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Aggressiveness
of the production environment is not always the limiting factor
in the selection of CRA materials. Specifically, production environments
with lower levels of H2S and chloride concentrations in the brine
commonly produce conditions where stainless steels (i.e., martensitic
and duplex alloys) are acceptable.
The severity
of several potential non-production environments can dictate the
need to use alloys with a greater degree of corrosion resistance.
Typical non-production environments include:
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Completion
and Workover operations, where concentrated clear brines are
used to counter balance the formation pressure and in some cases are
used for prolonged periods as packer fluids. CRAs used in these wells
must be resistant for short to prolonged service in the presence of
such fluids. |
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Stimulation
acid operations, fluid formulations used in these processes
typically have a combination of corrosion inhibitors and other chemicals
to control reaction rates in the formation and to modify flow characteristics.
More commonly used stimulation acids are based on HCl (15% and 28%),
HCl + HF mixtures (mud acid) or organic acids such as formic acid. |
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Injected
water operations, typically, water is injected into subterranean
formations for either secondary recovery (formation pressure maintenance)
or water disposal. Condition of this injected water may vary greatly
in condition from raw acid gas containing aerated brine to treated
(chlorinated and deaerated) fresh water. |
Environments
in petroleum production service typically contain a mixture of
CO2 and H2S in combination with brine and various liquid/gaseous
hydrocarbon species. In many cases, the composition of these environments,
along with the temperature and pressure characteristics of the
particular application determine the overall severity from a corrosion
standpoint.
However, it
is also true that the aggressiveness of the production environment
is not always the limiting factor in selection of CRA materials.
Specifically, production environments with lower levels of H2S
and chloride concentrations in the brine commonly produce conditions
where stainless steels (i.e., martensitic and duplex alloys) are
acceptable. The severity of several potential non-production environments
(completion and workover fluids, stimulation acids and/or injected
water) can dictate the need to use alloys with a greater degree
of corrosion resistance. In these situations, it is common to
require higher alloy materials such as nickel base alloys or high
alloy austenitic stainless steels to resist the greater corrosivity
of the non-production environment.
In this section,
some of the parameters for evaluation of CRAs in the three types
of non-production environments are described. A list of alloys
evaluated in the system can be found in Appendix
1.
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Completion
and Workover Fluids
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Parameters
that affect the severity of the completion fluid environment include
the following
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Fluid
composition (Cl- Br- content and other anion species) |
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Temperature |
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Acid
gas partial pressures, i.e., H2S and CO2 |
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Aeration |
Evaluation
for completion fluid environment service is characterized in terms
of the following steps:
- Determination
of the type of brine chemical and the specific gravity.
- Determination
of severity of operating environment in terms of acid gas concentrations,
pH and temperature.
- Determination
of potential corrosion rate for steel casing/tubing, used in
more than 50% of situations.
- Account
for steel inhibitor efficiency.
- Examine
acceleration of corrosion rate for aerated systems.
- Modify
environmental severity to account for inhibition efficiency.
- Find operating
temperature and apply limiting temperature and inhibitor efficiency
criteria.
Most completion
fluid environments do not pose a problem for CRAs with alloy content
higher than 25 Cr duplex stainless steel. The range of concern
here are lower alloy materials between 9Cr-1Mo and 25 Cr. For
these materials, resistance is typically a function of specific
gravity and temperature.
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Stimulation
Acids
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Parameters used
in assessing environmental severity for stimulation acids include
the following:
- Acid type
- Acid concentration
- Temperature
- Acid gas
concentration
- Fluid additives
(inhibitors, retarders, etc.)
The Socrates-B
system acidizing environment evaluation procedure can be summarized
in terms of rules that combine the severity of the acid environment
to a potential for pitting attack as a function of maximum temperature.
Each material in the alloys database has a pitting index (PI) number
determined as,
PI
= Cr + 3.3 Mo + 11 N + 1.5 (W + Cb)
where
Cr, Mo, N, W and Cb represent the percent composition of Chromium,
Molybdenum, Nitrogen, Tungsten and Columbium in the alloy, respectively.
The system determines a required minimum pitting index as a function
of the acidizing environment specified and selects those alloys
that meet the compositional requirements.
Environmental
Parameters
- Type of acid
- Temperature
- Acid gases
(CO2, H2S)
- Inhibition
Steps involved
in evaluating alloys for acidizing service are as follows:
- Determine
acid type, temperature, CO2 and H2S
- Determine
environmental severity to estimate weight loss corrosion in steels
- Determine
required pitting index from Environmental Severity factor (ESF)
and temperature
- Modify ESF
and pitting requirements to reflect duration effects and inhibition
concentration.
One of the most
important aspects of acidizing environments is obviously the acid
species used for well stimulation. Stimulation acids with HF are
extremely corrosive and more so with respect to titanium and zirconium
alloys that are typically corrosion resistant in other acidic media.
When HF is present, Socrates-B will not permit recommendation of
either titanium or zirconium materials.
The corrosion
rate of acidizing fluids on steel can be controlled to an acceptable
level with corrosion inhibitors. However, it has been found that,
in some cases, these inhibitors can be substantially less effective
in protecting CRA materials which have high levels of Cr,
Ni and Mo, leading to localized corrosion and stress corrosion cracking
(SCC). Therefore, stimulation acids are a class of environments
which constitute a major concern especially in terms of compatibility
with CRA equipment.
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| Injected
Water |
In the Socrates-B
system, the conditions of the injected brine are evaluated in
a manner similar to that described for the other environments.
The environment is assessed based on the following parameters:
- Chloride
concentration
- Acid gas
partial pressure
- Oxygen
concentration
- Chlorination
- Scaling
tendency
- Temperature
Injected water
environments in Socrates-B are classified into four broad categories:
- CO2
Brine -- high pressure CO2 injected into water for
recovery. Typical concern here is one of general corrosion and
primarily so in case of carbon and low alloy steels. Additional
factors in computing environmental severity include pH, chlorides,
temperature and fluid velocity. Oxygen levels are assumed to
be less than 100 ppb.
- Sour Brine
-- Essentially CO2 brine with H2S more
than 0.01 psia.
- Deaerated
Brine -- Sea water injected down hole, after deaeration and
treatment
- Aerated
Brine for systems exposed to sea water in a piping system or
sea water injected down hole
Environmental
severity in injected water environments is defined by how corrosion
rate of steel is affected as a result of chlorides, temperature,
velocity and pH in each of the four types of environments defined
earlier. In the Socrates-B system, severity in injected water
environments is determined as a four step process:
Determination
of application: The system contains a database of different materials
for standard applications that are typically used in water injection:
- Injection
pumps
- Valves
- Impellers
- Flowlines/Pipelines
- Downhole
Equipment
- Tubing/Casing
- Subsurface
Equipment
- Submersible
pumps
Determination
of corrosion rate of steel based on pH at ambient temperature.
The pH is determined from acid gas partial pressures, bicarbonates
and temperature.
The Corrosion
rate from step 2 is modified to reflect
(a) CO2, H2S effects
(b) Temperature effects
(c) Chloride effects
(d) Velocity effects
as per rules for different types of brines.
Determine
corrosion allowance and compare expected corrosion rates to allowable
rates prior to evaluation of stainless steels and CRAs.
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