Failure Analysis
Microbiologically Influenced Corrosion (MIC)
Some amount of microscopic life form is present in essentially
every producing well. Of primary concern to sucker rods are
the single celled organisms capable of living in all sorts
 of conditions and multiplying with incredible speed-commonly
referred to as bacteria or "bugs." Suspect fluids
should be monitored continuously for bacteria by sampling,
identifying and counting the bacteria. The extinction dilution
technique is commonly used to culture bacteria for an estimation
of the number of bacteria present in the well. Bactericide
should be used on all suspect fluids to control bacteria populations.
Bacteria are classified according to their oxygen requirements:
aerobic (requires oxygen), anaerobic (no oxygen), and facultative
(either). Some bacteria generate H2S, produce organic acids
or enzymes, oxidize soluble iron in produced waters, or any
combination of the preceding.
MIC has the same
basic pit shape characteristics
of H2S, often with
multiple stress cracks
in the pit base,
tunneling around
the pit edge and/or
unusual anomalies
(i.e. shiny splotches)
on the rod surface.
Bacteria are very
aggressive and all
sucker rod grades
corrode rapidly in
downhole environments
containing bacteria.
Sulfate reducers
(SRB's), those that
produce H2S, probably
cause more problems
to downhole artificial
lift equipment than
do any other bacteria
type. Multiple cracking
in the pit bases
results from the
hydrogen sulfide
by-product of the
bacterial lifestyle,
which corrode and
embrittle the surface
of the steel under
the colony.
Figure 23 shows
several examples
of microbiologically
influenced corrosion
(bacteria) on sucker
rod bodies.
|
|
|
