External Casing Corrosion
The external casing corrosion may be caused by one or a combination of the following:
• Corrosive formation water (having high salinity).
• Bacterially-generated H2S.
• Electrical currents.
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1 1 « Notive |
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stote |
j / E volues ^ / ofter C. P. ! / |
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■^reodmgs^ XindicoteY A /^current ‘Л. /у flowing — vX xJown cosin<£>v |
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Negative slope indicotes current is leaving casing S / bA / J Positive /slope indicates / current is entering / cosing / |
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<N Positive V004 av current annSA |
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— { |
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v/ / 1 1 1 ‘ / |
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Microvolts -400 -200 0 +200 |
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1000 |
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3000 |
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4000 |
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Microvoltmeter |
Fig. 6.1: Casing potential profile test equipment and example of plotting data. (After Jones, 1988. p.66. fig 1.8-2: courtesy of OGC’I Publications. Tulsa. OK.)
• Corrosive completion fluids.
• Movements along faults which cross the borehole (this gives rise to weak, damaged steel zones susceptible to corrosion).
Electrolytic corrosion is the main source of casing corrosion. The current flow may originate from either potential gradients between the formations traversed by the casing and between the well casing and long flowlines (> 1 V). or it may enter from the electrical grounding systems and connecting flowlines.
The origin of stray currents is not easy to determine. The use of a voltmeter across an open flowline-to-wellhead flange, however, will show whether or not the electrical current is entering the well. i. e.. whether or not electrons are leaving the casing.