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The user can
specify data for any of the parameters and watch the effect of that
parameter on the corrosion rate in the system instantaneously. The
system starts with a set of default values and calculates a corrosion
rate based on any changes to the displayed values. A typical consultation
will involve the following five steps:
Specification of pH related data: At the outset, the system
determines a corrosion rate only if the operating environment is
acidic or has aeration. If the specified environment has no acid
gases or there is sufficient buffering to produce a pH higher than
7.0, the system will predict zero or very low corrosion rates, except
under conditions of aeration. So, the first step in consulting the
system involves specification of the acid gas (H2S and
CO2) partial pressures as well as the bicarbonate and
acetate content of the environment.
Temperature/Gas-Water
ratios: Temperature has a significant impact on corrosion rates
since precipitation of corrosion products and scaling are functions
of temperatures. Corrosion rates typically increase with increasing
temperature, though in CO2 dominated systems, FeCO3 scaling
at higher temperatures can produce significant protection against
further corrosion. If the Gas to Oil Ratio indicates gas dominated
conditions (as opposed to an oil dominated system) the system uses
the water to gas ratio and the dew point as means to determine availability
of an aqueous medium to measure corrosion. So, depending on the
value entered for the Gas to Oil Ratio, the system will let you
specify the relevant water-related parameters. If the Gas to Oil
Ratio is less than 5000 scf/bbl (which denotes an oil well), the
system uses the water cut and oil persistency to determine the wetness
effect.
Chlorides/oxygen/ sulfur: Chlorides and sulfur typically
make corrosion worse if the process has been initiated by the presence
of acid gases. Their role, while not as critical as that of H2S
or CO2, is significant because these parameters can significantly
increase corrosion rates in mildly corrosive systems. Presence of
oxygen beyond 20 ppb even in mildly acidic systems can lead to significant
corrosion rates, especially with high chlorides and high flow rates
at elevated temperatures.
Velocity/Type of flow: Flow parameters are very critical
in both determining and controlling corrosion effects. Erosion corrosion
as well as the protection (or the lack of it) from corrosion films
is very much a function of fluid velocity. Velocity has a significant
impact on mass transport with in the corrosion boundary layer and
also impacts a corroding system's ability to form protective scales.
Inhibition/corrosion allowance: Inhibition choices in the system
allow the user to select applicable methods of inhibition for vertical
or horizontal flow and determine the extent of corrosion mitigation.
In some cases, the system might provide no protection due to inhibition
because of high velocities or chloride concentrations. The system's
rules assess the appropriateness of method of inhibition delivery
for a given set of conditions.
Multipoint
Sensitivity Analysis: While performing corrosivity analysis,
it is very helpful to understand the effect of a particular parameter
or a group of parameters. Using Multipoint Sensitivity Analysis,
users can study the effect of a number of parameters on the predicted
corrosion rates or computed pH. For instance, while analyzing a
particular well, it makes sense to check the effect of a change
in production rates and how such a change would affect the corrosion
rates. Or for instance, in case of a flow line, one may need to
see the effect of pipe diameter on the flow characteristics and
the predicted corrosion rate. Such sensitivity analyses can be easily
performed using the Multipoint Sensitivity Analysis Tool using MS
Excel.
Corrosion
Profile: It is essential to perform corrosion analysis over
a pipe length since many factors like change in temperature/pressure
occur along the length. These changes affect many parameters including
the water phase behavior which in turn affects corrosion rates.
In some cases the system may show very high corrosion rates at certain
points downstream due to the condensation of water. The systems
enhanced calculation rules for predicting the water of natural gas
and dew point calculations enable accurate predictions for water
condensation ans thus corrosion rates. A glance at the corrosion
profile informs the user of such problem spots in the pipe system
where there is a high possibility of water condensation and thus
very high corrosion rates.
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