Investigation of the impact of nitrate injection to control sourcing problem in oil reservoir : benefit and side effects on steel materials

Abstract

The successful control of reservoir souring by nitrate injection has been well documented in the literature. Recent interest has centred on how nitrate application can increase the corrosion risk in pipelines and metal equipment. This study was conducted to observe the impact of nitrate reducing bacteria (NRB) and sulphate reducing bacteria (SRB) on the extent of corrosion on UNS S31603 and ASTM-A572-50 carbon steel, two commonly used materials in the oil and gas industry. The bacteria used in this experimental study were indigenous bacteria, isolated from the crude oil and production water samples of an oil field off the northwest of Western Australia. The NRB were isolated using nitrate broth, targeting nitrate reducers while SRB were isolated using Starkey media, targeting lactate utilizing bacteria (Desulfotomaculum and Desulfovibrio). In this study, a mixture of corrosive production water which contains a high level of chloride (21000 mg/L) supplemented with 10% (v/v) crude oil was used as the testing solution. The crude oil was taken into account because in the oil field it serves as the carbon source for the bacterial growth and it may also influence the corrosion behaviour of any steel material. However, this study has limited its scope to observe the impact of SRB and NRB using electrochemical techniques in a closed batch culture system without any water/nutrient renewal.The basic experimental design for the two materials investigated, UNS S31603 and the ASTM-A572-50 carbon steel were similar. The experiments were conducted at 50°C in electrochemical cells containing 10% (v/v) crude oil in 700mL production water and purged with filter-sterilized nitrogen to keep the oxygen level as low as possible. Each of the two materials were evaluated under four different conditions: (1) control cell (no bacteria), (2) NRB inoculated cell, (3) SRB inoculated cell, and (4) mixed bacteria (NRB+SRB) inoculated cell. A small amount of 5 mM NaNO3, was added as the growth nutrient for the bacteria. The open circuit potential (Ecorr) of the corrosion coupons, and the redox potential (Eh) of the solution were monitored throughout the experiments. Additionally, the microbial populations were counted by Most Probable Number (MPN) method and direct counting method using a Helber Counting Chamber Z30000. A production water analytical analysis (nitrate, nitrite, sulphate and sulphide) was conducted before and after the experiment.However, the exposure time in the corrosive media and the electrochemical analysis were different for each of the two materials tested. The immersion time for the UNS S31603 was 28 days and at the end of the immersion period, Linear Polarization Resistance (LPR) and Cyclic Polarization Scan (CPS) were carried out to determine the uniform and localized corrosion behaviour, respectively. The immersion time for the ASTM-A572-50 carbon steel was 21 days; LPR and electrochemical impedance spectroscopy (EIS) were conducted at an interval of every five days to monitor the biofilm formation, corrosion product formation and corrosion rate changes.The results for both UNS S31603 and ASTM-A572-50 carbon steel demonstrated that: (1) The addition of nitrate, promoted the growth of NRB and suppressed the growth of SRB, hence H2S production can be eliminated; (2) The NRB was able to oxidize the sulphide and also maintain a redox potential of above -100 mV, a level which inhibits the growth of SRB; (3) An increase in Ecorr was observed in the presence and absence of bacteria.The results from the UNS S31603 investigation show that: (1) NRB forms patchy biofilms on the UNS S31603 surfaces that weaken steel passivity and decrease its Critical Pitting Potential (CPP) to a greater extent than SRB after 28 days, (2) On the contrary, the corrosion rate of the UNS S31603 in the SRB inoculated cell is higher than the corrosion rate of the coupons in the NRB inoculated cell. This can be due to the formation of metastable pits as observed in the non-polarized corrosion coupon of the SRB inoculated cell after 28 days. (3) In order to study the effect of immersion time on the NRB and mixed bacteria (NRB+SRB), additional tests were conducted in the same manner for a short period of seven days of exposure. The results show that the CPP for the corrosion coupons in both the NRB inoculated cell and the mixed bacteria inoculated cell is higher than the corrosion coupons in the control cell. Therefore, it is postulated that, in this particular corrosive biochemical environment (high chloride level), the NRB and the mixed bacteria (NRB+SRB), may give beneficial protection to UNS S31603 for a short immersion time, and may give detrimental effects in longer exposure times.The results from ASTM-A572-50 carbon steel investigation show that: (1) the EIS technique can be used to monitor the formation of biofilm and/or corrosion products layers on the steel surface, (2) LPR, EIS and weight loss results show that in the tested corrosive environment, the presence of bacteria give lower rate of corrosion of ASTM-A572-50 carbon steel coupons compared to the corrosion coupons in the control cell. This indicates the corrosion inhibition activity of bacteria in this particular environment. (3) However, in the presence of bacteria, localized corrosion is inevitable as pits may form underneath bacterial biofilm.In general, it is important to examine the corrosion impact of nitrate application on case by case basis because it is a complex mechanism and cannot be simply predicted based on electrochemical techniques alone. Further work is required to understand the biological materials that were produced by NRB, especially in the presence of other bacteria, which in a real situation in an oil reservoir may not only be SRB. Such studies can aid in understanding the fundamentals of the biochemistry of nitrate injection, thus resulting in important information for the process and design of field applications.