What Is Carbon Capture and Storage?

Carbon capture and storage (CCS) is a set of technologies that capture carbon dioxide (CO2) from industrial sources or directly from the air, transport it and store it permanently underground so that it does not enter the atmosphere. Because CO2 is the main driver of human-caused climate change, and some industries (such as cement, steel and fertiliser production) are hard to decarbonise by switching fuels alone, CCS is considered by many scientific and policy bodies to be one of the tools needed alongside renewable energy, efficiency and nature-based solutions.

Related terms:

• CCUS: carbon capture, utilisation and storage, where some CO2 is used in products or processes.
• DACCS: direct air capture with carbon storage, which removes CO2 already in the atmosphere.
• BECCS: bioenergy with carbon capture and storage.

This article focuses on the geological storage part, which is where geoscience matters most. Whether CO2 stays underground for centuries depends on the rocks, the fluids, the pressures and the engineering of wells. It is a subject that needs geologists, hydrogeologists, geophysicists and reservoir engineers.

The CCS Chain: Capture, Transport, Storage

1. Capture. CO2 is separated from other gases at the source. Main approaches include:

• Post-combustion capture: removing CO2 from exhaust (flue) gas, often using chemical solvents.
• Pre-combustion capture: converting fuel to hydrogen and CO2 before burning, then separating the CO2.
• Oxy-fuel combustion: burning fuel in oxygen to produce a CO2-rich exhaust.
• Industrial process capture: from cement, steel, ammonia and hydrogen production, where CO2 streams can be relatively concentrated.
• Direct air capture (DAC): removing CO2 from ambient air using chemical sorbents or solvents. It is energy-intensive because CO2 is dilute in air (around 0.04 percent), so it relies on low-carbon energy and heat. Geothermal heat and power are attractive for DAC, which is one reason Kenya’s Rift Valley has drawn interest.
• Capture from geothermal gas: some geothermal fields release CO2, which can be captured and reinjected.

2. Transport. Captured CO2 is compressed and moved by pipeline, ship, truck or rail to the storage site. In some projects, capture and storage are at the same site, avoiding long transport.

3. Storage. CO2 is injected through wells into deep underground rock formations, typically more than 800 metres deep for storage as a dense fluid, where it is kept for the long term. The remainder of this article explains how that works.

Why Depth Matters: CO2 as a Supercritical Fluid

CO2 becomes a supercritical fluid at pressures above about 73.8 bar and temperatures above about 31 degrees Celsius. This happens at roughly 800 metres depth in typical conditions. In the supercritical state, CO2 behaves partly like a gas and partly like a liquid, with a density much higher than at the surface (but still lower than water). That higher density means a large mass of CO2 can be stored in a given pore volume, making deeper storage more efficient. Shallower storage would hold much less CO2 and could allow it to expand as a gas.

For this reason, geological storage targets are usually deeper than about 800 metres, in rock formations with the right properties, below a seal.

What Makes a Good Geological Storage Site?

Geologists look for a combination of properties, often summarised as capacity, injectivity and containment:

1. Capacity: enough pore space. The reservoir must have large volumes of porous rock to hold the CO2. Porosity (the fraction of void space) and thickness and areal extent determine how much CO2 can be stored.

2. Injectivity: the ability to inject at useful rates. Permeability, the ease with which fluids move through the rock, must be high enough that CO2 can be injected at the required rate without excessive pressure build-up. A high-porosity rock with very low permeability is difficult to inject into.

3. Containment: a good seal and a stable system. A low-permeability caprock (seal), such as shale, mudstone, salt or tight carbonate, must overlie the reservoir to stop CO2 rising buoyantly. The seal must be thick, laterally continuous and free of leaks through faults or old wells. The reservoir should also be in a geologically stable setting without active faults that could leak or cause damaging earthquakes.

4. Other factors.

• Pressure and temperature conditions suited to supercritical storage.
• Fluid chemistry: salinity, composition and reactivity affect trapping and potential rock reactions.
• Geomechanics: the rock’s strength and stress state determine how much pressure it can safely handle without fracturing.
• Absence of competing resources and uses, such as potable groundwater or valuable hydrocarbons, and sensitive land use above.
• Proximity to emitters and infrastructure.
• Regulation, land rights and public acceptance.

Types of Geological Storage Formations

1. Deep saline aquifers. Porous and permeable rocks (often sandstones or carbonates) filled with very salty water that is unsuitable for drinking or irrigation. They are widespread and have the largest storage potential globally. The Sleipner project in the North Sea, which has stored CO2 in a saline sandstone formation since 1996, is the best-known long-running example. Characterising saline aquifers requires data from wells and seismic surveys.

2. Depleted oil and gas fields. Reservoirs that have already held hydrocarbons for millions of years demonstrate the existence of a seal. They have abundant well data and infrastructure, but older wells may be potential leak paths and must be properly sealed. CO2 injection into oil fields is also used for enhanced oil recovery (EOR), which has a different climate balance depending on how the oil is used.

3. Unmineable coal seams. CO2 can adsorb onto coal, displacing methane, but coal seams usually have low permeability and can swell when CO2 is adsorbed, which reduces injectivity. This option is limited.

4. Basalt and other reactive rocks (mineral storage). Basalts and some ultramafic rocks contain minerals rich in calcium, magnesium and iron that react with dissolved CO2 to form stable carbonate minerals such as calcite, magnesite and siderite. In effect, the CO2 is turned into rock. The CarbFix project in Iceland injected CO2 dissolved in water into basalt and found that most of the injected CO2 turned into carbonate minerals within about two years, much faster than expected, and a pilot in Wallula, Washington State, also demonstrated storage in basalt. This pathway offers very secure storage, because mineralised CO2 cannot leak back as a gas, but it requires large quantities of water and suitable basalt with enough permeability. Kenya’s volcanic geology has attracted interest here, and we discuss it in our article on whether Kenya is suitable for geological carbon storage.

5. Other options. Salt caverns, deep ocean storage and others are less common or not used for CO2 storage at scale.

How CO2 Is Trapped Underground: Four Mechanisms

Once injected, CO2 is held in place by several mechanisms that operate over different timescales. Combining them improves long-term security.

1. Structural and stratigraphic trapping. Buoyant CO2 rises until it meets the caprock and accumulates beneath it, trapped by geological structures such as folds, faults or pinch-outs. This is the main mechanism in the early years.

2. Residual (capillary) trapping. As CO2 moves through pores, some is left behind as tiny disconnected droplets, held by capillary forces, like oil in a sponge. This immobilises a significant share of CO2 over time, especially if CO2 and water are injected alternately to enhance it.

3. Solubility trapping. CO2 dissolves in the formation water, forming slightly acidic, denser water that tends to sink rather than rise, which reduces leakage risk. This takes decades to centuries but is substantial.

4. Mineral trapping. Dissolved CO2 reacts with rock minerals to form solid carbonate minerals. In sandstones and carbonates, this is slow (hundreds to thousands of years), while in basalt it can be much faster. Mineral trapping is the most permanent.

Over time, the risk of leakage generally decreases as CO2 moves from mobile free-phase to trapped and dissolved forms, which is why project monitoring focuses on the early period.

Site Characterisation: How Geologists Evaluate a Storage Site

Characterisation is a staged process similar to exploration for oil, gas or geothermal energy.

Stage 1: Regional screening. Use geological maps, basin studies, existing wells and seismic data to identify sedimentary basins or rock formations with suitable depth, thickness and seal.

Stage 2: Site selection and detailed geology. Collect and interpret 2D and 3D seismic reflection data to map structure, reservoir and seal, faults and heterogeneity. Use well logs, cores and cuttings to measure porosity, permeability, mineralogy and pressure. Characterise the caprock’s thickness, sealing capacity and integrity. Assess faults and fractures, which can act as leakage paths or conduits, and map shallow aquifers that need protection.

Stage 3: Geomechanical and geochemical assessment. Estimate the in-situ stress and fracture pressure, so that injection pressures stay below levels that could fracture the seal or reactivate faults. Examine how CO2-rich fluids will react with the reservoir and caprock minerals.

Stage 4: Reservoir modelling and capacity estimation. Build static (geological) and dynamic (flow) numerical models to simulate how CO2 will move and be trapped, estimate storage capacity and predict pressure build-up and plume extent over decades to centuries. Uncertainty analysis is essential.

Stage 5: Risk assessment. Identify and rank possible leakage pathways (wells, faults, seals), pressure effects, induced seismicity, impacts on groundwater and surface environment, and design mitigation and contingency measures.

Stage 6: Appraisal drilling and injection tests. Drill and test wells to confirm the geological model, measure injectivity and verify seal integrity. Pilot injection may be done before full-scale operation.

Monitoring: How We Know the CO2 Stays Put

CCS projects follow measurement, monitoring and verification (MMV) plans:

• Seismic monitoring (time-lapse or “4D” seismic): repeating seismic surveys to image the CO2 plume’s movement.
• Well-based monitoring: pressure and temperature sensors, fluid sampling and well integrity logs.
• Groundwater and soil gas monitoring: detecting any CO2 migration towards the surface, in monitoring wells and soil.
• Microseismic monitoring: listening for small earthquakes caused by injection.
• Surface deformation monitoring: satellite radar (InSAR) and GNSS to detect ground uplift linked to pressure increase.
• Atmospheric monitoring near wells and potential leakage points.
• Tracers: chemical or isotopic tracers added to CO2 to follow its movement and distinguish it from natural CO2.

Monitoring continues through injection and for a period after closure, until regulators are satisfied that the plume is stable, then long-term responsibility is typically transferred under regulatory frameworks.

Risks and How They Are Managed

Leakage. The main concern is that CO2 could escape through poorly sealed wells, faults or fractures, or a failed caprock. Risk is managed through careful site selection, well design and abandonment, pressure management and monitoring. Studies of mature projects and natural analogues indicate that well-selected, well-managed sites can hold CO2 securely, but the risk is not zero and must be assessed site by site.

Induced seismicity. Injecting fluid raises pore pressure and can reactivate faults. Most induced events are small, but larger ones are possible in some conditions. Mitigation includes avoiding critically stressed faults, limiting pressure and monitoring.

Groundwater contamination. Leakage of CO2 or displaced brine into freshwater aquifers could degrade water quality. Characterising shallow aquifers and monitoring are essential. Hydrogeologists play a crucial role.

Pressure interference. Pressure build-up can affect neighbouring operations or wells, particularly in basins with multiple users.

Well integrity. Wells are the most likely leakage pathway. Cement and casing quality, corrosion resistance and proper plugging of old wells matter.

Impurities and corrosion. CO2 streams with water and impurities can be corrosive, requiring careful material choices.

Cost, energy and permanence. Capture is energy-intensive and costly, and projects need long-term liability and financial arrangements.

Public acceptance and governance. Transparent communication, community engagement, clear regulation and independent verification are needed for trust.

Costs and the Role of Policy

CCS costs depend heavily on the CO2 source (concentrated streams are cheaper to capture than dilute ones), transport distance, storage characteristics and scale. Direct air capture is generally more expensive per tonne than capture from concentrated industrial streams. Policy tools such as carbon pricing, tax credits, regulatory frameworks, and voluntary or compliance carbon markets influence which projects go ahead. Because costs and policies change quickly, consult current data for specific assessments.

CCS is generally viewed as a complement, not a substitute, for cutting emissions at source through renewable energy, efficiency and other measures.

Where Does Geology Fit in the Bigger Picture?

Geoscientists contribute at every stage:

• Regional basin analysis and site screening.
• Structural geology and fault characterisation.
• Sedimentology and reservoir characterisation.
• Geophysics (seismic, gravity, electrical and electromagnetic methods) for imaging and monitoring.
• Hydrogeology and groundwater protection.
• Geochemistry for reactions and mineral trapping.
• Geomechanics and induced seismicity assessment.
• GIS, remote sensing and data integration.
• Environmental and social impact assessment.

Developing countries with limited subsurface data need careful, step-by-step characterisation and capacity building to evaluate potential storage safely.

Frequently Asked Questions

Is CO2 stored underground safe? With careful site selection, well design, monitoring and regulation, geological storage can be secure, based on decades of experience from projects and natural CO2 reservoirs. Risks exist and must be assessed and managed for each site.

How long does CO2 stay underground? The goal is permanent storage over thousands of years. Trapping becomes more secure over time as CO2 dissolves and mineralises.

What is the difference between CCS and direct air capture? CCS typically captures CO2 from point sources such as power plants and factories. Direct air capture removes CO2 from the atmosphere. Both can use geological storage.

Can CO2 be stored in basalt? Yes. Basalt can convert CO2 into carbonate minerals quickly, as shown in pilot projects, but it needs water and suitable permeability.

Does CCS cause earthquakes? Injection can cause small induced earthquakes if it activates faults. Site selection, pressure management and monitoring reduce the risk.

Is CCS a replacement for renewables? No. It is a complementary tool, especially for hard-to-abate industrial emissions and carbon removal.

Conclusion

Geological carbon storage relies on well-understood geology: porous reservoirs to hold CO2, seals to contain it, trapping mechanisms that make it more secure with time, and monitoring to verify performance. Successful storage depends on rigorous site characterisation, careful engineering, transparent regulation and community trust. As more countries explore CCS, local geological knowledge and skilled geoscientists will be essential.

For geological, hydrogeological, structural and GIS support on resource and climate-related projects, contact Reg. Geol. Mbugua Githae (MGSK) via consult.innercoregeoconsultants.co.ke.