Carbon Capture Utilisation and Storage (CCUS) plays a vital role in global climate change mitigation efforts, particularly in reducing hard-to-abate industrial emissions. The success of these projects hinges not only on accurate storage capacity assessment but also on building public confidence in the long-term safety of underground CO2 storage.
The deployment of CCUS solutions depends on a precise understanding of storage capacity, which requires a comprehensive evaluation of geological formations. This complex assessment process underpins critical decisions about project viability, regulatory compliance, and long-term safety. Engineers and scientists increasingly rely on modelling techniques to evaluate potential storage sites, considering not only the basic volumetric capacity but also the formation’s ability to contain CO₂ over extended periods safely.
When evaluating storage potential, experts employ a hierarchical assessment framework that progresses from theoretical possibilities to practical realities. This systematic approach, commonly represented as a storage capacity pyramid, helps stakeholders understand the different levels of certainty in storage estimates.
The assessment begins with theoretical capacity – the maximum amount of CO₂ that could fit in the geological formation under perfect conditions. This estimate is then refined to effective capacity by applying real-world geological and engineering limitations. The practical capacity further narrows this down by considering economic factors, infrastructure needs, and regulations. Finally, matched capacity provides the most realistic estimate by aligning the CO₂ source with the storage site’s specific capabilities.
Storage site selection typically focuses on two distinct geological systems, each offering unique advantages for CO₂ containment. Open systems, such as saline aquifers, allow CO₂ to migrate within vast underground formations while managing pressure distribution. Closed systems, including depleted oil and gas fields, provide proven containment structures with well-understood geological seals.
For saline aquifer formations (open systems), engineers assess storage potential through a systematic methodology developed by the U.S. Department of Energy (DOE). This approach considers formation characteristics, CO₂ density under reservoir conditions, and storage efficiency factors to calculate storage capacity.
The storage efficiency calculation is more complex than a single factor (GCO₂ = Aₖhₛϕₜₒₜρξsaline). It encompasses the fraction of geological area available for storage, the accessible portion of formation thickness, the effective porosity that can store CO₂, the volume that CO₂ can contact around injection wells, and the pore space limitations due to immobile fluids. Each of these components plays a crucial role in determining practical storage volumes.
The storage efficiency calculation incorporates the following five critical components that determine the practical storage volume:
- Net-to-total area: This component evaluates the usable portion of the geological formation for CO₂ storage. It accounts for structural compartmentalisation, fault zones, and heterogeneities that may limit storage potential. The calculation considers areas with suitable reservoir properties while excluding regions compromised by geological barriers or uncertainty in reservoir characterisation,
- Net-to-gross thickness: The assessment of formation thickness focuses on identifying intervals with appropriate storage characteristics. This involves detailed stratigraphic analysis to determine productive zones within the total formation thickness, excluding shale breaks, tight zones, or other non-reservoir quality intervals that would impede CO₂ storage,
- Effective-to-total porosity: The reservoir’s storage capacity is directly influenced by its effective porosity. This factor accounts for the connected pore network available for CO₂ storage, excluding isolated pores and microporosity that cannot contribute to storage capacity. The calculation considers variations in pore connectivity and their impact on CO₂ migration and trapping mechanisms,
- Volumetric displacement efficiency: This parameter quantifies the reservoir volume that can be effectively contacted by injected CO₂. It considers the impact of reservoir heterogeneity, flow patterns, and injection well placement on CO₂ distribution. The efficiency factor accounts for preferential flow paths and potential bypass zones that affect storage utilisation,
- Microscopic displacement efficiency: At the pore scale, not all space is available for CO₂ storage due to residual fluid saturation and rock-fluid interactions. This efficiency factor accounts for irreducible water saturation, trapped gas, and capillary effects that limit the pore volume available for CO₂ storage. Understanding these microscopic mechanisms is crucial for accurate storage capacity estimation.
In closed systems, such as depleted hydrocarbon fields, the assessment approach differs significantly. These sites offer natural advantages through proven seal integrity and existing infrastructure. Storage capacity calculations build upon historical data, considering original reservoir conditions, cumulative hydrocarbon production, and remaining void space.
The analysis, in these contexts, converts available pore space to potential CO₂ storage volume while accounting for:
- Structural traps: Geological formations play a crucial role in CO₂ containment through various trapping mechanisms. These include anticlines that form dome-like structures, fault blocks created by tectonic activity, and stratigraphic pinch-outs where permeable layers terminate against impermeable rocks. These natural configurations create secure zones for long-term CO₂ storage by preventing vertical and lateral migration,
- Formation pressure limits: The management of reservoir pressure is essential for safe CO₂ storage operations. This involves determining maximum allowable pressure thresholds that maintain formation integrity while preventing fracturing or reactivation of existing faults. The limits are established through detailed geomechanical studies and consider both initial reservoir conditions and projected pressure evolution during injection operations,
- Rock and fluid properties: The interaction between stored CO₂ and the reservoir environment is governed by multiple physical parameters. These include the rock’s porosity which determines storage volume, permeability that controls flow rates, wettability affecting fluid distribution, and various fluid-rock interactions that influence long-term storage stability. Understanding these properties is crucial for accurate storage capacity calculations and injection strategy design,
- Safety margins for long-term containment: a comprehensive approach to ensure secure storage requires careful evaluation of caprock integrity and establishment of buffer zones around injection points. This includes maintaining minimum distances from active faults, implementing robust monitoring systems, developing pressure management strategies, and verifying secondary containment mechanisms. These safety measures form an integrated framework that supports the long-term stability and security of CO₂ storage operations.
This comprehensive approach to storage assessment provides the foundation for developing large-scale CCUS projects. As monitoring technology and modelling capabilities advance, these estimates continue to become more precise, supporting both technical planning and policy decisions.
Ad Terra has conducted storage assessment studies across multiple sites:
Site A in France
Ad Terra Consultancy executed a comprehensive CO₂ storage assessment programme examining 31 depleted field concessions in France. The methodology integrated technical screening of storage capacity and injectivity parameters with economic and siting considerations, using sophisticated ranking matrices and spider charts to visualise candidate performance. The assessment delivered a road map pilot project selection strategy through detailed timeline planning and cost analysis benchmarked against the Lacq pilot project. This systematic approach enabled the client to make informed investment decisions based on optimised storage capacity and ranking scores.
Site B in Denmark
Ad Terra Consultancy implemented a four-step storage assessment for onshore CO₂ storage potential in Denmark. The programme began with thorough subsurface risk analysis covering reservoir integrity, seal integrity, CO₂ migration, and induced seismicity. Our evaluation of the targeted formation identified 14.5 MT CO₂ storage capacity distributed across crest and flank structures. The assessment incorporated detailed seal integrity analysis through integrated interpretation of logging data, seismic inversion, and fault studies. The project established a practical 2-year timeline from Final Investment Decision to Commercial Operation Date, complete with defined exit gates and contingency planning for three development scenarios.
Site C in Greece
Ad Terra Consultancy conducted an integrated assessment onshore Greece (gas accumulation), evaluating its potential for combined natural gas production and CO₂-Plume Geothermal development. The analysis determined technical recoverable volumes of 26 bscf and assessed the feasibility of both CCS and CPG applications. Through comprehensive geological and technical evaluation, our team concluded that the reservoir characteristics presented significant challenges for commercial development of combined energy transition projects, providing crucial insights for the client’s strategic planning.
The natural geological barriers beneath us serve as powerful climate allies in our action to remove hard-to-abate industrial emissions. Storage assessment methodology, supported by rapid advances in modelling and monitoring technology, demonstrates a remarkable promise in identifying and validating suitable storage locations. These underground storage solutions represent a vital component in building a sustainable future as we accelerate our response to climate change, backed by rigorous scientific assessment and cutting-edge technology.
About this article series
This article series highlights the role of Carbon Capture, Utilisation, and Storage (CCUS) in mitigating climate change and supporting the global energy transition. Each step of the process is examined, from CO2 capture technologies, transport challenges, utilisation opportunities, and geological storage options through caprock integrity considerations, operations monitoring and safety to commercialisation schemes. This series provides insights into how CCUS technologies can contribute to a sustainable future.
About Federico Games
Federico Games, Ad Terra’s Head of CCUS, is a recognised expert with over 20 years of international experience in technical, commercial, and strategic energy projects. He plays an active role in the United Nations CCUS Initiatives, serves as an expert on the EU CCUS Working Group, where he collaborates on Europe’s Industrial Carbon Management Strategy, and is the Global CCUS Director at the Society of Petroleum Engineers. He is also a member of the CO2-Plume Geothermal Research Consortium led by ETH Zurich, which explores innovative CCUS-geothermal integration. Federico’s academic background includes an MBA in renewable energy and circular economy (University of Bradford), an MSc in Reservoir Geoscience and Engineering (IFPEN, France), and a specialisation in Environmental Studies (University of Poitiers, France).
