Investigation of oil production well corrosion issues and prevention: Role of seawater ingress on the corrosion behavior of 1Cr carbon steel used as production tubing material

This thesis presents the research work aimed at understanding the effects of seawater injection on the corrosion behavior of carbon steel in CO2 atmosphere. The project is motivated by the increasing number of failures and reduced lifetime of oil & gas wells when higher fraction of water is produced because of seawater injection to improve oil recovery.

The purpose is to understand how the affected parameters influence the corrosion resistance of tubing materials. The obtained knowledge is of importance to operators for developing an operational framework to be used in corrosion prediction and mitigation of oil & gas wells. Among the various changes caused by transient seawater injection, the thesis focuses onchanges to lower temperature, presence of calcium ions in the brine and calcium carbonate precipitation.

All these effects were evaluated against the CO2 corrosion behavior of carbon steel. Specifically 1Cr carbon steel with a martensitic microstructure was investigated as representative of production tubing material. The assessment of the impact of these parameters was performed under accelerated conditions and under conditions relevant for oil & gas operations in the Danish sector of the North Sea.

Electrochemical techniques were used throughout this research to follow the corrosion process. Linear Polarization Resistance (LPR) was employed to follow the corrosion rate with exposure time. The evolution of the processes occurring at the metal – electrolyte interface were studied by Electrochemical Impedance Spectroscopy (EIS), whilst the electrochemical behavior of the material was investigated with Potentiodynamic Sweeps (PS).

High- resolution Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray analysis (EDS) supported the investigation of the morphologies and cross sections of the corrosion products and scale. These films were characterized by X-Ray Diffraction (XRD). Chemical analysis of the corroding solution was conducted using ICP and UV Spectrophotometer and were modelled using water chemistry and scale prediction software.

X-Ray Computed Tomography (X-Ray CT) was used for characterizing the corrosion products obtained at different temperatures. Almost all the experiments were performed at ambient pressure, while the effect of flow was evaluated in a Flow Loop system at the Institutt for Energiteknikk (IFE), in Norway.

Overall, the results indicate that the injection of seawater can compromise the resistance of the production tubing material. The lower temperature prevented the precipitation of protective FeCO3 corrosion product. It resulted in a porous and non-uniform morphology, which was not able to reduce the corrosion of the steel.

The precipitated film at high temperature was instead compact and uniform, which resulted in a reduction of the corrosion rate. The low  temperature was also reported to facilitate localized corrosion attack. Calcium carbonate precipitation was able to dissolve the protective FeCO3 layer because of acidification of the solution.

The presence of calcium in the solution resulted in the precipitation of a substitutional solution FexCayCO3 (x + y = 1). This precipitation was delayed compared to pure FeCO3; however, its protectiveness was not jeopardized by the presence of calcium. Precipitation of FexCayCO3 was further delayed by the flow.

However, the investigated flow velocity did not affect the composition of the substitutional solution, which was comparable to the one obtained in absence of flow. The combined results and understanding obtained during this research are discussed in lights of the existing literature. Their applicability to the field is introduced and explained.