In deep geological formations, pressure and temperature can bring CO₂ to a supercritical state, typically from a depth of about 800 to 1,000 metres. [5] This state appears when CO₂ exceeds its critical temperature of 31.1 °C and its critical pressure of roughly 74 bar. [1]
Supercritical CO₂ then exhibits a high density, a low viscosity and a diffusivity that falls between that of gases and liquids. [1] These properties explain its value for geological carbon storage, as well as for certain industrial processes, thermodynamic cycles and deep geothermal energy.
Understanding how it behaves therefore sheds light on the mechanisms that support its storage, its movement through the subsurface and the technical constraints tied to its use.
The Physics of the Critical Point
Supercritical CO₂ is not simply carbon dioxide under heavy compression. It is a distinct thermodynamic state that emerges when temperature and pressure both rise above specific values, whose combination defines the critical point. Beyond this threshold, the usual distinction between gas and liquid vanishes and CO₂ forms a single homogeneous phase.
This transformation accounts for several of its physical properties. Its density becomes relatively high, while its viscosity stays low and its diffusivity sits between that of a gas and that of a liquid. [1] These characteristics help clarify how it behaves in industrial processes as well as in deep geological formations.
The Critical Point of CO₂: Definition and Values
The critical point refers to the combination of temperature and pressure above which there is no longer a clear separation between the liquid and gaseous phases. For CO₂, the critical values are as follows: [1]
Critical temperature: 304 K, or about 31.1 °C
Critical pressure: 7.4 MPa, or about 74 bar
These conditions are relatively easy to reach compared with those required for some other supercritical fluids, which makes CO₂ easier to use in a range of technical processes. [3] CO₂ is also non-flammable and has low toxicity, two traits that add to its appeal in several industrial applications. [3]
What Happens Beyond the Critical Point?
When conditions exceed the critical point of CO₂, the distinction between the liquid and gaseous phases disappears. Below the critical point, a substance can shift from one phase to the other and display a distinct interface between liquid and vapour. Above this threshold, that liquid-vapour transition no longer exists. CO₂ is then in a single homogeneous phase. [3]
This state does not mean, however, that the fluid acts exactly like a liquid or a gas. Instead, it blends several properties associated with both:
a relatively high density, comparable to that observed in liquids;
a low viscosity, more typical of gases;
an intermediate diffusivity, greater than that of many conventional liquids. [1]
This combination directly affects how CO₂ flows and how mass is exchanged. In particular, it explains why the supercritical fluid is valued in extraction, separation and transport processes through certain porous media. [1]
In the context of geological CO₂ storage, one clarification matters, though. Even if a homogeneous supercritical phase has no internal liquid-gas interface, the injected CO₂ generally coexists with water or brine in the rock pores. An interface then forms between these fluids, and capillary forces continue to play a key role in how they move and in how CO₂ becomes trapped. [5]
Physical Properties That Explain Its Uses
The characteristics of supercritical CO₂ largely determine the applications in which it can be employed. Three properties stand out:
High density: it allows a large quantity of CO₂ to be held in a smaller volume than in the gaseous state, a useful feature for geological storage. [1]
Low viscosity: it promotes the fluid's mobility and its flow through various systems, notably porous media. [1]
Relatively high diffusivity: it eases mass transfer and contributes to the efficiency of several extraction and separation processes. [1]
These properties should not, however, be viewed in isolation. In a geological reservoir, for example, the behaviour of CO₂ also depends on the porosity and permeability of the rock, the reservoir pressure, the presence of brine and the interactions between the different phases. [2] [5]
In industrial processes, the pairing of low viscosity with a diffusivity higher than that of many liquid solvents facilitates mass transfer. [1] This feature supports the use of supercritical CO₂ in fields such as compound extraction, supercritical fluid chromatography and certain separation processes. [1]
Grasping these properties thus provides a useful foundation for the applications covered in the following sections, including geological carbon storage, industrial processes and certain energy systems.
Geological Conditions: When the Subsurface Creates the Supercritical State
Underground, pressure and temperature generally increase with depth. In the geological formations targeted for carbon storage, these conditions usually allow CO₂ to remain in a dense or supercritical state from roughly 800 to 1,000 metres down. [5] This change directly influences its density, its volume and its mobility within the rock pores.
A Depth of About 800 to 1,000 Metres as a Benchmark
A depth of 800 metres is often used as a rule of thumb for geological CO₂ storage. The U.S. Department of Energy notes that at around 800 metres or more, pressure and temperature can keep the injected CO₂ in a dense, supercritical form. [2] The IPCC likewise places this typical transition at around 800 to 1,000 metres, depending on the geothermal conditions of the site. [5]
At these depths, the density of CO₂ rises sharply and becomes comparable to that of a liquid. [5] This characteristic makes it possible to consider injecting it into various types of geological formations with suitable properties, including:
deep saline formations;
certain depleted hydrocarbon reservoirs;
certain geological formations rich in organic matter. [2]
Depth is not the only criterion, however. Local temperature, reservoir pressure, and the porosity and permeability of the rock also shape storage conditions. [2]
High Density That Reduces the Volume Occupied
The shift to a dense or supercritical state leads to a sharp increase in CO₂ density. For the same mass of carbon dioxide, this increase translates into a significant reduction in specific volume compared with gaseous CO₂ at surface conditions. [5] This behaviour helps make storage in the pores of deep formations far more efficient in volumetric terms.
This high density does not mean, however, that CO₂ becomes denser than the fluids already present in most storage reservoirs. Supercritical CO₂ generally remains lighter than the brine filling the rock pores. It therefore tends to migrate upward due to buoyancy. [2]
This migration is why a sufficiently impermeable caprock above the reservoir is so important. It acts as a geological barrier that restricts the vertical movement of CO₂ and contributes to the long-term containment of the stored volume. [2]
Table 1: General Evolution of CO₂ Behaviour with Depth
| Indicative depth | CO₂ behaviour | Implication for storage |
|---|---|---|
| Shallow depth | State strongly dependent on local temperature and pressure | Density generally less favourable for storing large quantities |
| About 800 to 1,000 m and deeper | Dense or supercritical state generally reached in storage formations | Higher density and greatly reduced specific volume |
| Deep reservoir beneath caprock | CO₂ generally less dense than brine | Upward migration controlled by containment mechanisms |
Sources: [2] [5]
Injection and Trapping: How Supercritical CO₂ Is Immobilized
Geological CO₂ storage does not involve filling an underground cavity. The fluid is injected into the pores of a rock formation, where its distribution depends in particular on porosity, permeability, the fluids already present and the properties of the reservoir. [2] Once injected, several mechanisms gradually help limit its mobility and strengthen its containment.
CO₂ Mobility in Rock Pores
In a formation suited to storage, supercritical CO₂ can travel through the rock's pore network. Its low viscosity and mass transfer properties influence its interactions with the water present in the porous medium. [4]
High-pressure microfluidic experiments, carried out in channels with a hydraulic diameter of about 50 µm, showed that CO₂ bubbles or droplets shrink rapidly at the start of their flow, as matter transfers into the water. [4] This pore-scale work offers a better understanding of certain transfer mechanisms between CO₂ and water, although on its own it cannot reproduce the full complexity of a geological reservoir.
At the scale of the storage site, the properties of the formation remain decisive. Sufficient porosity provides an adequate pore volume for storage, while appropriate permeability allows CO₂ to flow through the rock during injection. [2]
Four Key Trapping Mechanisms
After injection, CO₂ does not necessarily remain as a fully mobile plume. In deep saline formations, several processes contribute to its geological trapping and progressively reduce its ability to move. [5]
Four mechanisms are particularly important:
Structural trapping: CO₂ rises through buoyancy until it meets a sufficiently impermeable caprock, which limits its vertical migration.
Residual trapping: part of the CO₂ remains immobilized in discontinuous form within the rock pores once the plume has passed through.
Dissolution: CO₂ gradually dissolves into the fluids present in the formation, especially brine.
Mineral trapping: dissolved CO₂ can take part in geochemical reactions with the formation's minerals and, under certain conditions, lead to the formation of carbonate minerals.
Table 2: Main CO₂ Trapping Mechanisms
| Mechanism | Principle |
|---|---|
| Structural and stratigraphic | Containment of CO₂ beneath a low-permeability caprock |
| Residual | Immobilization of CO₂ fractions within the rock pores |
| Dissolution | Dissolution of CO₂ into formation fluids |
| Mineral | Gradual incorporation of carbon into carbonate minerals |
These mechanisms do not necessarily operate in isolation. Their relative importance shifts according to the geological characteristics of the reservoir, the properties of the fluids and the time elapsed since injection. [5] Together, they explain how a growing share of the CO₂ can become less mobile within the formation.
Containment does not rely solely on the reservoir's natural mechanisms, however. The behaviour of the CO₂ plume, the pressure and the integrity of the site must also be monitored during and after injection.
Industrial and Energy Applications of Supercritical CO₂
The properties of supercritical CO₂ allow it to be used in very different fields. Some applications, particularly as an extraction solvent, have been established for several decades. Others, such as supercritical CO₂ thermodynamic cycles and certain geothermal systems, are still undergoing development and optimization.
Established Industrial Applications and Emerging Processes
Supercritical CO₂ is already used in several industrial processes, notably for decaffeination and the extraction of plant compounds. Other applications, such as certain methods for producing micro- and nanoparticles, are still the subject of research and scale-up efforts. [1]
Documented applications include:
decaffeination of coffee and tea;
extraction of essential oils and other plant-derived compounds;
supercritical fluid chromatography;
production of micro- and nanoparticles;
certain impregnation, polymer foaming, drying and cleaning processes. [1]
Another advantage of CO₂ in extraction processes lies in how easily it separates from the treated product. Lowering the pressure causes the fluid to lose its solvent power, which simplifies its recovery once the process is complete. [3]
Power Generation: CO₂ as a Working Fluid
Supercritical CO₂ can also serve as a working fluid in thermodynamic cycles designed to convert heat into electricity. In these systems, the fluid circulates in a closed loop between various components, including heat exchangers, compressors and turbines.
The U.S. Department of Energy points out that supercritical CO₂ cycles offer the potential for higher thermal efficiency and lower capital costs than certain benchmark steam cycles. [6] That performance nonetheless depends on the system configuration, the temperature of the heat source and the operating conditions.
This technology can be paired with several heat sources, including:
nuclear energy;
geothermal heat;
certain concentrated solar power plants;
industrial waste heat recovery. [6]
These cycles nevertheless remain a developing technology compared with the conventional steam systems that are widely deployed.
Geothermal Energy: Using CO₂ to Carry Heat
Supercritical CO₂ in geothermal energy is also being researched, particularly in Enhanced Geothermal Systems (EGS). In this type of system, a fluid circulates through deep, hot rock to extract its heat. Supercritical CO₂ can be considered as an alternative to water for this circulation. [7]
Its behaviour depends heavily on reservoir conditions. A study dedicated to CO₂-based EGS shows, for instance, that higher circulation flow rates may be needed to achieve a heat extraction rate similar to that of a water-based system. [7] The potential appeal of CO₂ therefore rests not only on its ability to carry heat, but also on its hydrodynamic behaviour and on the possibility that part of the fluid stays stored in the rock formation. [7]
In some deep volcanic reservoirs, temperature and pressure conditions can also push water into a supercritical state, above roughly 374 °C and 22.1 MPa. [3] Modelling work suggests that, under these particular conditions, CO₂ may become denser than water and tend to sink rather than rise. [8] This behaviour thus differs from what is observed in conventional carbon storage reservoirs, where CO₂ is generally less dense than brine.
Table 3: Selected Applications of Supercritical CO₂
| Sector | Application | Main property or benefit |
|---|---|---|
| Food processing | Decaffeination, extraction of plant compounds | Solvent power and easy separation after depressurization ¹ ³ |
| Industry | Chromatography, particles, impregnation, drying | Low viscosity and high mass transfer ¹ |
| Power generation | sCO₂ thermodynamic cycles | Potential for high thermal efficiency and more compact equipment ⁶ |
| Geothermal energy | Circulation fluid in certain EGS | Heat transport and distinctive hydrodynamic behaviour ⁷ |
| Geological storage | Injection into deep formations | High density at great depth ⁵ |
These applications draw on the same fundamental properties of the fluid, yet they serve very different goals. In industry, supercritical CO₂ is mainly sought after for its solvent and mass transfer qualities. In energy and geological systems, it is rather its density, its mobility and its behaviour under high pressure that make it attractive.
Key Takeaways on Supercritical CO₂
Supercritical CO₂ forms when carbon dioxide simultaneously exceeds its critical temperature of 31.1 °C and its critical pressure of about 74 bar. [1] In deep geological formations, these conditions are generally encountered from around 800 to 1,000 metres, depending on the characteristics of the site. [5]
In this state, CO₂ combines a high density, a low viscosity and distinctive mass transfer properties. [1] These characteristics explain its relevance in fields as varied as geological carbon storage, industrial extraction, certain power generation cycles and deep geothermal energy.
Underground, however, its behaviour depends as much on its own physical properties as on those of the reservoir. Porosity, permeability, the presence of brine, the caprock and trapping mechanisms all govern how CO₂ moves and stays contained. [2] [5] Understanding these interactions makes it easier to assess the opportunities and constraints linked to the various applications of supercritical CO₂.
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Summary of Key Points
CO₂ reaches its supercritical state above 31.1 °C and about 74 bar (7.4 MPa). [1] In deep geological formations, these conditions are generally met from around 800 to 1,000 metres, depending on site characteristics. [5]
In this state, supercritical CO₂ has a high density, a low viscosity and a diffusivity intermediate between that of gases and liquids. [1]
At depth, the increase in CO₂ density sharply reduces its specific volume compared with its gaseous state at surface conditions, which favours its storage in geological formations. [5]
In deep saline formations, four mechanisms play a particularly important role in CO₂ containment: structural trapping, residual trapping, dissolution and mineral trapping. [5]
Beyond storage, supercritical CO₂ is used as a solvent in several industrial processes [1] [3], and is being studied as a working fluid in thermodynamic cycles [6] as well as a circulation fluid in certain geothermal systems. [7]
References
[1] Yang, Junyi, and Peichun Amy Tsai. "Microfluidic Supercritical CO₂ Applications: Solvent Extraction, Nanoparticle Synthesis, and Chemical Reaction." Biomicrofluidics, vol. 18, no. 5, 2024, 051301. AIP Publishing, https://doi.org/10.1063/5.0215567.
[2] U.S. Department of Energy, Office of Fossil Energy and Carbon Management. Reliable Carbon Transport and Storage: Roles for Artificial Intelligence in Support of FECM RDD&D Priorities. Mar. 2023, https://www.energy.gov/sites/default/files/2023-03/ai-role-in-reliable-carbon-transport-and-storage.pdf.
[3] Escobar, Estephanie Laura Nottar, et al. "Supercritical Fluids: A Promising Technique for Biomass Pretreatment and Fractionation." Frontiers in Bioengineering and Biotechnology, vol. 8, 2020, 252. Frontiers Media, https://doi.org/10.3389/fbioe.2020.00252.
[4] Ho, Tsai-Hsing Martin, Junyi Yang, and Peichun Amy Tsai. "Microfluidic Mass Transfer of CO₂ at Elevated Pressures: Implications for Carbon Storage in Deep Saline Aquifers." Lab on a Chip, vol. 21, 2021, pp. 3942–3951. Royal Society of Chemistry, https://doi.org/10.1039/D1LC00106J.
[5] Benson, Sally M., and Peter Cook. "Underground Geological Storage." IPCC Special Report on Carbon Dioxide Capture and Storage, edited by Bert Metz et al., Cambridge UP, 2005, pp. 195–276. Intergovernmental Panel on Climate Change, https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_chapter5-1.pdf.
[6] U.S. Department of Energy. "SCO2 Power Cycles." Energy.gov, https://www.energy.gov/sco2-power-cycles.
[7] Gładysz, Paweł, et al. "Process Modeling and Optimization of Supercritical Carbon Dioxide-Enhanced Geothermal Systems in Poland." Energies, vol. 17, no. 15, 2024, 3769. MDPI, https://doi.org/10.3390/en17153769.
[8] Parisio, Francesco, and Victor Vilarrasa. "Sinking CO₂ in Supercritical Reservoirs." Geophysical Research Letters, vol. 47, no. 23, 2020, e2020GL090456. American Geophysical Union, https://doi.org/10.1029/2020GL090456.
