Published corrosion studies have demonstrated the formation of a passive layer and assessed corrosion rate in a NaCl matrix fluid. However, a critical issue for corrosion laboratory tests is to simulate realistic downhole and reservoir brine composition and conditions, which includes a complex matrix of Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Fe2+, HCO3, CO2(aq), and others. Just as crucial, dissolved oxygen concentration should be controlled at reservoir conditions which are strictly anoxic (<< 1 ppb dissolved O2), especially for projects dealing with ferrous iron. Strictly anoxic conditions are crucial for evaluating efficiency and degradation of corrosion inhibitors because dissolved oxygen may interfere and even react with corrosion inhibitors under production temperatures and pressures.

Flow-through corrosion tests were performed for 3 weeks at 200 °C and 1,380 bar. The feed solutions were prepared to represent realistic field brines with complex mixtures of cations and anions, instead of using only NaCl, as is commonly done. All solutions were strictly anoxic (<< 1 ppb dissolved O2). Ferrous iron is from the dissolution of carbon steel 1010 due to corrosion. Mineral deposits on the inner surface of test tubings were collected for XRD and SEM analyses.

A single phase of ankerite (CaMg0.27Fe0.73(CO3)2) was observed instead of siderite (FeCO3), calcite (CaCO3), or magnetite (Fe3O4), which are commonly reported as corrosion products at temperatures above 150 °C. Strictly anoxic conditions allow iron to exist in its ferrous form causing it to incorporate into the ankerite structure, as would be seen in the reservoir and production stream. SEM images and XRD composition of the corrosion products showed that the inner surface of test tubing was uniformly covered by ankerite.

These results demonstrate that more complicated corrosion products can form under simulated downhole environment by using realistic brine compositions under strictly anoxic conditions. The surface properties, solubility, and density of this passive layer could be very different from siderite (FeCO3) and magnetite (Fe3O4), which could cause different protection properties. As a result, corrosion rates may be different than previously reported under extreme conditions and should be studied further.

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