September 22, 2010
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Home / Issue Archive / 2010 / September #9 / Optimization of a Microbial Control Program to Minimize the Risk of Microbiologically Influenced Corrosion

№ 9 (September 2010)

Optimization of a Microbial Control Program to Minimize the Risk of Microbiologically Influenced Corrosion

   Prevention of microbiologically influenced corrosion (MIC) is a significant challenge in the petroleum industry. Microbial risks usually increase as a field becomes more mature, making the cost of the program difficult to justify with decreasing revenue.

By Victor Keasler and Brian Bennett, Nalco Co.

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   However, from an asset integrity standpoint, minimizing the risk of corrosion from microbes is a critical factor in maximizing the life of an oilfield system. Testing to identify the optimal microbial control program is not an easy task. Mimicking the field conditions as closely as possible and testing biocide efficacy against biofilms as well as planktonic microorganisms is critical.

   Development of this type of testing is highlighted in this case study, which involved a complex pipeline network which transports full well-stream crude, emulsion and water from offshore platforms to an onshore treating facility. This field has been developed over the past 35 years, with no major corrosion issues during the first 25 years of operation. However, an increasing water cut and sand production during the past 10 years as well as contamination of the main fluid lines with bacteria from open drain tanks has led to increased corrosion in the system. In fact, intelligent pig inspection results from 2008 led to the shut-in of one of the main production fluid lines and subsequent replacement of a 20-kilometer section of pipeline.

   Although it was relatively clear that bacteria were the main cause of the corrosion, a root cause failure analysis was conducted and confirmed MIC as the primary corrosion driver. In order to optimize the biocide program and better understand the potential for MIC in this system, a comprehensive analysis was undertaken that involved construction and optimization of a dynamic flow loop that allowed for simulation of the pipeline conditions to provide an appropriate treatment recommendation. Together, this testing provided a guideline for a new treatment regimen in the field that is currently being employed to better control bacterial growth and to minimize MIC.

Isolating Microbial DNA to Identify Microorganisms Present

   Microorganisms throughout the system were characterized using molecular-based technology including species identification via prokaryotic speciation1. Briefly, fluid and solid samples from the sump tanks, separators, and main fluid lines were collected and total microbial DNA isolated. The microbial DNA was PCR amplified using synthetic primers that reside in a conserved region of the 16S rDNA gene2-3 and separated via denaturing gradient gel electrophoresis (DGGE). Separated DNA was sequenced and bioinformatics analysis performed to identify the organisms present in this system.

   The results of the microbial characterization revealed a diverse population and the need for an effective biocide that can minimize microbial growth and the risk of MIC. To address this need, six different biocides were selected for evaluation of their efficacy against bacteria present in the production system fluids. The products selected for analysis included non-oxidizing biocides, surface-active biocides, and combination products (non-oxidizing biocide plus surface-active component). Biocides were dosed at a low concentration of each product in a planktonic kill study to differentiate performance. This screening method allows for determination of the products that perform best against planktonic organisms and narrows down the biocides to be tested in the dynamic flow loops. Results of the planktonic screen identified one biocide (referred to as Biocide 1) in particular that was expected to be effective in this system, particularly against the sulfate-reducing organisms that were identified in the molecular characterization. This biocide contains both a non-oxidizing component and a surface-active component to provide enhanced biofilm penetration and microbial kill.

   Based on the results of the planktonic kill study, a further evaluation of Biocide 1 versus the incumbent biocide in a dynamic flowing system was undertaken. This was done by utilizing a recently developed in-house dynamic flow loop system that allowed for simulation of many of the field conditions in a controlled, laboratory environment. The system contains approximately 1.5 liters of production fluid that are continually circulated over biostuds located in the 6 o’clock position of a modified Robbin’s device (Fig. 1A).

Intelligent Pigging Pin-Points Pitting Where Bacteria Can Hide

   Recent intelligent pigging (IP) data from this system revealed that significant pitting had developed in the pipe walls, most likely due to MIC. Because of this, it is no longer sufficient for an effective biocide to simply penetrate biofilms and kill bacteria, but it must also be able to kill the bacteria deposited within these pits. In order to evaluate this as well, artificial pits were machined in the biostuds used in the dynamic flow loop. The artificial pits were designed to match the ratio to the most severe pits identified during the IP runs (Fig. 1B).

   Two dynamic flow loops were assembled and run for approximately four weeks in order to allow sufficient time for biofilm development. Following biofilm formation, each of the two systems was dosed with a biocide (incumbent or Biocide 1). Before and after biocide dosage fluids (analysis of planktonic bacteria) and biostuds (analysis of sessile bacteria) were collected and enumeration performed according to NACE Standard TM01944. More specifically, fluid was removed from each flow loop before addition of biocide and immediately after the 4-hour biocide treatment. In addition, biostuds were removed before addition of biocide, immediately after the 4-hour biocide treatment, and 24 hours after removal of the biocide from the system. The dose rate for each biocide was 400 ppm of product, with each product having a nearly identical concentration of active biocide.

   Both the incumbent biocide and Biocide 1 showed good performance in the dynamic kill study versus planktonic organisms (Fig. 2A). Microbial kill was measured against three metabolic types of organisms including SRB, acid producing bacteria (APB), and general heterotrophic bacteria (GHB). The planktonic kill was similar with both biocides, although Biocide 1 did show increased performed against SRBs. Enumeration of sessile organisms was also performed before and after treatment with the incumbent biocide and with biocide 1 (Fig. 2B). Biostuds were removed and bacteria enumerated before addition of biocide, immediately after the 4-hour biocide treatment, and 24 hours after removal of biocide from the system. Biocide 1 outperformed the incumbent for all organisms tested at both the 4 and 24 hour time point, although the enhanced performed was most dramatic at 24 hours.

   A second sessile kill study was performed to determine whether addition of a biostat would provide extended kill in our dynamic flow loops. The longer microbial control can be maintained, the more cost effective the program becomes. To determine the potential impact of a biostat on biofilm regrowth, two dynamic flow loops were started as before to allow for evaluation of biocide efficacy against sessile organisms. However, this time biocide treatment was performed with either Biocide 1 (as before) or Biocide 1 in combination with 40 ppm of a biostat. Biostuds were removed and enumeration performed before biocide treatment and at 4, 24, 48, 72, and 192 hours after treatment. Fig. 3 shows the change in microbial activity in the biofilm at each time point. As is clearly illustrated by the data, the microbial activity is dramatically reduced with the addition of a biostat for a significantly longer period of time. This suggests that the addition of a biostat can retard biofilm growth and aid in extended microbial control between biocide treatments.

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