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.
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.
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.
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.
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.
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
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.
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.