Socrates-B System Description

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:

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.
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.
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.

Completion and Workover Fluids

Parameters that affect the severity of the completion fluid environment include the following:

1. Fluid composition (Cl- Br- content and other anion species)
2. Temperature
3. Acid gas partial pressures, i.e., H₂S and CO₂
4. Aeration

Evaluation for completion fluid environment service is characterized in terms of the following steps:

1. Determination of the type of brine chemical and the specific gravity.
2. Determination of severity of operating environment in terms of acid gas concentrations, pH and temperature.
3. Determination of potential corrosion rate for steel casing/tubing, used in more than 50% of situations.
4. Account for steel inhibitor efficiency.
5. Examine acceleration of corrosion rate for aerated systems.
6. Modify environmental severity to account for inhibition efficiency.
7. 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.

Stimulation Acids

Parameters used in assessing environmental severity for stimulation acids include the following:

1. Acid type
2. Acid concentration
3. Temperature
4. Acid gas concentration
5. 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:

1. Determine acid type, temperature, CO₂ and H₂S
2. Determine environmental severity to estimate weight loss corrosion in steels
3. Determine required pitting index from Environmental Severity factor (ESF) and temperature
4. 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.

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:

1. 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.
2. Sour Brine -- Essentially CO₂ brine with H₂S more than 0.01 psia.
3. Deaerated Brine -- Sea water injected down hole, after deaeration and treatment
4. 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.