This study describes a laboratory test system which was specifically developed to assess the ability of biocides to lower microbial corrosion rates. It was found that the common oilfield biocides THPS and glutaraldehyde, dosed at concentrations of 300 ppm for 4 hours weekly over 5 weeks, could reduce MIC rates from 109.7 mpy to as low as 4.3 mpy
Biocides are used to control problematic microorganisms in the oil and gas industry. High doses of biocides cause environmental and operational problems. Therefore, using biocide enhancers to make biocides more effective is highly desirable. 2,2-dibromo-3-nitrilopropionamide (DBNPA) is a popular biocide because it is broad-spectrum, effective, kills microorganisms immediately upon addition, and it degrades rapidly. D-amino acids are natural chemicals that have been used in lab tests to enhance biocides to treat biofilms. In this work, D-tyrosine was used to enhance DBNPA against Desulfovibrio vulgaris biofilm on C1018 carbon steel. After 7 days of incubation, the weight loss of coupons without treatment chemicals in culture medium was found to be 3.1 ± 0.1 mg/cm2. With a treatment of 150 ppm (w/w) DBNPA, the weight loss was reduced to 1.9 ± 0.1 mg/cm2 accompanied by a 1-log reduction in the sessile cell count. The combination of 150 ppm DBNPA + 1 ppm D-tyrosine achieved an extra 3-log reduction in sessile cell counts and an additional 30% reduction in weight loss compared with 150 ppm treatment of only DBNPA. The combination also led to a smaller maximum pit depth. Linear polarization resistance (LPR), potentiodynamic polarization and electrochemical impedance spectrometry (EIS) tests corroborated the enhancement effects.
By far, the microbiological species most associated with corrosion has been Sulphate-Reducing Bacteria (SRB). Majority of Microbiologically Influenced Corrosion (MIC) research has focused on the activities of this type of bacteria. One of the primary reasons for this has been the presence of iron sulfides in corrosion products associated with MIC. SRB reduce sulfates to sulfides, which then react with iron and steel. However, an accepted fact is that MIC is also caused by the action of the biofilm produced by bacteria, in a similar way to under-deposit corrosion.
The primary method used to prevent MIC in the oil and gas industry is by use of biocides. The criteria used for selection of biocides is often their proficiency to kill SRB. The danger with this is that one can neglect the ability of other bacteria frequently found in oil and gas environment, such as general aerobes and general anaerobes to cause corrosion by biofilm production. This became evident when severe general & pitting corrosion was observed in two oil and gas separators in one of the facilities in Kuwait Oil Company (KOC), where SRB levels were zero but significant numbers of sessile and planktonic general aerobes and general anaerobes were found to be present in the process.
Using microbiological and chemical analysis, the mechanism of this type of MIC, specially the relationship between the quantity of various biofilm-forming bacteria and nature and magnitude of corrosion has been studied and the findings are presented in this paper.
MIC-causing microorganisms were investigated in a 16” diameter and 9.6 km long injection water pipeline. Nitrate was added to the water and pigging debris from the pipeline showed that both sulfate-reducing bacteria (SRB), nitrate-utilizing bacteria, and methanogens were present in numbers of 105 – 106 cells/g.
The lengthy laterals of horizontal wells often pose microbiological challenges, as they provide more area to become microbially contaminated and require larger volumes of fluid and biocide for treatment. A Permian Basin oilfield has been experiencing MIC-related failures in its horizontal wells, which is of concern due to the associated high workover cost.
Laboratory biocide challenge testing identified several common oilfield chemistries and combinations thereof as being effective against this field’s population of microbes. However, aggressive applications of these products in the field neither delivered an effective microbial kill nor prevented the treated wells from experiencing further MIC and failures.
An acrolein field trial was conducted on a set of problematic, microbially contaminated horizontal wells over a time period of approximately one year. During this timeframe, these wells experienced microbial control for the first time, defined as meeting and maintaining microbial KPIs. Additional benefits were realized as a result of acrolein, including a dramatic improvement in water quality evident as a decrease in iron sulfide and suspended solids, a clean-out of the wells inferred by an initial increase of solids post-acrolein, a decrease in the corrosion rate as indicated by a significant reduction in iron and manganese counts, a decrease in the well failure rate, an increase in production, and an overall cost savings associated with the application of acrolein.
Zinc and its alloys are used as sacrificial anodes because zinc is an active metal. Carbon steel can be coated with zinc to protect against corrosion. These metals are known as galvanized steel. In this work, microbiologically influenced corrosion (MIC) of pure zinc and galvanized steel caused by a sulfate reducing bacterium was investigated. After 7 days of incubation in 125 mL anaerobic vials with 100 mL culture medium and 1 mL inoculum, the sessile cell count on the galvanized steel was slightly higher than that on pure zinc. The abiotic weight loss for pure zinc was 1.4 ± 0.1 mg/cm2 vs. 4.6 ± 0.1 mg/cm2 for galvanized steel after 7 days of anaerobic incubation at 37oC. The weight losses for galvanized steel and pure zinc were 31.5 ± 2.5 mg/cm2 and 35.4 ± 4.5 mg/cm2, respectively, which were 10X larger than the previously reported carbon steel weight loss in the same SRB broth. Electrochemical corrosion tests confirmed the severe corrosion of these two metals. The corrosion current densities of galvanized and pure zinc were 25.5 µA/cm2 and 100 µA/cm2, respectvely at the end of the 7-day incubation with SRB, confirming that pure zinc was more prone to SRB MIC than galvanized steel. In both cases, the corrosion product was mainly ZnS. Three MIC mechanisms were possible for the severe corrosion. Extracellular electron transfer MIC is thermodynamically favorable for Zn. Furthermore, the detection of H2 evolution in the vials suggest that proton attack and H2S attack occurred against Zn in the SRB broth with neutral pH after passive film damage by the SRB biofilm.