One of the most conventional methods to mitigate CO2 corrosion of pipelines in the oil and gas industry is to use chemical corrosion inhibitors. Probably the most commonly used inhibition method in the upstream oil and gas industry involves the continuous injection of an appropriate chemical formulation package to maintain a constant concentration of inhibitor in the fluid flowing through the pipeline. Selection of the inhibitor package is typically done through a tedious laboratory testing campaign, which involves the determination of the inhibitor efficiency and persistency in simulated production conditions. Developing laboratory methodologies that can help assessing these properties is of prime importance. This study presents methodologies that could simulate continuous inhibitor treatment (for the determination of efficiency), and also interruption of inhibitor injection (for the assessment of persistency). The study was performed using a model, yet representative, compound benzyldimethylammonium type CO2 corrosion inhibitor (BDA-C14). Its adsorption/desorption behavior was characterized. Furthermore, the influence of several key aspects of the experimental methodologies, such inhibitor contact and pre-corrosion times were examined using BDA-C14, mostly to investigate the effects of the iron carbide layer on inhibitor persistency. A three-electrode system using a rotating cylinder electrode (RCE) for the working electrode was used in this study with the ability for continuous flow-through dilution. All experiments were divided into three main steps: pre-corrosion, inhibitor addition and corrosion rate stabilization, then inhibitor dilution. The corrosion rate was monitored using linear polarization resistance (LPR) in all experiments. Langmuir isotherm model was used to model inhibitor adsorption and desorption behavior of BDA-C14 inhibitor using calculated kinetic coefficients.
CO2 corrosion of the internal walls of mild steel pipelines has always been a significant problem in the oil and gas industries. Different methods have been implemented to mitigate internal pipeline corrosion. The use of corrosion inhibitors provides advantages compared to other mitigation techniques as inhibitor treatment costs are seen to be lower and can be easily adjusted over time. Thus, chemical inhibitors are widely used as a conventional method to mitigate internal pipeline corrosion. Inhibitors are injected inside producing pipelines using two main methods : continuous corrosion inhibitor (CI) injection and batch inhibitor (BI) treatment.1-7 In continuous CI treatment (which is of interest in this study), water-soluble inhibitor solutions are injected into the flow stream at a specific volumetric flow rate to obtain the desired concentration, which in typically below 2000 ppm(v). These inhibitors are not expected to form a tenacious film on the pipeline inner surface. The properties on the inhibitor film present on the metal surface are controlled by the inhibitor bulk concentration and the adsorption/desorption equilibrium. Therefore, if an interruption to the continuous injection occurs, the bulk inhibitor concentration in the flow stream decreases rapidly and the inhibitor is expected to desorb from the surface, resulting in a loss of corrosion protection. Adsorption and desorption behaviors of inhibitor molecules depend on several parameters: the physicochemical characteristics of the CI, characteristics of the metal surface, and flow stream conditions (temperature and flow rate).2-12
