Understanding and control of microbially influenced corrosion in simulated oilfield seawater injection systems

Abstract

Microbially influenced corrosion (MIC) is the main cause of the localized corrosion, causing pipeline system leakages during the water injection process in secondary oil recovery. Biocides are often dosed to inhibit and kill the microbes which cause MIC. However, the MIC development in the early stages of water injection process was barely covered, and the demand for more innovative, effective and cost-efficient solutions to deal with MIC is still high. The overall objective of this thesis is to gain a better understanding of MIC process in secondary oil recovery and develop an effective MIC controlling strategy using free nitrous acid (FNA i.e. HNO2), which could be applied in practical oilfield operation either alone or combined with other corrosion inhibitory chemicals.A continuously-fed biofilm reactor simulating the water injection process was operated to allow biofilm to develop on the carbon steel coupons to study the MIC development. The development of MIC process was monitored for 5 months with open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), scanning electron microscopy (SEM), 3D optical profiling and weight-loss measurement. MIC development was found to comprise 3 phases: Initialization, Transition, and Stabilization. The Initialization phase involved the formation of the corrosion product layer and the initial attachment of the sessile microbes on the metal surface. In the Transition phase, the MIC process gradually shifted from charge-transfer-controlled reaction to diffusion-controlled reaction. The Stabilization phase featured mature and compact biofilm on the metal surface, and the general corrosion rate decreased due to the diffusional effect, while the pitting corrosion rate was enhanced at a lower carbon source level, which supported the mechanism of direct electron uptake from the metal surface by SRB.Intermittent dosages of FNA, which was previously found to be a biocide, were applied to a simulated water injection system containing carbon steel coupons with mature biofilm, to study its effect on MIC mitigation. In each treatment, 0.49 mg-N/L FNA was dosed by using 200 mg-N/L nitrite at pH 6 for 24 h. The corrosion properties were monitored by OCP, EIS, LPR, 3D optical profiling, and weight-loss measurement. The biofilm viability was monitored by measuring cellular ATP level. The general corrosion rate (calculated by weight-loss measurement) was decreased by up to 31%, which was supported by LPR tests and reduced ATP levels of the corrosion-inducing biofilm. The 3D optical profiling results showed that FNA decreased the average pitting corrosion rate by 59%, with 2 intermittent treatments with an 82-day interval over 304 days. Intermittent dosing of FNA has strong potential to be an effective and efficient strategy for controlling MIC in oil recovery infrastructure.Imidazoline and its derivatives are widely used corrosion inhibitors for the protection of oilfield pipelines. As a typical imidazoline derivative, N-b-hydroxyethyl oleyl imidazoline (HEI-17) was selected to be applied with FNA to investigate the combined effect on the MIC behaviour of carbon steel. The effect of the combined application was compared with pure HEI-17 treatment and with no treatments. The corrosion properties were monitored with OCP, EIS, LPR, 3D optical profiling, and weight-loss measurement. Following a single dose of FNA, the general corrosion rates in the experimental reactor dropped up to 50% of that in the reactor receiving continuous HEI-17 dosing (0.27 ± 0.04 vs. 0.54 ± 0.08 mm/y), but gradually recovered to 93.4% of that in the Control + HEI-17 reactor in 2.5 months. After the FNA treatment, the pitting corrosion was decreased by 64.6% compared with Control + HEI-17 for a month from measuring the cumulative distribution of the pitting depth. HEI-17 treatment alone showed moderate pitting corrosion inhibition effect (approx. 27%), and the FNA treatment inhibited the formation of deep pits effectively. The combined application of HEI-17 and FNA has shown synergetic effects and high efficiency in mitigating MIC in the simulated water injection system. This treatment strategy has strong potential to be applied in the practical oilfield operations.Overall, this thesis focuses on MIC, the most common and detrimental problem in oilfield operations, and supplies future MIC understanding and combating with comprehensive outcomes from thorough laboratory experiments. The FNA-based strategy has the potential to be an economical and green substitute for the current traditional MIC treating procedures in real oil recovery infrastructures.