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Geological Storage

Geological storage injects compressed CO2 into porous formation at 800–2,500 metres depth, where an impermeable seal holds it in place. The strongest field evidence in the sector sits here: Sleipner has stored more than 19 million tonnes since 1996 with no leakage detected across nearly thirty years of continuous monitoring.

One of eight durable carbon removal pathways presented by CDR Türkiye, all intended to store carbon for centuries to millennia.

Overview

Open a heading to read the detail.

  1. The last link in the chain Geological storage is not a removal pathway but the last link in a removal chain.

    It is the shared outlet for DAC, BECCS and BiCRS: captured CO2 is compressed into a dense, liquid-like state, moved to site by pipeline or ship, injected at 800–2,500 metres depth, and trapped beneath an impermeable seal.

  2. The evidence base The evidence base is solid.

    Sleipner has stored more than 19 million tonnes since 1996, and seismic surveys repeated over nearly thirty years have detected no leakage; the IPCC finds it likely that 99% or more is retained for 1,000 years. Cumulative storage from 1996 to 2024 reached 384.6 Mt. Separately, 77 commercial CCS facilities were operating as of July 2025, with 64 Mt a year of capture capacity. One caveat: the cumulative total includes CO2 injected to increase oil production.

  3. The binding constraint The binding constraint is not pore volume but the rate at which storage can be appraised, permitted and drilled.

    The global catalogue records 14,256 Gt of storage resource: 13,579 Gt undiscovered, 675 Gt sub-commercial and only 1.8 Gt commercial, with commercial resource only in Australia, Canada, Norway and the United States. Typically only about 2% of the rock’s pore volume can actually be filled with CO2, which is where the gap between theoretical and usable capacity comes from. Cost runs from a few dollars to under $25/t onshore and roughly $5–55/t offshore.

  4. Türkiye: Batı Raman experience Türkiye has subsurface CO2 operating experience: the Batı Raman field has taken roughly 1 Mt/yr of injection since 1986.

    But that operation uses naturally occurring CO2 to increase oil production; it is not capture, not removal, and no monitoring programme to carbon storage standards has been made public. About 108 Mt of capacity has been calculated for oil and gas fields. For saline aquifers there are only formation-scale academic estimates, and Türkiye has no national CO2 storage atlas. This is the country’s weakest link.

Bochi do Amarante et al., Advances in Geosciences IPCC AR6 WG3, Ch. 12 London Register of Subsurface CO2 Storage, 2025 Global CCS Institute, Global Status of CCS 2025 OGCI, CO2 Storage Resource Catalogue, Cycle 5 State of CDR, 3rd edition Sahin, Kalfa and Celebioglu, SPE Economics & Management REC Türkiye Engüçlü and Yıldırım, SPE Europe Energy Conference

Technologies and applications

Technology types The technical methods that capture, convert or store the carbon. Application areas The sectors, facilities and end uses where the method is applied.

Technology types

The technical methods that capture, convert or store the carbon.

  • Technology type

    Well completion and monitoring

    Completing the injection well so that it seals, and tracking the movement of stored CO2 through repeated measurement.

    How it works
    1. Well completion

      The well is completed with steel casing and cement to form a seal.

    2. Baseline

      Seismic and chemical baselines for the site are measured before injection.

    3. Injection monitoring

      Pressure, temperature and flow are recorded continuously.

    4. Repeat seismic surveys

      Seismic surveys repeated every few years show how the stored CO2 spreads underground.

    5. Post-closure

      Monitoring continues after injection has ended.

    Durability
    Measured

    Depends on well quality

    Storage durability depends on the quality of this step; monitoring is the sole basis for any leakage statement.

    What it rests on
    Sleipner’s evidential value comes less from the storage than from the monitoring: without nearly thirty years of repeated seismic surveys, the sentence "no leakage detected" could not be written. Abandoned or poorly plugged legacy wells are the most likely leakage path at any site.
    IPCC AR6 WG3, Ch. 12
Next tab Application areas

Application areas

The sectors, facilities and end uses where the method is applied.

  • Application area

    Saline formation

    CO2 injected into deep porous rock saturated with water too saline to be a drinking resource.

    How it works
    1. Captured CO2

      CO2 arrives from a capture facility.

    2. Compression

      It is compressed into a dense, liquid-like state.

    3. Transport

      It travels to the site by pipeline or ship.

    4. Injection

      It is injected at 800–2,500 m depth.

    5. Retention

      It is trapped beneath an impermeable seal.

    6. Monitoring

      Well pressure is logged continuously; a seismic survey that images the rock is repeated every few years.

    Durability
    Measured

    >1,000 years

    The IPCC considers retention of 99% or more over 1,000 years likely; the measured record is Sleipner’s nearly thirty years, the strongest field evidence in the sector.

    What it rests on
    This is the strongest field evidence in the sector: seismic surveys repeated since 1996 have detected no leakage, and the IPCC finds it likely that 99% or more is retained for 1,000 years. Suitability criteria are narrow: a depth of 800–2,500 m, a porous reservoir rock at least 20 m thick, a seal at least 10 m thick that extends over a wide area, and formation water far too salty to drink.
    IPCC AR6 WG3, Ch. 12
    Türkiye context
    Türkiye has no national CO2 storage atlas. Capacity calculated for oil and gas fields is about 108 Mt; for saline aquifers there are only formation-scale academic estimates.
    REC Türkiye

  • Application area

    Depleted oil and gas field

    CO2 injected into the reservoir of a produced-out hydrocarbon field whose geology is already characterised.

    How it works
    1. Field appraisal

      Reservoir and seal properties are derived from the production history.

    2. Well remediation

      Old production wells are inspected for integrity and plugged.

    3. Injection

      Compressed CO2 is pumped into the reservoir.

    4. Retention and monitoring

      It is held beneath a seal that trapped hydrocarbons for millions of years, and the site is monitored.

    Durability
    Measured

    Depends on the seal

    Seal integrity is demonstrated by the field’s own production history.

    What it rests on
    The advantage here is low uncertainty: the reservoir held hydrocarbons on geological timescales and the geology is known from production data. The risk concentrates in legacy wells. A critical distinction: injection to increase oil production is not storage, and the cumulative total stored since 1996 does not separate the two.
    London Register of Subsurface CO2 Storage, 2025
    Türkiye context
    The Batı Raman field has taken roughly 1 Mt/yr of CO2 injection since 1986, but that operation uses naturally occurring CO2 to increase oil production: not capture, not removal, and no monitoring programme to carbon storage standards has been made public.
    Sahin, Kalfa and Celebioglu, SPE Economics & Management

  • Application area

    Basalt formation

    A storage setting in which CO2 reacts with the host rock and becomes solid carbonate rather than being held as a gas in pore space.

    How it works
    1. Dissolution

      CO2 is dissolved in water before injection.

    2. Injection

      The solution is pumped into reactive basaltic formation.

    3. Reaction

      CO2 reacts with calcium and magnesium in the rock.

    4. Solidification

      In the Carbfix pilot, over 95% of the CO2 turned into solid carbonate mineral within two years.

    Durability
    Measured

    Mineral carbonate

    More than 95% of the injected CO2 becomes carbonate in under two years.

    What it rests on
    The conversion was measured, not modelled, by tagging the CO2 with a traceable marker. Storage in this setting removes the dependence on seal integrity, because the carbon ends up solid. The constraints lie elsewhere: water consumption, and wells that take less injection over time.
    Matter et al., Science
    Türkiye context
    Türkiye’s ophiolite and peridotite belts are extensive, but no assessment of their storage capacity has been published.

Frequently asked questions

What is geological storage?

Geological storage is the injection of compressed CO2 into porous formation at 800–2,500 metres depth, where it is held beneath an impermeable seal. It is a storage step that works as the last link in a removal chain.

Does geological storage count as removal on its own?

It does not. Where the stored CO2 is atmospheric or biogenic in origin, the chain produces a removal; storing fossil CO2 is an emission reduction. Most operating capacity today stores fossil CO2.

Does stored CO2 really stay in place?

The strongest field evidence is Sleipner: more than 19 million tonnes stored since 1996, with no leakage detected across nearly thirty years of seismic surveys repeated over time. The IPCC finds it likely that 99% or more is retained for 1,000 years.

How much storage capacity is actually usable worldwide?

The global catalogue records 14,256 Gt of storage resource: 13,579 Gt undiscovered, 675 Gt sub-commercial and only 1.8 Gt commercial, with commercial resource only in Australia, Canada, Norway and the United States. The binding constraint is not pore volume but the rate of appraisal, permitting and drilling.

Who holds liability once a storage site closes?

This is a regulatory question rather than a geological one, and the answer differs by jurisdiction. Under the EU regime, responsibility transfers to the competent authority once the operator has closed the site and met the required conditions; until transfer, monitoring and financial security stay with the operator. A thousand-year durability claim therefore rests not only on the caprock but on who will watch the site for centuries.