggressive rehabilitation schemes using chemicals and mechanical means to increase production and efficiency. Understanding casing integrity enables the water well manager to make informed decisions throughout the life of the well.
Pacific Surveys has conducted casing inspection surveys since 2010 after introducing the first electromagnetic (EM) tool designed specifically for water wells. That inspection tool emitted a low-frequency EM field to measure remote eddy currents. Remote eddy currents exhibit a change in the phase when encountering damage or thinning of the casing wall. This method employed a single transmitter and receiver spaced 4ft apart.
Since then, pulse eddy current (PEC) has allowed greater resolution and the inspection of multiple concentric pipes by having the receiver coil embedded in the transmitter coil. According to Faraday’s law of Induction, the transmitter coil emits an electromagnetic pulse from which eddy currents in the casing are generated. The receiver coil will have a time-varying induced electromagnetic force (EMF). With changes in wall thickness, flaws, or damage to the casing, the induced EMF will change over time from what would normally be expected. The processing of this signal allows the measured results to be presented in a traditional geophysical log format, where Wall Thickness, EM Caliper, and Metal Loss are plotted against depth.
Pacific Surveys’ PEC tool has two sensors aligned with the axis of the tool, parallel to the casing. The “A” sensor has the deepest investigation with 42 channels of time domain data. The “C” sensor is shorter in coil length, influenced primarily by the innermost pipe. This sensor has 36 channels of time domain data. Six lateral sensors are located at a right angle to the casing, with 8 channels each, that inspect 60° of the inner pipes for a full 360° of the casing. With this configuration, 126 channels of time domain data aid in the understanding of the electromagnetic properties of the surrounding casing.
Analyzing the data can determine whether the corrosion/damage is inside or outside the casing wall. The analysis of the data can also pinpoint where precisely where the most damage occurs. With this understanding, the water well manager can make informed decisions about maintaining the current conditions of their wells along with asset management over the life of the well.