Download Biocide injection as a means of internal corrosion control of oil

Indian Journal of Chemical Technology
Vol. 14, September 2007, pp. 536-538
Biocide injection as a means of internal
corrosion control of oil pipelines
D F Aloko1 & A D Mohammed2*
Department of Chemical Engineering, Federal University of
Technology, P. M. B 65, Minna, Niger State, Nigeria
Email: [email protected]; [email protected]
Received 15 January 2007; revised received 1 May 2007;
accepted 17 July 2007
This research work is aimed at investigating the effect of
biocide injection in the treatment of internal corrosion of oil
pipelines. In this research, carefully formulated laboratory
experiments are performed on a sample of sump tank discharge
from Mobil Producing Nigeria (MPN) offshore facilities using
formaldehyde (methanol) as the biocide. This is to assess its effect
on bacterial proliferation as well as hydrogen sulphide (H2S)
formation, under both aerobic and anaerobic conditions. The
result from the turbidity observed in untreated cultures showed
that bacterial growth had occurred in them and such growth must
have been hindered by the formaldehyde in the treated aerobic and
anaerobic cultures. A blackening of lead acetate paper observed
with emanating rotten egg smell gas is an indication for the
formation of hydrogen sulphide gas. Biocide (formaldehyde) has
proved to be efficient at combating bacterial growth and hydrogen
sulphide production by the bacteria. These results show that
biocide can be used to prevent internal corrosion of oil pipeline
since it inhibits hydrogen sulphide formation from metabolism of
the bacteria.
Keywords: Corrosion, Biocide, Hydrogen sulphide, Aerobic
condition, Anaerobic condition
IPC Code (s): F16L57/00, A01N25/00
Today, severe corrosion of critical oil pipelines is
causing a major concern in the worlds oil industries.
Corrosion audit has been carried out by many
consulting firms in order to investigate the root cause
of the corrosion and recommend a suitable solution
for corrosion control, to ensure asset integrity and
maintain flow assurance1.
Corrosion in its broadest sense may be defined as
the deterioration of a substance or its properties
because of a reaction with environment. Primarily in
the oil field, it refers to the destruction of metal by a
chemical reaction with a given environment, caused
by existence of an electrochemical mechanism2.
Many forms of corrosion of metals are known,
based on the principal causative substance: oxygen,
water, carbon dioxide, hydrogen sulphide and
microbiologically induced corrosion (MIC)2. Hydrogen
sulphide is soluble in water at pressures and
temperatures common in oil field operations and when
dissolved, behaves as a weak acid and usually causes
pitting in a process termed as sour corrosion. Hydrogen
sulphide can be generated by sulphide reducing
bacteria (SRB). These bacteria contribute to corrosion
due to their ability to flourish in the absence of oxygen
and their ability to change sulphide ions into hydrogen
sulphide3.
Microbiologically induced corrosion (MIC) has
been identified as one of the major causes of
corrosion of oil pipelines4.
Technically, biocide is any substance that is
poisonous to organisms and can inhibit their
metabolism or annihilate them. According to Mobil
Producing Nigeria, biocide refers to a preparation of
formaldehyde that betters biocide physical properties
but maintains its chemical property and fungicidal
capacity5.
The objective of this work seeks to investigate the
efficacy of biocide in the treatment of internal
corrosion of oil pipeline by laboratory experiments
using sump sample obtained from MPN offshore
facilities alongside formaldehyde which happens to be
the major constituent of biocide used in almost all oil
companies.
Experimental Procedure
Sample collection
The sample to be tested was collected from the
sump tank discharge system of production platform of
Mobil Producing Nigeria Unlimited. The sample was
collected in clean, transparent plastic container and
was covered properly until ready for test in the
laboratory.
Preparation of lead acetate papers
Filter paper was cut into strips of 50 – 60 mm
length and 8 – 10 mm width. The cent – o- gram
balance was used to weigh 0.25 g of lead acetate
crystals. The crystals were then dissolved in 10 mL of
water. Cut strips of filter paper were dipped into the
lead acetate solution and allowed to soak for about
3 min before they were removed, placed in a flat glass
plate and allowed to dry in the oven before being used
as lead acetate paper.
NOTES
537
Preparation and sterilization of nutrient medium (broth)
Incubation of sub – cultures
2.5 g of nutrient powder was measured and taken
into a glass beaker. 100 mL of distilled water taken in
a glass beaker into which the nutrient powder was
dispensed. The powder was allowed to soak for about
10 min before being swirled to ensure a proper mix.
Afterwards, 10 mL of the solution was measured out
and dispensed into each of the 10 clean test tubes and
their openings were sealed off with aluminium foil.
The test tubes were secured in the metal gauze stand
and placed in the autoclave for wet sterilization for
about 15 min at 125oC and 15 Ibf/in2 (1.09 kgf/cm2).
The media in different test tubes were then
refrigerated.
Out of the two sets of four inoculated test tubes
(the one further treated with formaldehyde and the
one with no formaldehyde), a test tube was taken
from each set and carefully placed in the anaerobic
jar. A candle was lit and placed in the same jar, the lid
was then carefully replaced and the edge lined with
grease. The flame from the candle reduced in glow
until it finally extinguished, that is an indication that
all the oxygen within the anaerobic jar had burned
out. Carefully, the jar with its contents was placed in
the incubator alongside the other two subs - cultures.
The incubator was adjusted to 32oC and the
experimental set–up was left in it for 12 days.
Preparation and incubation of primary culture
Collection of cultures
Two of the test tubes containing nutrient broth
were used in this stage. The aluminum foil was
removed from the brim of one test tube while
holding it very close to the flame of a spirit lamp to
ward off every bacterium or micro – organism that
might enter the medium. The wire loop was held in
the flame until it turned red hot, then it was dipped
in the sump sample to remove a loopful of it. The
loopful of sump sample was dispensed into the open
test tube. Again, aluminum foil was used to seal off
the test tube opening. This procedure was repeated
for a second medium and both inoculated test tubes,
were put in the incubator which was left at about
32°C. The set–up was left for 36 h to allow for
proper bacterial growth, if any. The following day,
the media were ready for inocula to be drawn from
them.
At the expiration of 12 days, the incubated cultures
were carefully removed from the incubator. The smell
which each culture exuded, the appearance of each
lead acetate paper collected, and the turbidity of each
culture were carefully noted. The lead acetate papers
were collected in separate sterilized bottles. The
bottles were then labeled appropriately: Untreated
anaerobic culture, treated anaerobic culture, untreated
aerobic culture, treated aerobic culture, lead acetate
papers – aerobic and lead acetate papers - anaerobic.
Results
The results obtained have been summarized in
Table 1. Following observations can be made from
the results obtained.
Appearance of lead acetate papers
•
Preparation of sub – cultures and biocide injection
Four fresh media in four different test tubes were
inoculated by the same procedure as in case of
incubation of primary culture.
Two out of the four freshly inoculated media were
further treated with formaldehyde. A strip of lead
acetate paper was hung up on the inside of the brim
of each of the four test tubes with the aid of
aluminum foil which also served as lid for each test
tube.
•
•
lead acetate paper withdrawn from the
untreated anaerobic cultivation had blackened
at the tip hanging close to the culture,
lead acetate paper from treated anaerobic
cultivation remained white even at the end of
12-day incubation just like it was at the
beginning,
lead acetate paper withdrawn from untreated
aerobic cultivation blackened at the tip just
like the one withdrawn from the untreated
anaerobic cultivation, and
Table 1⎯Appearance of lead acetate papers, smell and turbidity of cultures
Cultivation
Aerobic
Anaerobic
Formaldehyde treatment
Yes
No
Yes
No
Lead acetate papers
Initial
White
White
White
White
Final
White
Blackened
White
Blackened
Turbidity
Initial
Negligible
Negligible
Negligible
Negligible
Final
Negligible
High
Negligible
High
Smell
Initial
No
No
No
No
Final
No
Yes
No
Yes
INDIAN J CHEM. TECHNOL., SEPTEMBER 2007
538
•
lead acetate paper withdrawn from the
formaldehyde- treated aerobic cultivation was
observed to have remained white at the end of
the incubation period like it was in the
beginning.
Turbidity of cultures
•
•
•
•
•
primary culture in two test tubes was
observed to be very turbid after a day of
incubation,
formaldehyde- treated culture incubated in the
anaerobic jar was observed to have remained
clear after 12-days of incubation,
culture which was not treated with
formaldehyde and incubated in the anaerobic
jar was observed to have turned very turbid
after 12-days of incubation,
formaldehyde-treated aerobic culture was
observed to have remained clear optically as
observed in the treated anaerobic culture, and
intense turbidity was observed in the
untreated aerobic cultivation.
Smell from cultures
A very disagreeable smell, of rotten egg was observed
from both anaerobic and aerobic cultivations but not
from the sample treated with formaldehyde. This was
observed at the end of the 12-day incubation period
taking off the aluminium foil seal.
Discussion
Turbidity of the primary culture after incubation
showed that bacteria within some samples were still
alive and were infact, capable of growth. This was a
necessary step to ensure non-wastage of experimental
material in subsequent stages. The turbidity observed
in untreated cultures showed that bacterial growth had
occurred in them, such growth must have been
hindered by formaldehyde in the treated anaerobic
and aerobic cultures. In short, increased turbidity
indicates bacterial growth.
For this research however, the non-growth of
bacteria in the treated cultures is attributable to the
formaldehyde because the control set–ups were
similarly handled at every stage, other than the
injection of formaldehyde. A blackened lead acetate
paper observed along with emanating rotten egg smell
in the experimental results can, be an indication for
the formation of hydrogen sulphide gas.
Conclusion
Formaldehyde has proved to be efficient at
combating bacterial growth, bacterial metabolism and
by extension, hydrogen sulphide production by
bacteria as well as pipeline internal corrosion as
caused by H2S, other factors remaining the same. The
efficiency is irrespective of whether or not the
condition is anaerobic.
Furthermore, growth in the untreated aerobic
culture implies that, though sulphate - reducing
bacteria do exist under anaerobic environments
such as that obtainable inside pipelines, they may
also be able to thrive in aerobic environments-an
ability that would make them facultative and not
full anaerobic. Another possibility is the existence
of other bacteria than SRB in the sump tank
discharge. Ultimately, it has been shown that
biocide can be used to combat pipeline internal
corrosion since it inhibits H2S formation from
metabolism of the bacteria.
It is recommended that other factors within the
pipelines such as viscosity, flow of oil and pressure
within the pipeline should be investigated to ascertain
their influence on the biocide injection program. A
mathematical formula to define the amount of biocide
to be injected in each pipeline and other methods of
combating internal corrosion are attractive areas for
future research.
Acknowledgement
The authors would like to acknowledge the time,
effort of Mobil Producing Nigeria Unlimited, Ibeno,
Akwa-Ibon State, Nigeria and also dedication of
Mr. Ogo Nwaoche Godfrey during the research work.
References
1 NALCO/Exxon Energy Chemicals, L.P., 2001, Corrosion
Audit for Exxon Mobil Empire fields at Eket, Nigeria.
2 Corrosion types overview web page, http: //
httd.njuct.edu.cn/mat
web/
corrosie/c.htmhttp://www.
bd.com/ds/technical Center/inserts/Nutrient_Gelatin.pdf.
3 Sulphur bacteria web page, http://www.reef.educ/asp pages/
seeb.asp.
4 NACE, 1999, Biologically Induced Corrosion.
5 Mobil producing Nigeria unlimited, 1999, Corrosion
Principles OIMS – PR – 1.0.
6 Mobil producing Nigeria unlimited, 2001, Joint Venture
Operations Overview.
7 Richard M B, “Black powder” in the gas industry sources,
characteristics and treatment, gas machinery Research
council, 1998, 3 – 1, 3-2.