Preliminary study on the benefits and side effects of nitrate injection on pitting corrosion of UNS S31603

Abstract

Biogenic production of H 2S in oil reservoir (souring) by sulphate reducing bacteria (SRB) is a serious concern for the oil industry. Nitrate injection has been widely used as an approach to control this problem. This method relies on the ability of nitrate reducing bacteria (NRB) to suppress the growth of SRB. This experiment was conducted to observe the effects of bacterial metabolites and biofilm on localized corrosion of UNS S31603. Anaerobic tests were performed by immersing UNS S31603 samples for 7 days at 50°C in electrochemical cells containing 10% crude oil in brine water, supplemented with 5mM sodium nitrate (NaNO3). The brine was a corrosive field sample containing high level of sulphate and chloride. Three different conditions were evaluated: control (no bacteria), NRB inoculation, and Mixed (NRB and SRB) bacteria inoculation. The Open Circuit Potential (OCP) of the steel was monitored throughout the test and Cyclic Polarization Scan (CPS) was carried out at the end of the test. OCP results demonstrated that ennoblement occurred both in the presence and absence of bacteria. CPS results showed the Critical Pitting Potential (CPP) of the steel decreased for all the samples and were most severe when the biofilm was damaged.