Identifying Pathways for CO₂ Migration
Underground CO₂ storage is ultimately a containment challenge. Operators must be confident that the storage formation, wells and engineered barriers will continue to perform—not only during injection, but over the long term after active operations have ended. That requires understanding current asset condition, anticipating how materials may change over time and identifying potential pathways that could allow CO₂ to move outside the intended storage zone.
Existing oil and gas infrastructure can create an opportunity to accelerate a storage project, particularly when established wellbores provide access to a depleted reservoir. However, those wells were not necessarily designed for CO₂ service and may have accumulated damage or degradation during production. Operators must determine whether each well can be used for injection or monitoring, or need to be modified or permanently abandoned to ensure integrity of the storage reservoir.
An evidence-based understanding of the storage system that combines well inspection, materials testing, full-scale equipment evaluation, engineering analysis and risk assessment is needed. C-FER helps operators bring that evidence together so they can make defensible decisions about reuse of legacy equipment, qualification of new equipment and long-term containment risk.
CO2 Storage Complicates the Integrity Question
Many of the principles used to manage conventional oil and gas wells remain relevant to underground CO₂ storage. The well casing must maintain structural integrity, cement must preserve zonal isolation, and downhole and wellhead equipment must continue to perform their intended functions.
The difference from oil and gas operations is the environment in which those equipment and barriers must perform. When CO₂ encounters water in the storage reservoir, the resulting conditions can be corrosive to casing, cement and other well components. Impurities in the CO₂ stream can introduce additional compatibility concerns by forming other corrosive products, and the types and concentration of the impurities can vary depending on the capture process.
Possible impurities include water, hydrogen, hydrogen sulphide, oxygen, nitrogen oxides and sulphur oxides. Depending on the combination of CO₂, water and impurities, operators might need to evaluate corrosion, environmental cracking, cement degradation and the compatibility of non-metallic materials such as elastomers.
C-FER has proposed the development of a new super-critical CO2 flow loop that will investigate how impurities affect the formation, transport and deposition of corrosive products in a pipe. The results of these tests will help operators set reasonable limits on impurities in CO2 streams and will help develop methods to minimize corrosion
Different Storage Settings Require Different Data and Decisions
Underground CO₂ storage is not a single application. There are a range of storage scenarios, each with different challenges such as the availability of information, the condition of existing infrastructure and operational uncertainties. The four primary storage scenarios are:
- Saline aquifers offer permanent storage capacity but they can have limited historical reservoir information because they have not previously been developed for oil or gas production.
- Depleted gas reservoirs provide permanent storage and typically have extensive production history, reservoir data and existing wellbores. Information could be sparce, however, on the post-gas production condition of the wellbores, reservoir and cap rock and how these might affect the integrity of the storage reservoir
- Mature oil reservoirs used for enhanced oil recovery provide permanent storage CO2 as some of the CO2 and like the depleted gas reservoir scenario there is likely abundant data available on the initial conditions but information on current conditions of the wells, reservoir and cap rock could be lacking. The storage process is more complex than simple storage operations as oil production must be balanced with CO2 injection to ensure reliable CO2 storage
- Salt caverns provide temporary CO2 storage during operational interruptions in the transportation pipeline or at the permanent storage facility. Managing the cavern pressure cycles caused by each fill and empty cycle is critical to minimizing damage to the cavern and wellbore and ensuring storage integrity
Each storage scenario presents a different set of challenges that require different sets of data to inform decisions on reservoir performance, well integrity and operational practices.
Legacy Wells Are Both an Opportunity and an Integrity Challenge
Reusing an existing well can reduce the need for new infrastructure, but reuse is not simply a change in operating purpose. A legacy well might contain materials, cement systems, elastomers, connections, pumping equipment or packers selected for conditions that differ from the proposed CO₂ environment.
Its operating history matters as well. A well that has experienced a long production life may contain corrosion, cracks, cement damage or other conditions that affect its future performance. Nearby wells that are not being reused may intersect the storage formation or its pressure-affected area and create potential pathways for migration.
The operator’s decision extends beyond a simple pass-or-fail assessment. A well may be suitable for one function but not another. Relevant questions include:
- Can the well safely support CO₂ injection?
- Can it be used as a monitoring well?
- What modifications or remediation would be required?
- How could its condition change under future service?
- If it cannot perform an intended function, should it be further abandoned to protect long-term containment?
Small-scale testing can provide useful information about how steel, cement or elastomer materials in wells respond to CO₂, water and expected impurities. However, material test data alone might not demonstrate how a complete product will perform. For example, the performance of defects in the equipment such as corrosion or cracks that occurred while in service can be difficult to assess with small-scale material tests.
C-FER conducts full-scale tests to addresses this gap by evaluating the complete product under representative loads, pressures and environmental conditions. Used together, small-scale tests, full-scale tests, field observations and engineering analysis help compensate for the limitations of any one evidence source.
Inspection Technologies are Critical for Decision Making
Before operators can determine whether an existing well is suitable, they need reliable information about its current condition. Well-logging and casing-inspection technologies can contribute to that understanding, but different tools may not detect or characterize defects in the same way.
Side-by-side evaluations of casing-inspection technologies and fiber optic-based well monitoring systems using full-scale tubular strings containing known defects in C-FER’s test well allowed operators to compare how different vendor technologies performed under controlled conditions and gave vendors information to benchmark and improve their technologies.
Connecting Evidence to Long-Term Risk
Inspection and testing establish what is known about the well. Risk assessment connects that information to potential outcomes and operator decisions.
A storage-integrity assessment may need to consider migration into another subsurface formation, movement toward potable groundwater or a release to surface. These scenarios can have different environmental and life-safety consequences and should be managed accordingly.
C-FER applies risk-based methods that combine historical information with engineering models to estimate potential leak probabilities and flow rates, evaluate consequences and help operators compare mitigation options.
For operators, this evidence can support decisions involving:
- Reuse, modification or abandonment of existing wells
- Selection of casing, cement, elastomers and downhole equipment
- Qualification of well designs and casing connections
- Selection of inspection and logging technologies and schedules
- Prioritization of wells for additional assessment
- Evaluation of potential leak paths and consequences
- Comparison of prevention, monitoring and mitigation options
The objective is not to eliminate every uncertainty before a project proceeds, but to identify which uncertainties could have the largest impact on containment, safety and project viability
How C-FER Helps Operators
C-FER helps operators evaluate underground CO₂ storage systems by combining engineering, risk assessment and testing in independent technology assessments.
C-FER can help operators:
- Assess the current condition and potential future performance of existing wellbores
- Evaluate whether wells are suitable for planned operations
- Validate material performance with small-scale material tests in representative service environments
- Verify equipment performance with full-scale operational tests under controlled conditions
- Evaluate logging and monitoring technologies in controlled, full-scale tests
- Develop quantitative risk models for well and reservoir integrity
C-FER helps operators move from assumptions about long-term storage performance to decisions supported by unbiased evidence and rigorous risk-based assessments by combining four decades of experience in well integrity, equipment qualification and severe-service oil and gas operations with laboratory and full-scale testing.
Contributor
Brian Wagg, MSc, PEng.
Director, Testing Services
With over 35 years with C-FER, Brian Wagg’s current focus is on applying C-FER’s analytical and full-scale testing capabilities to projects in clean energy including CCUS, hydrogen, geothermal and small modular nuclear reactors.
