Artificial Pipeline Model Enables Comparison of Biocides in a Dynamic System
The pipelines and vast infrastructure required for oil and gas production and transport are constructed largely of carbon steel, which is highly susceptible to damage and failure as a result of direct or indirect microbially influenced corrosion (MIC). One approach to reduce or remediate MIC is by applying microbicides to affected assets such as storage tanks, pipelines, heat exchangers, and pumps. The objective of this paper is to generate efficacy data for biocidal formulations against corrosion-associated biofilms grown under anoxic and flowing conditions. The data are difficult to obtain because of the distinct nature of biofilms and growth conditions. It can be more representative when mimicking petroleum- and water-transporting pipelines.
To facilitate testing and comparison of products that successfully attenuate MIC under field-relevant conditions, a method was developed that allows consistent corrosion measurements to be made for corroding biofilms grown on steel surfaces under flowing and anoxic conditions. This method uses microorganisms from a low-tide, anoxic North Sea sample (black sand). When enriched on carbon steel and grown in recirculating flow circuits, the sediments can generate corrosion rates of up to 65 mils per year (mpy) after 4 weeks of growth. This paper describes the method and its application to the benchmarking of field-relevant biocidal chemistries in MIC control experiments. Attention is also given to the reproducibility of the method. Distinct differences can be observed in biocidal performance against biofilms developed on metal surfaces.
Petroleum reservoirs are serviced by complex networks of stainless or carbon steel pipelines, tanks, and other industrial assets that are susceptible to various corrosion processes. The total annual costs of corrosion to the upstream petroleum industry in the US are estimated in the billions of dollars.
One particular type of corrosion is biocorrosion, which is caused or influenced by the presence of prokaryotic cells. It is estimated that 13% of all pipeline failures can be linked to this detrimental process. MIC is defined by the presence and influence that prokaryotic cells have on the corrosion process, which makes it “biologically influenced.” Often the presence of microbial cells and their activities can heavily accelerate and exacerbate the chemical corrosion process. Biogenic processes that significantly impact corrosion rates include the following.
• Formation of H2S via sulfate reduction—sulfide reacts readily with metal
• Biofilm formation and creation of corrosion cells because of disturbance of the electron cloud in the metal
• Accumulation of organic acids in a biofilm, lowering the pH and leading to an acidic attack on metal
• Direct use of the metal as an electron source
The first three mechanisms indirectly influence the corrosion; e.g., H2S reacts favorably with metallic iron, producing iron sulfide scale. The fourth mechanism is direct biocorrosion, because the microorganisms directly attack the metal and integrate electrons derived from the metal into their primary metabolism. This latter mechanism is particularly hostile for metal surfaces, displaying corrosion rates that far exceed those of chemical corrosion processes.
Oil companies typically use large mechanical brushes, or “pigs,” to remove biomass and organic and inorganic deposits from pipelines to reduce the risk of biocorrosion impairment caused by the presence and activities of prokaryotic cells. The pig scrapes the inner walls of the pipe and is often followed by a slug of organic microbicides. However, many pipelines cannot be cleaned with pigs and can only be controlled with chemical treatments. In these cases, an organic microbicide must be carefully selected based on the environmental parameters of the system. Consideration must be given to microbicide dose quantity and application recurrence. The developed scheme will be critical to successfully remediate potential damage caused by biocorrosion.
To carefully investigate which microbicide will perform the best, laboratory studies need to be performed in systems that closely resemble a real pipeline. These studies require an anoxic environment, flowing conditions, presence of a metal surface, and a relevant corroding microbial culture. Since these parameters represent a challenge when it comes to reproducibility of experiments, all four elements are discussed in paper SPE 190901.
The complete paper presents a detailed discussion of the experimental methods used to develop the artificial pipeline model system. Discussion topics include enrichment of a corroding culture and its growth conditions, determination of culture composition via retrieval of genetic material (DNA) analysis techniques, construction of the artificial pipeline model, and collection and examination of the C1018 steel coupons used in the system.
After sampling sand from a beach in the North Sea and sending cores to a laboratory for enrichments, coupons were scraped with a cleaned spatula after 2 weeks of biofilm development. DNA from the scrapings was extracted following a modified extraction protocol using a chelating chemistry to ensure that the iron ions would not interfere with the polymerase chain reaction. Similar extractions were performed for the liquids.
The enriched corroding culture was applied to the artificial pipeline system, which included a coupon-holding device that allowed the positioning of C1018 steel grade metal coupons, a large bottle serving as a media storage and overflow, and connecting hoses. The system is engineered to allow operation of multiple coupon holders in parallel during one experiment. The entire device was placed in an anoxic atmosphere. Flow conditions in each coupon holder mimicked the pipeline system model and were controlled by a peristaltic pump.
Organic microbicides were introduced by first removing the liquids after biofilm development and subsequently refilling with a liquid matrix containing the respective microbicides of interest at a concentration of 125 ppm. Flow with this matrix occurred for 4 hours, followed by a complete replacement of the liquids and a new inoculation with the enriched culture.
The C1018 steel coupons were removed from the drained pipeline systems and examined, analyzed, and weighed according to guidelines described in NACE Standard RP0775-2005.
Results and Discussion
The results and discussion section of the paper cover development and validation of the pipeline model system, outcome description of the denaturing gradient gel electrophoresis (DGGE) and next-generation sequencing (NGS) analysis, validation of reproducibility of the artificial pipeline model system, and evaluation of microbicides for preventive treatment. Numerous photos, tables, and charts illustrate the discussion.
Baseline biocorrosion must be first demonstrated before microbicide evaluation. The enriched culture of the anoxic sediment displayed corrosion visible to the naked eye under both the static and subsequent flowing conditions and had the potential to biocorrode at a rate which far exceeded 10 mpy.
To determine the relevance of the enriched culture for the petroleum industry, the microbial population distribution was determined using DNA sequencing techniques. NGS of partial 16s RNA genes was applied to investigate the microbial composition structure from a coupon in more detail. The findings reconfirmed what was found using DGGE analysis.
The entire artificial pipeline system was placed under anoxic conditions. The coupon-holding devices allowed for longitudinal removal of coupons in time and allowed for the determination of the corrosion rate at different time points including the viability of the cells causing the biocorrosion.
To determine if sufficiently high biocorrosion rates could be achieved for the enriched corrosion culture in the artificial pipeline model, a run in one of the circuits was conducted and the corrosion was evaluated over 2 months with biweekly sampling. After the first evaluation, it was determined that there were more than 107 cells per steel coupon and a corrosion potential of over 40 mpy. This corrosion level remained throughout the run.
Prior to microbicide testing, tests were performed to evaluate whether the corrosion rate in the separate circuit runs were consistent and reproducible. Results indicate that the corrosion rates are comparable, allowing reliable comparison of biocides in their ability to mitigate or prevent biocorrosion.
To ensure maximal comparability, treated and untreated circuits were run simultaneously in one experiment. Treated model pipelines received 125 ppm microbicide for 4 hours, which was considered preventive. After the application of microbicide, the flow circuits were filled with new media, spiked with enriched culture, and again placed under dynamic conditions for the next week before a new treatment of microbicide was applied. The corrosion rate was determined in each loop. Mass loss at week 4 corresponded to a corrosion rate at 64 mpy untreated and 5 mpy treated.
Evaluation of Microbicides for Remedial Treatment
To determine whether treatment with a chemical can remediate or avert significant biocorrosion that has already taken place, circuits inoculated with the corroding culture were placed under dynamic flow conditions without any treatment for 2 weeks. Four different biocides were evaluated, so five different circuits were run in parallel, with one control. The outline of the experiment remained largely the same as the pre-biofilm development. The untreated pipeline models reached 60 mpy. The treated pipeline models displayed a corrosion rate slightly above 20 mpy.
In the same experiment, four different microbicides were evaluated for their curative efficacy. All circuits received a dose of a 125-ppm active ingredient each for 4 hours each week, and corrosion rates were determined based on C1018 steel coupon analysis. One of the biocides brought down the corrosion to common galvanic corrosion levels. Conversely, another increased the biocorrosion potential compared to the untreated circuit. The data shown in the complete paper point out the heavy differentiation in microbicide performance and call for detailed evaluation of biocorrosion reduction potential for microbicides under more realistic field conditions.
The performance of microbicides applied in a curative manner differs significantly and therefore, requires expert opinions when evaluating them for petroleum-related systems needing treatment. When a biofilm and biocorrosion process are already established, reducing the biocorrosion rate is more difficult than for prevention. Consequently, applying microbicide preventively in the field could be a more cost-effective strategy of MIC management.
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12 February 2020
12 February 2020
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12 February 2020