When screening, ranking, and evaluating either saline aquifers or depleted oil and gas fields for potential carbon dioxide (CO2 ) storage, three key technical factors must be considered: capacity, injectivity, and containment. For containment, artificial penetrations (or wells) are considered the primary risk for fluid migration. This risk is twofold: the risk of CO2 leakage and the risk of complex remedial operations. If not addressed effectively, these risks could make the project unviable.
Therefore, when comparing storage site candidates, the risk posed by legacy wells is an important variable for ranking the feasibility of potential locations. This risk is not only a result of the number of wells, but also of their potential to leak and how complex it would be to correct that.
Unfortunately, it’s quite common for only the density (number of legacy wells per square kilometer) to be evaluated, disregarding other risky elements such as well status, age, architecture, and mechanical state.
However, it’s more than well density that’s critical—it’s their integrity.
To robustly rank potential carbon storage sites, a comprehensive set of both technical and nontechnical criteria—organized in a clear hierarchy and weighted by importance—must be considered, along with a list of well integrity factors that include repurposing feasibility.
What does well integrity mean?
The International Organization for Standardization defines well integrity, or mechanical integrity, as “(the) mechanical condition of a well, such that engineered components maintain their original dimensions and functions, solid geological materials are kept out of the wellbore, and fluids including CO2 are prevented from uncontrolled flow into, out of, along, or across the wellbore, cement sheath, annulus, casing, tubing, tubing and/or packers.”
Failures in well integrity may result in the unintended migration of CO2 out of the storage formation, the consequences of which can include
- Environmental damage
- Contractual penalties
- Affecting other parties’ interests (e.g., pore space rights, waste disposal operations, mineral extraction, hydrocarbon exploitation)
- Loss of public trust.
For this, it’s crucial to adequately manage well integrity throughout the different phases of your CO2 storage projects, including both legacy and newly drilled wells. How? These are the activities that generally take place during each phase:
- Preinjection
a. Screening—initial legacy well screening
b. Evaluation—legacy well integrity risk assessment and acquisition of new logging or integrity data if necessary
c. Detailed design—design of well integrity monitoring and remediation plans based on risk assessment
d. Construction—remediation of legacy wells and acquisition of baseline integrity data and logging of newly drilled wells - Injection—integrity monitoring of legacy and newly drilled wells, along with remediation when needed
- Postinjection—barrier evaluation and verification, well intervention and remediation, and final plug and abandonment.
Steps taken during the initial assessment of well integrity
Because most carbon storage projects are pre–final investment decision or in early development, we’ll focus on the preinjection phase of an initial well integrity assessment. The following steps must be considered when assessing the integrity of existing or legacy wells in an area of interest:
- Data collection, extraction, and analysis
- Barrier analysis
a. Well grouping
b. Well schematics
c. Barrier assessment
d. Barrier failure probability - Wellbore leakage analysis
a. Leakage probability
b. Leakage risk assessment - Recommendations.
Data collection
We begin by collecting, organizing, and reviewing all the data necessary (and available) for assessing the well’s current integrity. Input files can include daily operations reports, end of well reports, cement details, well schematics, formation evaluation, cement and corrosion logs, and well history reports.
The quality and quantity of available well records determine the effectiveness of the well integrity assessment. It’s recommended that all sources of information—including public and private databases, data from previous owners, visual inspections, and site surveys—be investigated. Running new logs to acquire well integrity data may also be necessary.
Digital tools powered by artificial intelligence can be used to efficiently extract the relevant data from these reports, which typically includes
- Well identification, name, or number
- Location (to identify its position and any surface constraints)
- Wellhead status (e.g., in place, removed, damaged)
- Spud and abandonment date
- Casing shoe depths
- Top off
- Well integrity history
- Mechanical status (e.g., single or multiple wellbores, cemented tubulars, fish).
Barrier analysis
To assess them by category rather than individually, wells are grouped for analysis based on type, architecture, status, or other factors as needed. Depending on the scope of the assessment, barrier schematics are then created for each well or well type.
Barriers are assessed based on a Failure Mode, Effects, and Criticality Analysis (FMECA). It evaluates the documented state of the well barrier elements, the most likely leak paths, failure mechanisms, and resulting consequences. It’s crucial to utilize a standardized and normalized scoring system aligned with regulatory well barrier classifications. This helps ensure objectivity and repeatability while integrity is compared across wells and potential sites. Depending on risks or project needs, the probability of barrier failure can be evaluated with different levels of confidence, based on expert advice and semiquantitative or quantitative modeling.
Wellbore leakage analysis
Once the potential leak paths, failure mechanisms, and likely consequences are understood, the next step is to quantify the onset, probability, and potential magnitude of a CO2 leak. At its simplest, this assessment can be based on expert opinion, but a more comprehensive analysis can also be conducted. This would be based on either an integrated leakage probability model (using constant barrier and permeability conditions) or on a model considering evolving barrier status.
In general, this step would require:
- FMECA
- Analyzing the most likely flow paths
- Reservoir model extraction
- Determining overburden property petrophysics
- Flow modeling
- Quantitative leakage modeling.
Leakage risk assessment can then be conducted using a defined severity scale or consequence modeling.
Recommendations
Individual risk levels will determine the most appropriate approach for each well, including whether any action, monitoring, or remediation is required. At this stage, wells that are good candidates for repurposing can also be identified. Project teams should make sure to establish technical limits for abandonment or repurposing operations, including maximum allowable time, effort, and expense. For this, experience in plug and abandonment, remediation, and workover will be key.
What successful risk assessment looks like
Well integrity risk assessment is not only a requirement for a prospective CO2 storage site, but it’s also fundamental for understanding containment risks. It can be conducted with different levels of detail depending on site-specific risks and project needs. And it can also provide operators with qualitative and quantitative data to make more informed decisions.
The results of the assessment will enable operators to decide whether well integrity and leakage risks are acceptable, manageable, or, in some cases, unacceptable. The sooner they understand these risks, the more successful their carbon storage projects will be.