Carbon storage

In order to have an impact on man-made emissions, very large volumes of captured CO2 must be safely stored and permanently excluded from the atmosphere.

The Global CCS Institute (GCCSI) have produced an excellent 101 explainer on carbon storage.  Below, we give more detail, including deeper dive links to presentations at UKCCSRC events or Flexible Funding projects that have undertaken further research into specific aspects of carbon storage.

The largest available carbon storage sink we have is subsurface geological formations, where CO2 can be stored in the pore space of sedimentary rocks. These formations mainly consist of highly porous sandstones which are capped by low permeability rock formations such as shales. In order to maximise the security of geological storage reservoirs are typically greater than 800m below the Earth’s surface.

At this depth, CO2 is in a dense supercritical state which is less buoyant than the gas phase.  Being in a dense phase is also advantageous to storage capacity. A tonne of CO2 at STP (0˚C and 1 atm) has a volume of 509m2, if the geothermal and lithostatic gradients are 35 ˚C/km and 22.5MPa/km respectively then the same mass of CO2 will have a volume of only 2.5m2 if it is stored 1km below the surface.

Where can we store carbon?

The UK has huge potential for offshore geological storage with numerous potential storage reservoirs within the northern and central North Sea Basin, the southern North Sea Basin and the East Irish Sea Basin. It has been estimated that the UK has 16-20Gt (i.e. 16 – 20,000,000,000 tonnes) and 19-716Gt worth of storage capacity in abandoned hydrocarbon fields and saline aquifers respectively. This is enough to store over 500 years of the UK’s annual emissions. Important aspects of geological storage not only include storage security but also injection strategies and monitoring techniques.

Investigations of potential storage sites in the UK and Europe are well underway, with 37 exploration licences currently issued (27 of those in the UK).  Only three sites currently have storage permits (two at Porthos (Netherlands), 1 in Norway), with six more pending.  Storage sites can only proceed if there is a supply of captured carbon for them to store – a bit of a chicken and egg situation.  Sleipner is the only currently operational storage site – read Norway’s Sleipner: Where CO2 has been buried in the rock since 1996

The Clean Air Task Force have useful interactive maps highlighting storage (and other) facilities – UK and Europe, the USA and the Middle East/North Africa.

The Net Zero Industry Act and Industrial Carbon Management Strategy will oblige members states to publish information and data about their potential storage sites.  An investment atlas of potential storage sites should also be created, as well as a knowledge sharing platform of CCUS projects.

How do we store carbon?

There are four principle geological processes which can physically or chemically trap injected CO2 within the storage reservoir. Structural and stratigraphic trapping involves low permeability layers, such as a shale caprock, or geological structures, such as anticlines. These low permeability layers prevent the buoyant ascent of CO2 as they have a high capillary entry pressure, which basically means that the pore fluid in the low permeability layer is at a significantly higher pressure than ascending CO2.

Solubility trapping occurs when CO2 dissolves into brine (pore water containing large amounts of salt) and becomes an aqueous phase. This brine/CO2 mixture is denser than the surrounding brine and so will sink towards the bottom of the reservoir.

Dissolution of CO2 into brine produces a mild acid which can then undergo chemical reaction with silicate minerals rich in Ca, Mg and Fe to form solid carbonate minerals. This process, known as mineral trapping, is the most stable and permanent form of storage. However, it is a slow process that takes place over hundreds to thousands of years.

Residual trapping occurs when blobs of CO2, at a range of scales, become isolated as reservoir brine flows into the tail of a migrating CO2 plume. This trapping mechanism could well prove to be the most important as experimental work has shown that up to 70% of injected CO2 can be immobilised in this manner.

Stuart Haszeldine (UKCCSRC Co-Investigator and Professor of Carbon Capture and Storage at the University of Edinburgh) also provides an overview of carbon storage in this video (recorded June 2022).

Deeper Dive

“CO2 Storage” parallel session from the UKCCSRC Spring 2023 conference (recording and slides available).  Speakers were:

  • Clare Bond (CSRF and University of Aberdeen)
  • Vahid Niasar (University of Manchester)
  • Chris Holdsworth (University of Edinburgh)

Rockit – the geochemistry of turning carbon to rock via geological CO2 storage in basalts” – Amir Jahanbakhsh, Heriot-Watt University (Flexible Funding 2022)

Evaluation of Caprock Integrity for Geosequestration of CO2 in Low Temperature Reservoirs” – Efenwengbe Nicholas Aminaho, Robert Gordon University (Flexible Funding 2022)

Sensor Enabled Seabed Landing AUV nodes for improved offshore Carbon Capture and Storage (CCS) monitoring” – Anna Lichtschlag, National Oceanography Center Southampton (Flexible Funding 2021)

TERC amine capture plant

Carbon Capture

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Transportation

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Systems, Policy & Public Perception