In the selection of materials for Carbon Capture and Storage (CCS), there is a tendency to rely upon existing knowledge and experience from oil & gas production. However, CCS presents a unique set of conditions and challenges in identifying suitable and cost-effective materials.
CCS vs. Oil & Gas Wells
CO2 streams typically include oxygen and other impurities not found in oil and gas production and much lower temperatures may be experienced requiring the use of materials with low-temperature toughness. While there has been a lot of experience with CO2 injection for enhanced oil recovery, this has typically been from a very clean source of CO2 with well-controlled water content. For CCS, CO2 can originate from a variety of industrial sources and therefore contain a wide range of impurities. Reduction of these impurities may not be cost-effective so materials need to be selected which are resistant to the potential conditions at the extreme limits of the anticipated impurities and temperature.
Contaminants & Impurities
For corrosion to occur, water must be present. During CO2 injection, the presence of water may be the result of condensation from the CO2 or flow back from the reservoir. Depressurization due to upsets in the process often results in water dropout. Contaminates such as methanol and glycol may also trigger the drop out of water. The CO2 dissolves in the water to form carbonic acid. SOx, NOx, H2S, and other contaminants may also react with each other to form strong acids, including nitric and sulfuric acids, and potentially elemental sulfur. In addition to the contaminates mentioned, there is a wide range of potential impurities which have not been studied.
Existing Industry Guidelines: NACE MR0175
While NACE MR0175/ISO 15156 provides guidance for the selection of material for oil and gas production environments containing H2S, it may not be applicable to CCS. The pH of water that drops out can be significantly lower than what is covered by NACE MR0175/ISO 15156. In addition, the partitioning of the H2S to water from CO2 is different than that from a hydrocarbon. The H2S limits provided by NACE MR0175 are largely based upon the partial pressure of H2S, while for CCS the concentration of H2S in the water would provide a better indicator.
Corrosion Factors in CCS
The low pH of the condensed water may also lead to the depassivation of corrosion-resistant alloys, resulting in localized corrosion and potentially stress corrosion cracking. Oxygen, H2S, and chlorides are also drivers of SCC. CCS conditions typically include oxygen and high chlorides may occur due to water flow back from the reservoir. This may require the use of highly alloyed corrosion-resistant alloys.
An additional consideration with CCS is the potential for extremely low temperatures resulting from a drop in pressure. This may drive the selection of materials with low-temperature toughness, such as austenitic stainless steels or nickel-based alloys.
Each CCS project differs in the potential composition of the gas stream and operating temperature range. Industry guidelines for the selection of materials have yet to be developed and much research is still required. It is therefore recommended that specific qualification testing be performed to ensure the safe selection of material. Testing needs to consider the full range of environments the material may be exposed to during operation, shut-in, or interruptions.
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