Materials Performance

JUN 2019

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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14 JUNE 2019 W W W.MATERIALSPERFORMANCE.COM MATERIAL MATTERS Continued f rom page 13 but does not measure dormant or dead bacteria and archaea, is not affected by other organisms (e.g., fungi and algae), is quick and accurate, is not affected by variations in water chemistry, and can easily be used in the f ield at the well site. According to Jenkins, Fajt, and Mur- phy, the new technique determines total living planktonic bacteria in a sample of water, and utilizes a bacteria metabo- lism-based assay approach using f luores- cence spectroscopy to obtain a f luores- cence value that is proportional to the total number of living bacteria present in a sample. Fluid samples to be tested for bacteria can be processed within 30 min of collection. Samples can be collected and pro- cessed aerobically or anaerobically, but are required to be cooled to <38 °C for analysis. Ty pically, 50 mL of water is iso- lated from the water source in a syringe. The sample is then passed through a 0.2-µm f ilter that captures the bacteria. After the bacteria are captured, they are exposed to a solution that contains an enzyme substrate dye complex. If the bacteria are living, a phospho- rescent dye will be released. The dye is then extracted by rinsing the f ilter into a cuvette where dye concentration is read with a f luorometer. Each batch of tests includes a set of control bacteria samples to ensure the method is being performed correctly. Ty pically, it takes 30 min to analyze a water sample; however, 10 to 20 samples can be analyzed concurrently. The test results can be visualized using a heat map. This method is unique because it con- verts the f luorescence value into mean- ingful data such as colony forming units per mL and the number of equivalent serial dilution bottle turns. The conver- sion is based on the results of thousands of experiments comparing the f luores- cence value with accurate laboratory bac- teria measurements (e.g., plate counts and nucleic acid-based testing) per- formed on f ield waters from a wide vari- ety of sources. This new f luorescence spectroscopy- based technique has been implemented in many oil f ields throughout the world, and several case histories illustrate how the technique was used to successfully opti- mize biocide treatments. Jenkins, Fajt, and Murphy comment that nucleic acid- based testing provides greater accuracy over the new technique and presents information on the specif ic species of bacteria or archaea present; however, a higher level of technical expertise and signif icant ongoing capital investment infrastructure are required to perform that ty pe of testing. In addition, nucleic acid-based testing currently cannot be performed in the f ield, which leads to delays in analyzing samples. In a paper presented at CORROSION 2019, 2 Jenkins, Fajt, and Murphy describe three case studies where the new f luores- cence spectroscopy-based technique was used to determine bacteria levels in oil- f ields. In one study, the operator of an off- shore oil platform in DR Congo in Central Africa was concerned that MIC might be occurring in the oil production system and seawater injection system. Bacteria surveys of these assets were performed to determine the location and concentration of any bacteria in the f ield, develop bacte- ria management programs to reduce the risk of bacteria corrosion, and optimize the biocides used in the f ield. In the oil production system, no bio- cides were being applied at the time of the surveys. Water samples were taken from the topsides header, high-pressure (HP) separator inlet, HP separator outlet, and produced water pipeline. For the seawater injection system, water samples from the seawater uptake—upstream and down- stream of the f ilters and prior to the injec- tion wells—were analyzed. The seawater was treated intermittently with hy po- chlorite (ClO − ) before f iltering, and the water exiting the deaeration tower was dosed with week ly applications of alter- nating biocides [tetrakis(hydroxymethyl) phosphonium sulfate (THPS) and glutar- aldehyde]. Bacteria measurements were performed before and after application of the biocides. Low levels of bacteria (as def ined by the operator) were found at all monitor- ing locations in the oil production system, although bacteria levels did increase as the f luids proceeded downstream through the production system. In the seawater injection system, however, bac- teria concentration started at near zero and increased to high levels as the water progressed through the water f iltration and injection equipment. The highest lev- els of bacteria were found in the seawater immediately before injection into the for- mation. The authors note that an increase in bacteria levels as water passes through an injection system is evidence that the system is colonized with a biof ilm and requires treatment. Bacteria testing before and after bio- cide application in the seawater injection system showed that the biocides had no effect on the planktonic bacteria popula- tions, which indicated the biocide pro- gram was ineffective. Based on the results of the surveys, it was apparent that a modif ied biocide program was required for the seawater injection system. To con- trol the planktonic bacteria in this sys- tem, it was recommended to increase the ClO − dose rate, perform regular chlorine residual measurements, and only use glutaraldehyde at the outlet of the deaera- tion tower instead of alternating treat- ments with THPS. It was also recom- mended to increase the dose rate of the glutaraldehyde treatment and increase the frequency to twice a week. Consider- ing it was likely that sessile bacteria were present in the f ilters, the operator was advised to clean this equipment since bio- cides tend to have low eff icacy against biof ilms. Additionally, since the bacteria levels increased as the f luids passed through the oil production system, it was recom- mended to initiate a biocide treatment in this system as a preventive measure. References 1 D.G. Bennet, "Oilfield Microbiology : Detec- tion Techniques Used in Monitoring Problem- atic Microorganisms Such as Sulfate Reducing Bacteria SRB," Offshore Technology Confer- ence Asia 2016, paper no. OTC-26788-MS (Kuala Lumpur, Malaysia: OTC Asia, 2016). 2 J. Fajt, A. Jenkins, A. Murphy, "Development and Field Application of a new Bacteria Mon- itoring Technique," CORROSION 2019, paper no. 13158 (Houston, TX: NACE International, 2019). —Kathy Riggs Larsen

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