28 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION
sion. This is not always the case, and several authors have
discussed how MIC can be identifi ed. MIC can be con-
fi rmed only when all other possible explanations for the
observed corrosion have been eliminated.
41
MIC occurs within biofi lms that form on metal sur-
faces. These fi lms start out microscopically thin but can
become much thicker. It might be more accurate to
describe this corrosion as being the result of biofouling,
which includes macroscopic growths, for example,
mussels and barnacles. One defi nition of MIC is “ an
electrochemical type of corrosion in which certain micro -
organisms have a role, either enhancing or inhibiting. ”
40
Like most other forms of corrosion, MIC is an elec-
trochemical process. Microorgnanisms can affect the
extent and severity of corrosion and, like all organism -
related phenomena, water is necessary for microbial life
and therefore is necessary for MIC.
Bacteria attach to metallic surfaces and start to form
thin biofi lms consisting of cells, living or dead. These
fi lms also incorporate water and debris from the envi-
ronment. Growth of these fi lms can change the chemical
concentrations of the water at the biofi lm/metal sub-
strate interface. Films as thin as 12 μ m can prevent dif-
fusion of oxygen and produce localized areas that are
anaerobic enough to promote the growth of sulfate -
reducing bacteria (SRB). One result of biofi lm forma-
tion is the creation of concentration gradients that
produce electrochemical cells that can be explained by
the Nernst equation discussed in Chapter 2 , Chemistry
of Corrosion.
Biofi lms can form in minutes to hours, and MIC can
be detected within 10 – 20 days in stagnant waters, for
example, improperly drained and dried equipment that
has been hydrotested with microbe - containing water.
There are many ways of classifying bacteria, but two
distinctions are important to understand for control of
oilfi eld corrosion:
The carbonate minerals shown in Figures 3.24 and 3.25
are soluble in mineral acids, and the same mineral acids
used for descaling downhole formations can be used to
removed scale from the inside of piping. If the acid is
left in the equipment too long, or if the corrosion inhibi-
tors added to the cleaning acid are inadequate, then
rapid corrosion can occur, resulting in perforated tubing
within a matter of hours.
Microbially Infl uenced Corrosion (MIC)
MIC is a phenomenon that has been recognized for
many years and is the subject of a number of books,
39 – 48
as well as numerous reports on the subject. It is a
growing problem in the oil and gas industry and, unfor-
tunately, many of the problems are introduced by
improper water handling of surface waters. One of the
problems associated with MIC is that most oilfi eld engi-
neers and technicians have very little understanding of
biology and, therefore, are likely to believe “ experts ” or
“ rules of thumb ” whether or not they have validity.
A number of terms have been used for MIC including
“ microbiologically induced corrosion, ” “ biocorrosion, ”
and others. NACE standardized on the term MIC in the
early 1990s, and this term emphasizes that microbes can
increase or decrease corrosion. Both phenomena have
been reported, although increased corrosion is obvi-
ously of more technical and economic interest.
The following recent developments apply to MIC:
39
•
MIC can occur in environments where corrosion is
not expected, for example, in downhole pumping
equipment removed from any sources of oxygen or
other apparent corrodents.
•
MIC corrosion rates can be very rapid.
•
Liquid culture techniques, the long - standing stan-
dard method of identifying the biological sources
of MIC, do not provide accurate assessments of the
numbers and types of organisms involved in fi eld
situations.
•
Mitigation and control strategies have shifted from
the widespread use of biocides to manipulation of
the environment, for example, introduction of
smooth surfaces where biofi lm attachment is diffi -
cult and MIC is less likely.
MIC is not the only oilfi eld problem associated with
biofi lms and bacteria. Table 3.7 shows some of the other
oilfi eld problems caused or accelerated by bacteria.
49
There are a number of possible mechanisms involved
in MIC, and some have been better described than
others.
50
Many advocates of MIC claim that whenever
high microbe populations are found in the presence of
corrosion, this is evidence that MIC caused the corro-
TABLE 3.7 Examples of Operational Problems That May
Be Caused by Bacteria
49
Increased frequency of corrosion failures
Increasing H
2
S concentrations
Reservoir souring
Rapid production decline
Metal sulfi de scales
Failure of downhole equipment due to metal sulfi de deposits
Ineffi cient oil/water separation
Ineffi cient heat exchange
Black water
Black powder in gas transmission lines
Filter plugging
Loss of injectivity
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