Socrates assists you in determining the viability of carbon steels versus CRAs through a rigorous evaluation of general corrosion susceptibility of steels with or without inhibition in the operating environment. The system determines a steel corrosion severity index, a measure of the overall corrosive severity of the environment.

In material selection it should be realized that conventional steels can be significantly more economical than CRAs in low severity environments or in greater severity environments when used with proper chemical inhibition. Therefore, before considering CRAs, it is important to first evaluate whether steels can be used in oilfield environments without being susceptible to sulfide stress cracking.

Socrates uses a large number of parameters to determine the steel corrosion severity index. The system uses the de Waard - Milliams' CO₂-based corrosion prediction model to obtain an initial assessment of corrosion severity in carbon steels. The system generates a number that is directly proportional to corrosion rate of steel in mpy using the de Waard-Milliams model. This number is modified to account for the effects of other factors such as temperature, H₂S, chlorides, velocity and inhibition. Relevant parameters in steel evaluation include:

Parameters

Hydrogen Sulfide (H₂S)

Corrosion generally increases with H₂S partial pressure. H₂S is an acid gas and the term acid refers to its ability to depress pH when it is dissolved in an aqueous solution. This increased aggressivity results from the decrease in the pH of the aqueous phase as the partial pressure of H₂S increases. An added effect of H₂S in CO₂/brine systems is a reduction in corrosion rate of steel when compared to corrosion rates under conditions without H₂S. This reduction in corrosion rate is primarily a low temperature effect and predominates system corrosivity at temperatures less than 175 F (80 C) due to the formation of a meta-stable iron sulfide film. At higher temperatures the combination of H₂S and chlorides will usually produce higher corrosion rates than just CO₂/brine systems, since stable iron carbonate films usually do not occur as readily in systems with H₂S as they do in systems without H₂S.

Carbon Dioxide

Corrosion severity generally increases with CO₂ partial pressure. CO₂ is an acid gas and the term acid refers to its ability to depress pH when it is dissolved in an aqueous solution. This increased aggressivity results from the decrease in the pH of the aqueous phase as the partial pressure of CO₂ increases.

Chlorides

Under normal circumstances, the chloride content of the aqueous phase does not directly affect the hydrogen charging conditions in steel. However, it can have an effect on the effectiveness of chemical corrosion inhibitors. Therefore, in many cases, more careful selection of inhibitors and inhibition procedures must be performed where high levels of chlorides (>30,000 ppm) are present.

In naturally deaerated production environments, corrosion rate increases with increasing chloride ion content over the range 10,000 ppm to 100,000 ppm. The magnitude of this effect increases with increasing temperature over 150 F (60 C). This combined effect results from the fact that chloride ions in solution can be incorporated into and penetrate surface corrosion films which can lead to destabilization of the corrosion film and increased corrosion. This phenomenon of penetration of surface corrosion films increases in occurrence with both chloride ion concentration and temperature. Corrosion rates of steel in oil and gas production generally increase with increasing chloride content. The chloride species in the aqueous phase can work to penetrate and destabilize protective surface films. Typically, brines with low chloride content (i.e. <10,000 ppm) less aggressive than those having higher chloride contents provided that they are compared at the same pH. In some cases, the presence of salts can reduce the solubility of acid gases or buffer the water therefore affecting the solution pH.

Bicarbonates

The bicarbonate ion is a buffering agent used in aqueous solutions to increase the pH of the solution. Its presence is typically measured in milli-equivalents/liter (meq/l). One meq/l represents 0.061 grams of HCO3 in one liter of solution. The reduction in pH in turn decreases the corrosivity of the environment. Hence, presence of HCO3 is beneficial from the standpoint of corrosion. Typical quantities of HCO3 in production environments range from 1 meq/l to 100 meq/l.

Maximum Operating Temperature

Temperature affects the corrosion rate of steels in several ways which must be taken into account for estimation of corrosion severity:
Increasing aggressivity of chloride corrosion reactions with thermal activation.
Decreasing solubility of dissolved gases with increasing temperature which increases pH.
Formation of a protective carbonate scale in aqueous CO₂ environments at elevated temperature.
Reduction in CO₂ corrosion rate with addition of H₂S.

Type of Flow

The flow conditions (i.e. static, stratified, turbulent, etc.) are dependent on the nature of the produced gases and fluids and if the flow is primarily horizontal (surface production) or vertical (subsurface production). Horizontal flow is usually more prone to static and stratified conditions which limits the amount of mixing of oil and water phases at low flow rates. Vertical flow typically exhibits these types of conditions only during period of shut-in of the well. See Superficial Gas Velocity for more information.