Water Survey Assesses Souring and Corrosion at Production/Injection Plant
One sulfate-reducing bacteria (SRB) mitigation measure is the injection of biocide into produced and/or injected water. However, determining the suitable injection spot and the required biocide concentration can be a challenge. At a water treatment plant for an onshore oil field in northern Germany, formaldehyde injection was started at the end of 2015. The goal of this study was to understand the chemical parameters and microbial distribution in the water system and whether formaldehyde injection was achieving the desired effect. The data obtained by the authors not only can help operators to optimize biocide injection, but also to better understand the complex microbial growth occuring in the production facilities.
Oil and water are produced from three fields and processed in a joint processing plant. The separated water is subsequently injected into the reservoirs to support pressure maintenance. The circulated high-saline water (~20% salt) contains not only approximately 60–90 mg/L sulfate, but also a microbial burden originating from the produced reservoirs. To mitigate the risk of hydrogen sulfide (H2S) formation and minimum inhibitory concentration (MIC), a continuous injection of 70-ppm formaldehyde (active concentration, with weekly batch treatments of 1,000 ppm) was initiated. Microbial analyses were not performed routinely, but ad hoc samples gave varying results regarding microbial activity.
Materials and methods of the experiments (sampling and analytics methods) are provided in the complete paper.
Chemical Analysis of Water Samples. Sulfate measurements show that water from Field B and Field C do not contain measurable amounts of sulfate. All sulfate in the system seems to originate from Field A. The cause of an additional increase in sulfate concentration (from 61.3 to 80.2 mg/L) before Separator A/B is not known yet. Salt content in Field C (18.4 %) is slightly lower compared with that of Field A and Field B (20.2%), with a slightly higher pH. Formaldehyde was measurable after the formaldehyde injection point but in lower concentrations (14 and 8 ppm) compared with the theoretically injected 70 ppm. Approximately 10-ppm formaldehyde is still measurable at the injection pump at the end of the facility and only very low amounts (approximately 3 ppm) reach the injectors in the fields. The water chemistry of the injector pump indicates an even mixture of the water from both separators. This is confirmed by facility personnel. The water chemistry does not change significantly during the transportation from the pump to the injectors.
Microbiologically Relevant Parameters. According to the generated data, organic acids (only acetate and lactate are shown), which are known carbon sources for SRB and other bacteria, are distributed in the system. A significant decrease from approximately 50 to 10.7 mg/L between the separators and the injector pump indicates microbial consumption. Interestingly, organic acid content increases in the lines going to Field A and Field B, without any known injection of chemicals containing organic acids into these lines.
Low amounts of H2S could be measured in the gas phase of the sampling bottles containing produced water from Field B. After formaldehyde injection, no H2S is measurable, which might be caused by the fast chemical reaction between H2S and formaldehyde. H2S could again be measured in the water of the booster pump and all three injection waters. It must be considered that H2S was measured after 1.5 days in the lab under a pressure of 0.7 bar and room temperature. Therefore, the obtained H2S values are not directly transferable to the actual amounts in the water system, where the pressure and temperature have a direct influence. Still, the measured concentrations give a good qualitative indication about H2S potential.
Bacterial and archaeal total cell numbers showed the same trend. Total bacterial cell numbers coming from Field B were under the detection limit of less than 2E+02 cells/mL. After the pigging stations onsite, the authors monitored the increase in the cell numbers. After the formaldehyde injection points, before the separators, a significant decrease in cell numbers going below 6.5E+01 cells/mL in the line from Field C was observed. However, before Separator A/B, cell numbers do increase to 3E+05 cells/mL despite formaldehyde injection. The same trend is visible in the SRB cell numbers determined by most-probable numbers. SRB cell numbers are generally low, no biocidal effect of the formaldehyde between pigging station A/B and separator A/B was observed. A further increase in both bacteria and SRB cell numbers in water taken from the pump and from the injectors in the fields indicates microbial growth in the lines and possibly in the injection-water tanks, which are located between the separators and the pump. The highest SRB cell number (4.2E+04 cells/mL) was detected in a sample from the injector well in Field A. By contrast, SRB cell numbers in the injector well in Field C were lower, indicating that absolute cell numbers cannot always be correlated with microbial activity.
The high-saline waters from the three fields can be distinguished by salt and sulfate content and pH. Field A seems to be the primary source of sulfate in the system, which becomes available for all SRB when reaching the main facilities. Indeed, H2S formed after the separators in the surface facilities indicates microbial activity. The authors measured H2S at the main pump with a concurrent decrease in organic acids; however, there was no significant decrease in the sulfate concentration, and major amounts of sulfate were injected into all fields. It is unclear whether additional sulfate sources in the system existed to explain these observations.
Especially important for this study was the distribution of formaldehyde after the formaldehyde-injection point. The theoretically injected concentration of 70 ppm was not measured, which indicates a fast reaction of the biocide in the first meters of the system. Only minor amounts of formaldehyde reached the injector wells. Formaldehyde will react not only with microbial membranes but also with oxygen scavengers and H2S. Therefore, a system containing both sulfide and microbial contamination will place a high demand on the formaldehyde. In the line for Field C, formaldehyde shows a very good reduction of cell numbers. In contrast, formaldehyde shows no biocidal activity in the line for water from Field A and Field B, although starting cell numbers are similar to the ones in the line for Field C. The main observed difference is an input of H2S from Field B, which reacts with formaldehyde before it can target cells. Another possibility is the presence of a stable biofilm in the lines running to Separator A/B, which can cause an ongoing proliferation of cells that inoculate the water system. Biofilms can be very resistant to biocides because of slime formation, which limits diffusion of chemicals inside the biofilm matrix. The operator confirmed that Separator C was cleaned recently in contrast with Separator A/B. Therefore, this separator might be a microbial hotspot reinforced by a major cell input from Field A and Field B. Because of the lack of sampling points between the separators and the pump, it cannot be determined with certainty if only the separator carries a high microbial contamination. It is evident that, between the pump and the injecting wells in the fields, the cells further increase. In the injection water lines not treated with pigging, formaldehyde concentration is low, which leads to the conclusion that, over the past decade, active biofilms have formed in these pipes without causing any plugging issues.
All these data, of course, only depict a snapshot of the system. The authors cannot rule out the possibility that, during the sampling, parts of biofilms or debris were washed out of the lines and influenced the microbial data, despite the precautionary measure of discarding the first liter of the sampled fluid before the sampling. Also, it must be considered that water samples mostly contain planktonic cells, which are only part of the overall microbial burden in the system. Many bacteria, especially those related to MIC, are surface-associated and only can be sampled using metal coupons. This, however, requires costly in-line installations, which generally are not feasible, especially at multiple spots in the system. To verify results, a second sampling campaign is planned, with additional sampling points between the separator and the pump (Fig. 1).
The authors’ assumption is that Lines A/B are undertreated with formaldehyde and that sulfate-reducing activity is the cause for H2S formation in the operational facilities.
Sampling and analysis allowed mapping and tracking of the water flow in the surface facilities and provided a conclusive picture of the chemical and microbial parameters.
Only low amounts of H2S originate from the producing fields. H2S is formed mainly inside the surface facilities.
H2S formation, organic acid consumption, and concurrent bacterial and SRB cell number increase can be observed between Separator A/B and the main pump.
Formaldehyde injection seems not to have an optimal effect before Separator A/B.
On the basis of these results, a cleaning campaign will target Separator A/B. A second survey with additional sampling points, which will show the effectiveness of the cleaning measure, is planned.
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01 April 2020
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