August 6, 2024 Decarbonization  |  Sustainability

CO₂ Capture, Transportation, and Storage (CCTUS) projects face significant technical challenges, including avoiding severe corrosion, stress corrosion cracking and pitting due to low pH levels, high chloride content, and impurities like SO₂, NO₂, H₂S, and oxygen. These factors necessitate the use of Corrosion Resistant Alloys (CRAs) for long-term durability. CRA selection must consider pH, chloride content, temperature, pressure, and impurities. Proper metallurgical processing and expert consultation are essential. Regulatory compliance, as detailed in the UIC Program under 40 CFR Part 146 subpart H, requires computational modeling and effective corrosion management for CO2 injection in Class VI wells. OLI Systems’ advanced tools are critical to compliance with these guidelines – they accurately predict impurity levels and corrosion rates, optimize materials selection, and ensure well integrity and safety.

Technical Challenges and Material Selection Criteria for CO2 Capture, Transportation, and Storage (CCUS) Projects

Some of the biggest technical challenges and risks associated with CO₂ Capture, Transportation, and Storage (CCUS) projects include severe corrosion, stress corrosion cracking, and pitting due to low pH levels, high chloride content, and impurities like SO₂, NO₂, H₂S, and oxygen. Additionally, variable temperature and pressure conditions complicate material selection, necessitating the use of Corrosion Resistant Alloys (CRAs) for long-term durability and integrity of injection, transportation and storage equipment.

The selection of Corrosion Resistant Alloys (CRAs) for CO₂ Capture, Transportation, and Storage projects is influenced by several factors. CRAs may be necessary when free water is present, either during surface injection or from a saline reservoir, to ensure long service life. The pH of injected CO₂ and its interaction with saline formations are crucial, as lower pH levels increase corrosion risks. Other factors such as chloride content, temperature, pressure, and impurities like SO₂ and NO₂ significantly impact CRA selection. Hydrogen sulfide (H₂S) and oxygen levels can cause stress corrosion cracking and pitting, affecting material choice. Water content and chemistry, including total dissolved solids and buffering capacity, play a role in CRA selection. Annular wellbore fluids, service life expectations, and industry experiences, especially in acid gas injection, are also important for ensuring material durability and performance.

Defining CRA limits involves multiple factors. Temperature and other dependent variables are key. Traditional CRA selection guidelines are based on partial pressures of acid gases, but supercritical CO₂ (sCO₂) is better defined by fugacity. Limited fugacity data mean current limits use partial pressures conservatively. Injection wells must consider reservoir fluid composition, including salinity and pH, and CRA limits that are often guided by oil and gas industry practices, emphasizing water content and corrosive environments.

While there is no established standard method for designing CO₂ injection equipment, design considerations include well location, depth, and regulatory requirements. CO₂ injection wells require upgrades for higher pressure ratings and corrosion resistance. Temperature changes due to CO₂ processes must be considered, with surface equipment needing to withstand up to 200°F and injection well equipment up to 300°F. Carbon steel is suitable for dry conditions, while CRAs are necessary for water-bearing environments. Proper metallurgical processing and expert consultation are essential for material suitability and longevity.

Regulations for Underground Injection Control (UIC) of CO2 – 40 CFR Part 146

Maintaining the integrity of injection wells and protecting underground sources of drinking water are critical aspects of the Underground Injection Control (UIC) Program. This program, regulated under 40 CFR Part 146, outlines stringent criteria and standards to safeguard environmental and public health for Class VI wells. A key component of this regulatory framework is the use of computational modeling and effective corrosion management, which play vital roles in planning, monitoring, and managing injection well operations.

Most importantly, corrosion modeling is now a requirement for the use of lower Cr alloys (generally less than 22% Cr) under the Environmental Protection Agency (EPA) guidelines.

Understanding the Regulations

The UIC Program, detailed in 40 CFR Part 146, establishes criteria for various classes of wells, including Class VI wells used for carbon dioxide (CO₂) geologic sequestration. The regulations cover a broad range of requirements, from construction and operating procedures to monitoring and reporting obligations.

The Importance of Computational Modeling

1. Delineating the Area of Review (AOR)

The “Area of Review” (AOR) is a critical concept within the UIC regulations. It defines the region around a geologic sequestration project where underground sources of drinking water (USDWs) may be at risk due to injection activities. Computational modeling is used to delineate this area by considering the physical and chemical properties of the injected CO₂ and displaced fluids. This modeling ensures that the AOR is accurately mapped based on site-specific data, helping to protect USDWs from contamination​.

2. Developing Monitoring and Testing Plans

Effective monitoring and testing are essential for maintaining integrity and environmental safety. Computational models simulate the potential impacts of injection on the subsurface environment, including the movement of the CO₂ plume and the pressure front. These simulations help operators design robust monitoring plans that comply with regulatory requirements and ensure the long-term stability of the injection site.

3. Predictive Analysis and Risk Assessment

Predictive modeling is a powerful tool for assessing the long-term behavior of injected fluids and the integrity of confining zones. By integrating geological, geophysical, and geochemical data, these models provide valuable insights into potential risks and help in the development of effective mitigation strategies. This proactive approach is crucial for meeting regulatory standards and preventing environmental hazards.

The Importance of Corrosion Modeling

Most importantly, this guideline calls out that in instances where an application proposes to use less corrosion resistant alloys (generally less than 22% Cr) in saline storage environments, applicants will receive a request for additional information (RAI) requiring project-specific justification for the use of the lower Cr alloy. In order to support the use of a lower Cr alloy, applicants will be required to submit a demonstration that must include, at a minimum, corrosion modeling over the timescale of the project in addition to provision of site-specific information required by 40 CFR 146.82.  The guideline stipulates that any corrosion modeling must consider the site-specific chemistry, including the CO₂ stream and formation fluids, as well as consider possible stress cases in addition to normal operations and any other relevant factors.

Mitigating Corrosion Risk with OLI Systems’ Solutions

OLI delivers proven software-based process simulation solutions based on first principles that accurately predict chemistry, corrosion and mineral scaling behavior for the most complex industrial applications. Through a decade of extensive research & development innovation, OLI has gained the capability to accurately characterize the behavior of impurities in CO₂ streams and their impact on corrosion. Some of these capabilities are highlighted below.

1. Corrosion Prediction and Simulation

Corrosion is a significant challenge in maintaining the integrity of injection wells. OLI Systems specializes in advanced corrosion prediction and simulation tools that can forecast corrosion rates under various environmental conditions. These tools simulate interactions between injected fluids and well materials, identifying potential corrosion issues before they occur. This predictive capability helps in designing wells that meet the stringent construction standards outlined in §146.86 and ensures long-term operational safety.

2. Material Selection and Compatibility

Selecting the right materials for well construction is critical to minimizing corrosion. OLI’s modeling tools evaluate different materials’ compatibility with the injected fluids and the geological environment. This evaluation aids in selecting corrosion-resistant materials that comply with regulatory requirements and enhance well longevity.

3. Real-time Monitoring and Data Management

OLI’s data management solutions provide chemistry and corrosion insights based on first principles models for real-time monitoring to detect early signs of corrosion and other integrity issues. These tools support the creation of comprehensive monitoring plans that meet the requirements of §146.90 and ensure that wells maintain their structural integrity throughout their operational life.

4. Geochemical Analysis and Risk Mitigation

Understanding the geochemical environment of the injection zone and the injected fluids is crucial for predicting and managing corrosion. OLI’s software delivers first principles-based chemistry and corrosion insights for detailed geochemical analyses to assess the corrosive potential of the environment, providing data to support the design and implementation of effective corrosion mitigation systems.

5. Training and Support

OLI Systems offers training programs for engineers and operators on best practices for corrosion prevention and management. These programs cover material selection, corrosion monitoring techniques, and the implementation of effective corrosion control measures, ensuring that personnel are well-equipped to maintain the integrity of injection wells and comply with regulatory standards.

OLI has developed the MSE Corrosion Model to predict corrosion rates in systems where water is not the primary solvent. This model uses a thermodynamic approach to calculate properties like pH and an electrochemical model for predicting corrosion rates. The MSE model accurately predicts corrosion rates for duplex stainless-steel alloys in acid environments, aiding in material selection for CO₂ transport and injection, ensuring asset integrity and safety. Read this recent article to learn more about how the OLI technology can be applied to accurately predict the behavior of impurities in CO₂ that can lead to the formation of corrosive acid phases that risk equipment and pipeline integrity.

Conclusion

Without free water, sCO₂ is non-corrosive, allowing for carbon steel construction. If free water is present or CO₂ is injected into a water-bearing formation, appropriate CRA selection depends on water chemistry, CO₂ impurities, H₂S, oxygen presence, and well conditions. Expert consultation and lab tests are essential for accurate material choices.

Computational modeling and effective corrosion management are indispensable tools in the UIC Program for Class VI wells, helping to protect underground sources of drinking water and ensure the safe operation of injection wells. By leveraging OLI Systems’ advanced modeling, corrosion prediction, and data management tools, operators can enhance their compliance efforts, optimize materials selection, and contribute to environmental sustainability.

References:

1. Craig, Bruce, Adam Rowe, Michael Warmack, Thomas E. Doll, Catherine Stevens, and Kevin C. Connors. “Guidelines for the selection of corrosion resistant alloys for CCS and CCUS injection wells.” International Journal of Greenhouse Gas Control 129 (2023): 103988.
2. 40 CFR Part 146 subpart H