Casing Potential Surveys - Casing Inspection

illustrates a well casing - figure 1

 Casing potential surveys detect electrochemical corrosion as it occurs; hence, these tools indicate where damage from corrosion is imminent. The schematic of Figure 1 illustrates a well casing in which certain parts of the casing act like anodes relative to other sections of pipe, which act like cathodes.

Those sections that appear as anodes are undergoing electrochemical corrosion and metal loss. Casing potential surveys locate these intervals and assist in developing cathodic protection operations to protect these wells. A log showing the casing voltage or potential profile. A positive slope indicates a cathode, and a negative slope indicates an anode (interval of metal loss) Run 1 indicates that the intervals from 1495 to 1650 ft (456 to 503 m) and from 1700 to 1850 ft (518 to 564 m) are anodes, and hence are corroding. In Run 2, cathodic protection is being used by putting a current to the wellhead of five amps. Two areas are still found to remain anodes by the casing potential survey. At eight amps, Run 3 shows that the whole casing string is now a cathode, and therefore such electrochemical corrosion is minimized or eliminated.

Acoustic Casing Inspection
Acoustic casing-inspection tools are basically modifications of the pulse-echo cementbond tools. The geometrical parameters indicated in that section are presented, plus an analysis of the frequency response of the signal appearing in the gate W2. The frequency analysis is conducted to determine the wall thickness of the pipe. The kind of information available from these tools, (e.g.Schlumberger CET) includes measurements of ovality, minimum and maximum radius, internal diameter, and minimum and maximum wall thickness.

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Newer-generation tools, such as Halliburton’s Circumferential Acoustic Scanning Tool (CAST-VTM ) and Schlumberger’s Ultrasonic Imager (USITM ), provide full 360 degree coverage of the wellbore profile in a range of presentation formats. Cased hole applications include both ultrasonic cement evaluation and pipe inspection.