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Capturing carbon dioxide is only half the challenge. Once CO2 is captured from an industrial source, it has to go somewhere — permanently, safely, and verifiably. That “somewhere” is almost always underground, which means the success of any carbon capture and storage (CCS) project ultimately hinges on geology as much as it does on capture technology. Understanding subsurface conditions, containment integrity, and long-term monitoring requirements is what separates a credible storage project from a costly and potentially risky one.
This article looks at the geoscience underpinning carbon capture and storage, what a storage site assessment involves, and why this work has become increasingly central to climate strategy across industry and government.
What Is Carbon Capture and Storage?
CCS refers to the process of capturing carbon dioxide emissions from industrial sources — power generation, cement production, steel manufacturing, natural gas processing — and permanently storing it underground rather than releasing it into the atmosphere. A related approach, carbon capture, utilization, and storage (CCUS), captures CO2 for use in industrial processes such as enhanced oil recovery before eventual storage, while direct air capture (DAC) removes CO2 directly from ambient air rather than from a point-source emitter.
Regardless of the capture method, the storage component relies on injecting CO2 deep underground into geological formations capable of holding it securely for hundreds to thousands of years — a requirement that places geoscience at the center of any credible CCS project.
Where CO2 Gets Stored
Depleted Oil and Gas Reservoirs
Depleted hydrocarbon reservoirs are attractive storage targets because their geology is already well understood — decades of exploration and production data typically exist, reducing subsurface uncertainty. These reservoirs have also demonstrated, by definition, that they were capable of trapping buoyant fluids for geological timescales, which is a strong positive indicator for CO2 containment.
Saline Aquifers
Deep saline formations — porous rock layers saturated with brine far too saline for drinking water or agricultural use — represent by far the largest potential CO2 storage capacity globally. Unlike depleted reservoirs, saline aquifers often require more extensive site-specific characterization, since they typically lack the decades of production data available for hydrocarbon reservoirs.
Basalt Formations
Basaltic rock offers a fundamentally different storage mechanism: rather than simply trapping CO2 as a buoyant fluid beneath a caprock, CO2 injected into basalt can react chemically with the rock to form stable carbonate minerals — effectively converting the CO2 into solid rock over a period of months to a few years in favorable conditions. This mineralization pathway offers a compelling permanence argument, though basalt storage projects are generally less mature and less widely deployed than saline aquifer or depleted reservoir storage.
Unmineable Coal Seams
Deep coal seams too thin or deep to mine economically can adsorb CO2 onto the coal matrix, sometimes displacing methane that can itself be recovered as a valuable byproduct. This pathway is less commonly pursued at scale compared to saline aquifer and depleted reservoir storage but remains relevant in certain geological settings.
Core Geoscience Components of a Storage Site Assessment
Reservoir Characterization
Understanding the storage reservoir itself is fundamental: porosity and permeability, which determine how much CO2 can be stored and how easily it can be injected; reservoir thickness and lateral extent, which determine total storage capacity; and geological heterogeneity, since variations in rock properties across the reservoir affect how CO2 plumes migrate once injected.
Caprock and Containment Assessment
The caprock — an impermeable or low-permeability layer overlying the storage reservoir — is what prevents injected CO2 from migrating upward toward the surface or into shallower groundwater. Assessing caprock integrity involves evaluating its thickness, lateral continuity, permeability, and capillary entry pressure (the pressure required to force CO2 through the caprock’s pore network), along with a careful review of any faults or fractures that could compromise containment.
Structural and Fault Analysis
Faults represent one of the most significant risk factors in CO2 storage projects, since they can potentially provide pathways for CO2 migration if not properly characterized and avoided or managed. Structural geology assessment identifies fault locations, evaluates whether they’re likely to be sealing or open, and informs injection well placement and pressure management strategies designed to avoid reactivating fault movement.
Geomechanical Assessment
Injecting large volumes of CO2 underground changes subsurface pressure conditions, which can affect rock stress states in ways that matter for both containment and induced seismicity risk. Geomechanical modeling assesses how the reservoir and surrounding rock will respond to injection over the project’s operational life, helping define safe injection rates and pressures that avoid fracturing the caprock or reactivating faults.
Hydrogeological Assessment
Where storage reservoirs are located near or connected to freshwater aquifers, hydrogeological assessment evaluates the risk of CO2 or displaced brine migrating into groundwater resources, and informs monitoring well placement designed to detect any such migration early.
Site Screening and Selection
Before detailed characterization begins, broader site screening typically uses regional geological data, existing well records, and seismic survey data to identify and rank candidate storage sites based on capacity, containment likelihood, proximity to CO2 sources, and access to existing infrastructure such as pipelines or depleted wells that could be repurposed.
Monitoring, Verification, and Reporting
Long-term monitoring is a defining feature of CCS projects, both for regulatory compliance and to build public and stakeholder confidence that stored CO2 remains safely contained. Common monitoring approaches include:
- Seismic monitoring, using repeat 3D or 4D seismic surveys to track how the CO2 plume migrates through the reservoir over time.
- Pressure and temperature monitoring in observation wells, providing direct data on reservoir behavior during and after injection.
- InSAR surface deformation monitoring, since CO2 injection can produce subtle surface uplift detectable from satellite radar data, offering a cost-effective way to monitor large storage sites remotely alongside ground-based instrumentation.
- Groundwater monitoring, to confirm that shallow aquifers remain unaffected by injection activity.
- Soil gas and atmospheric monitoring, providing a final line of detection in the unlikely event that CO2 were to migrate all the way to the surface.
This monitoring data feeds into measurement, reporting, and verification (MRV) frameworks required by most regulatory regimes, providing the ongoing evidence base needed to demonstrate that stored CO2 is behaving as predicted.
Why Geological Rigor Is Non-Negotiable
CCS projects carry a unique burden of proof compared to many other infrastructure projects: they need to demonstrate not just that they will work, but that they will continue working — safely and verifiably — for centuries. This makes thorough, conservative geological characterization essential from the earliest stages of site selection through to long-term post-closure monitoring.
Underestimating geological risk at any stage — overlooking a sealing fault’s true transmissivity, underestimating caprock heterogeneity, or basing capacity estimates on insufficiently dense subsurface data — can undermine project viability, regulatory approval, or public confidence, sometimes long after significant capital has already been committed.
The Growing Role of Geoconsulting in CCS
As CCS moves from a relatively niche technology to a mainstream component of industrial decarbonization strategy, demand for rigorous, independent geological assessment has grown substantially alongside it. Site screening, storage capacity estimation, containment risk assessment, geomechanical modeling, and long-term monitoring program design all require genuine subsurface expertise — not just familiarity with CCS as a concept, but deep, region-specific geological knowledge of the formations being considered for storage.
Evaluating a potential CO2 storage site or developing a CCS project’s geological assessment and monitoring program? Our geoscience team can help you build the subsurface case your project needs. Get in touch to discuss your site.
