Appendix II:
Steel Evaluation in Socrates
An Integrated Approach to Corrosion Assessment
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,
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.
Method of Inhibition
For horizontal flow systems the following types of inhibition method
are commonly used:
Continuous Inhibition - Inhibitor is continuously
injected into the flow stream. This may be conducted in both downhole
and surface production systems. It is preferred where the flow
velocity is greater than 10 ft/sec (3 meter/sec) or where the amount
of water is high.
Batch Inhibition - Inhibitor is added in the flow
system periodically in batch treatments usually between two pigs. A
strongly persistent filming inhibitor is usually used which can
reduced corrosion rates effectively during the period between batch
treatments. This technique is usually effective where the chloride
concentration is high but the velocity is low. It is commonly used to
supplement other inhibition techniques.
Pigging - Pigging is the use of flowline
pigs to assist in (a) application of batch inhibitors and (b) removal
of accumulated water, solids and other deposit in the flow system. In
many applications, pigging is required to get proper distribution of
inhibition chemicals through the flow system. In cases where flow
velocity is low, pigging is used to remove water and deposits from the
bottom of the pipe which can promote corrosion at this location.
For vertical flow systems the following types of inhibition
method are commonly used:
Batch Inhibition - Inhibitor is added in the flow
system periodically in batch treatments and usually added to the
tubing bore in a process where the fluids in the well bore are
displaced with the inhibitor and its carrier. A strongly persistent
filming inhibitor is usually used which can reduce corrosion rates
effectively during the period between batch treatments. This technique
is usually effective where the chloride concentration is high but the
velocity is low. However, conditions of high temperature (>250 F;
120 C) and high flow rate generally limit the use of this technique.
Squeeze Treatment - Squeeze treatments are a
modification of batch inhibition used for controlling downhole
corrosion. Instead of just displacing the tubing with inhibitor and
its carrier fluid, the squeeze treatments also forces the fluid under
pressure into the surrounding formation. This has the benefit of
extending the duration between batch treatments in some wells.
However, in other cases, squeeze treatments can also interfere with
the well's production by plugging the formation.
Continuous Inhibition - Inhibitor is continuously
injected into the tubing at bottom of the string or through a
subsurface injection valve. The rate of injection is regulated to
provide the inhibitor at a required concentration to mitigate
corrosion. While more costly and requiring more equipment than batch
inhibition, continuous inhibition has been shown to be more effective
particularly in deeper high temperature wells and at more severely
corrosive conditions. At high flow rates, continuous inhibitor
injection may become costly and possibly ineffective.
|
Socrates
Interface
What's New in v7.0?