Today, the world has a large set of theoretical underground storage sites for CO2 distributed across all major sedimentary basins with different properties. But not all storage sites are equal. In fact, some of them are completely unsuitable for long-term storage, which is why the process of screening and ranking potential storage sites plays a crucial role in determining the success and risk levels of CCUS projects.
Opting for a well-suited site, characterized by reliable sealing layers, suitable geological conditions, and strategic spatial positioning, can significantly mitigate the potential risks associated with CCUS. Thorough site screening and selection involves evaluating various criteria, both technical and nontechnical, quantitative and qualitative.
Employing a scientific methodology and workflow is imperative to yielding sound and reliable outcomes in terms of site ranking. Moreover, given that CCUS projects often return little data in their early phases and rely on inputs from diverse data sources, it’s essential to consider a probabilistic approach and analyze how different uncertainties influence your screening and ranking results. The objective ranking of potential sites is necessary for validating which storage sites merit further evaluation for project viability and final investment decisions.
What should you screen and rank when it comes to carbon storage?
The process of site screening and ranking requires evaluating technical and nontechnical criteria, some of which can conflict. Based on decades of experience with CO2 injection in saline aquifers (e.g., Sleipner carbon capture and storage project) and depleted oil and gas reservoirs, the technical factors are generally understood to be
- Capacity—the amount of CO2 that can be securely stored in a particular site. This is mainly controlled by rock properties such as porosity, reservoir extent area, and reservoir gross thickness.
- Injectivity—the ease with which CO2 can be injected. This is mainly affected by rock permeability, pore pressure, host rock thickness, and in situ stresses.
- Containment—the ability to store CO2 securely and permanently (for hundreds of years). This is affected by the host rock and caprock properties, the presence of fractures or faults, and the integrity of existing and new wells.
It’s worth noting that these factors are interdependent. For instance, avoiding containment risks generally results in reducing the theoretical injection rate and capacity.
While technical criteria are common to all sites, nontechnical criteria can vary widely depending on geographic, political, legal, and economic conditions, among others. These include the presence or absence of regulations pertaining to CO2 emissions and CCUS, proximity to sensitive facilities or populated areas, and the ease of monitoring the storage complex (including groundwater and legacy wells).
It's because of this combination of various factors, availability of data, and early-stage uncertainties that a rigorous methodology and consistent workflow are required to reliably rank potential sites in the most objective manner. The question is, how can you achieve that?
The challenges of screening carbon storage sites the traditional way
CCUS site screening and selection is a multidomain decision-making exercise. And because human brains have difficulty thinking in multiple dimensions, attempting to generate consistent ranking using traditional methods—methods such as spreadsheet-based weighted score cards, which we subsequently discuss—only introduces subjectivity.
There are multiple challenges associated with the screening and ranking process. First, you must ensure that all relevant aspects are considered. This can be achieved by creating a comprehensive list of criteria based on existing regulations and directives, along with previous experience in the field.
These criteria must be considered in a holistic manner, as some can be interdependent. Not to mention that selecting your criteria depends heavily on the site itself and the data you have available. You must then assign each criterion a relative weight, given they don’t all have equal importance. This exercise becomes increasingly challenging as more criteria are added and a hierarchy is created. In our experience, we found that a typical site selection exercise needs 100–200 distinctive criteria. That’s a massive hierarchy!
Today, the most common method for site selection is spreadsheet-based weighted score cards. This manual process calculates raw evaluation scores using expert inputs or published information combined with the weight given to each criterion (including quantitative and qualitative ones). Users enter these numerical scores, along with their estimates of uncertainty, into a spreadsheet. In some cases, these uncertainty analyses are carried out by varying assigned candidate scores, a process which is prone to a high level of subjectivity.
Our suggestion? Take the steps necessary to adopt a more objective and accurate approach to site screening and selection.
The spreadsheet-based method relies heavily on experts’ input for both assigned scores and weighting factors, but it’s difficult to accurately assign consistent weight values for different criteria simultaneously (especially because they describe distinct aspects of a site).
In many circumstances, a given criterion has dependencies (subcriteria) that contribute to its overall impact—and the impact of other criteria as well. Injectivity, for example, depends on permeability, host rock thickness, pore pressure, and in situ stress. That same in situ stress also controls containment (a different criterion), making it infinitely harder to manually assess one criterion at a time.
It’s for this reason that other methodologies have been introduced for CCUS site screening and selection. Evidence support logic (ESL) is one of them and has been used to assess the suitability of an offshore CO2 storage site in a depleted gas field in the North Sea. But while ESL can handle multi-wise comparisons (i.e., comparing the same criteria across multiple sites), the truth is that you won't always have enough evidence to score a given criterion properly.
A method for objectively screening potential carbon storage sites
Let’s go back to what we mentioned earlier for a moment: CCUS site screening and selection is a multicriteria decision-making (MCDM) problem. Of all available MCDM methods, the analytical hierarchic process (AHP) and technique for order of preference by similarity to ideal solution (TOPSIS) are probably the most popular. Why? They’ve been successfully applied to other domains and industries to facilitate informed engineering decisions under uncertainty. And when it comes to engineering, a tech company is a tech company regardless of industry.
We propose integrating both the AHP and TOPSIS methods to build a new methodology for CCUS site screening and selection that incorporates uncertainty analysis. This proposed methodology can be summarized in two steps:
- Leverage AHP to determine the relative weight of each decision criterion. AHP can evaluate attributes both quantitatively and qualitatively and enables a simple, effective, and objective determination of relative weights by comparing attributes in pairs.
- Use the TOPSIS method to score and rank sites by comparing each weighted criterion simultaneously for all location options. At this point, Monte Carlo simulation is used for analyzing the uncertainty level of each site, while sensitivity analysis is provided to show how a site’s ranking would change if various inputs changed.
While still relatively new, this method has already been used to complete the industry’s first screening study for offshore carbon storage in the Nile Delta. The study evaluated and ranked 16 potential CO2 storage locations in the area, identifying a maximum estimated storage capacity of 622 million metric tons.
The same meticulous, science-based methodology was also used to successfully identify which licensing blocks in Trinidad and Tobago offer the best return on investment given the goals of the potential licensee (as presented by Holloway et al. at the 17th Greenhouse Gas Control Technology Conference in 2024). This included using 67 key criteria to evaluate potential sites based on a breadth of technical and nontechnical data.
The findings were compared with well-known carbon storage site characteristics from the Illinois Basin and the Utsira Formation in offshore Norway, after which the East Coast Marine Area (ECMA) was identified as the best region for carbon sequestration based on the selected criteria. ECMA ranked comparably to offshore Norway, indicating it has the potential to host a successfully operating CCUS site like the Sleipner project (the longest ongoing CO2 storage project in the world).
The best part? While this is a fit-for-purpose site selection methodology for CCUS projects, we’re confident it can be adapted and used to accurately screen and rank sites in adjacent industries such as geothermal, blue hydrogen, green hydrogen, and radioactive waste sites.
