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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.