Material Selection Considerations for CO2 Sequestration Projects
tables. Consideration should be given to the compatibility of the alloys chosen for tubing and casing with the annulus fluids to be used. Corrosion testing might be required to verify the compatibility of the fluids and metals.
Casing
General corrosion is controlled by the use of pH adjustment and oxygen scavengers and/or biocides in the annulus fluids. Grade selection should be based on the carbon steel entries in Table 5. Where the casing extends below the packer (and comes into contact with the production fluids), the corrosion conditions will differ from the upper section of casing. It might be acceptable in corrosive conditions to allow the casing below the packer to corrode if this does not affect the well integrity or production rate; otherwise a corrosion-resistant alloy suitable for this service should be selected.
Christmas Trees
General guidelines on suitable materials for Christmas trees are given in Table 6.
Pipelines
Suitable materials for process pipelines are given in Table 7. Practical Considerations for Pipelines
For longer pipelines, carbon steel is about the only economically
feasible material choice for dense phase transport of CO2, balancing material cost with the mechanical strength needed to
Table 6: Christmas Trees
Temp pH2S (ºC)
Any <140 <200 <3.5 <3.5
<2001 <10 <2001 <20
Conditions NaCl
(mbar) (g/l) <3.5
<200 <165 <20
<250 <250
Material* Other
SLC <3 Carbon steel with alloy 625 trims
Martensitic stainless steel (F6NM/CA6NM) with alloy 625 trims
Martensitic stainless steel (F6NM/CA6NM) with alloy 625 trims
22Cr duplex** 25Cr duplex**
*The use of alloy 625 for trims arises from the need to prevent even minor corrosion of valve seals and sealing faces that could impair their function.
**Duplex stainless steels are most susceptible to SCC between 80 and 130°C. Although the temperature quoted is 200°C, the pH2S and NaCl limits are based on the most susceptible temperatures; this assumes that the duplex stainless steel might experience these temperatures at some stage during service life.
withstand the internal high pressures and the external loads. Dry
CO2 (all water is dissolved in the CO2) has been shown both in laboratory experiments and from pipeline operating experience to give very low corrosion rates for C-Mn steel. For example, in a study conducted in connection with the design and engineering of the SACROC pipeline, the experimental corrosion testing programme reported corrosion rates of less than 0.0005mm/yr on X-60 ERW steel when there is no liquid water present. When liquid free water is present, corrosion of carbon steel will definitely occur.
Corrosion reactions are electrochemical in nature. For the CO2–Fe corrosion system there are several anodic and cathodic reactions
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