ABSTRACT

In the oil and gas industry, water flooding, often with a polymer additive, is used in enhanced oil recovery (EOR) to increase reservoir pressure because reservoirs are aging. This practice causes operational concerns because water flooding brings nutrients and microbes downhole, which may allow microbes to flourish. Polymers such as carboxymethyl cellulose sodium are used in EOR to increase the viscosity of the injection water. However, there is a possibility that EOR polymers may be utilized as a carbon source by microbes downhole causing reservoir souring and microbiologically influenced corrosion (MIC). In this work, carboxymethyl cellulose sodium (3,000 ppm by mass) was found to be utilized by an oilfield biofilm consortium containing various microbes including sulfate reducing bacteria (SRB) and biodegradation microbes during a 30-day anaerobic incubation test at 37°C. The polymer utilization increased the planktonic cell count and SRB sessile cell count in anaerobic vials with 100 ml artificial seawater. After the 30-day incubation, the polymer utilization led to 16% viscosity loss. The utilization also slightly increased weight loss and pitting corrosion on C1018 carbon steel.

In the oil and gas industry, enhanced oil recovery (EOR) is utilized very often nowadays because reservoirs are aging.1 Water flooding with added EOR chemicals is used to increase reservoir pressure.2 Seawater is often injected. This practice brings oxidants (e.g., sulfate), nutrients and microbes downhole.3 A reservoir is strictly anaerobic.4 Oxygen is usually removed during water flooding to avoid oxygen corrosion of downhole tubing. In this environment, anaerobic microbes such as sulfate reducing bacteria (SRB) flourish.5 SRB can produce biogenic H2S causing reservoir souring.6 Fermentative microbes can also grow.7 In the field environments, microbes live in synergistic biofilm consortia.8 Biofilms can cause biocorrosion in many industries, especially in the oil and gas industry.9-12 This kind of corrosion is also known as microbiologically influenced corrosion (MIC).13 MIC is gaining more concerns since the Trans-Alaska Pipeline leak in 2006.14 It is important to investigate whether EOR polymers can be used by microbes as organic carbons thus worsening reservoir souring and MIC.

Adding a polymer in EOR is an efficient way to increase fluid viscosity.15 Xanthan gum, partially hydrolyzed polyacrylamide (HPAM) and cellulose-based polymer are commonly used in EOR. Xanthan gum was popular in the old days. However, it was found that it is susceptible to microbial degradation which leads to the loss of viscosity.16 It is a polysaccharide17 that is widely used as a food additive.18 HPAM is inexpensive and commonly used in recent years. Reports showed that HPAM can also be utilized by microbes.19 It was found that HPAM can be utilized by a sulfate reducing bacterium as a carbon source resulted in viscosity loss.20Bacillus cereus was found to utilize the amide group and the carbon backbone of HPAM as their nitrogen and carbon sources, respectively.21 Cellulose-based polymers have been adopted as polymer flooding materials in EOR in recent years.22 Some reports showed that the fermentation products of cellulose such as organic acids can be used as organic carbon and contribute to SRB growth.23,24 There is no literature reporting direct utilization of cellulose- based polymers by SRB. However, microbes live in biofilm consortia in the field. It is expected that other microbes can digest it and then provide organic nutrients to SRB. So far, there is a lack of reports showing the utilization of cellulose-based polymers that affects microbial growth and MIC.

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