Why rocks matter: exploring the science of geological storage

Imagine a world where the rocks under our feet can be used in the fight against climate change and work to safeguard our future? This is the value that geological storage brings to the world of carbon capture and removal. When we discuss topics of carbon capture and storage (CCS) or carbon dioxide removal (CDR), we often explore the various ‘capture’ technologies that exist to remove carbon dioxide (CO₂) from the atmosphere but give little attention to methods used to permanently store that CO₂, despite this being crucial in ensuring the CO₂ remains isolated from the Earth’s surface and our atmosphere. In this series of articles, we will explore this aspect of CCS and CDR projects looking at the science, the safety and the economics of geological storage and how it compares to other carbon storage sinks. So, let’s ask the question: Why are rocks an essential component of the carbon equation?

Part 1: The Basics of Geological Carbon Storage

The process of geological storage has a whole host of names which can lead to confusion: geostorage, sequestration, subsurface carbon storage, and the list goes on, with terms often being related to the industry doing the CO₂ storage (e.g. CCS or CDR). Ultimately, they all refer to the same process, which is injecting captured CO₂ into underground rock layers for safe, long-term storage. Keep this in mind as we explore the science, economics, and societal aspects of geological storage throughout this series of articles. So which rocks make the most suitable hosts for our captured CO₂?

For those unfamiliar, there are three main rock types: sedimentary, igneous, and metamorphic rocks. Sedimentary rocks, which are made up of small grains of sediment fused together, like sandstones, are the most common rock type used to store CO₂. The actual space where the CO₂ is stored is commonly visualised as large underground caverns, but this is (usually) far from the truth! Instead, we use the microscopic pore space within rocks as our storage space. We use the term ‘porosity’ to describe how much volume within the rock mass is void space where we might be able to store CO₂, with more porous rocks having more storage space while less porous rocks have more limited storage space. In sedimentary rocks like sandstones, this is the space between the individual sand grains which make up the rock, just like the grains of sand you might find at the beach but packed tightly together as the grains have been buried underground and subject to higher temperatures and pressures.

Sedimentary and igneous rocks are the two main rock types which we use as reservoir rocks. The first image shows layered sandstone at outcrop, while the second is a microscope image of sandstone. The blue colour shows the pore space in the sandstone, which is where we store injected CO₂. The third and fourth image show basalt at outcrop and under the microscope respectively. Note the lack of porosity between the individual mineral crystals which make up the basalt. The fifth image shows a fracture network, with a climber for scale. Note the regular geometric patterns of the fractures. The first, third, and fifth photos are from Mathias Reding, Tamara Bitter, and Jose Ruales respectively on Unsplash. Photos two and four taken from the Virtual Microscope project (www.virtualmicroscope.org)

Igneous rocks, like basalts (cooled lava flows) and peridotites (the main component of oceanic tectonic plates), are the second most commonly used rock type for CO₂ storage; these rocks form from the cooling of magma migrating up through the Earth’s crust, forming a variety of different mineral crystals as it cools. Unlike sedimentary rocks, there is usually no pore space between these crystals, meaning these rocks have no natural space where we might store CO₂ and therefore has low porosity. However, they can be crosscut by linear cracks, called fracture networks, which from as the rock is stressed by regional or local processes, like the movement of the Earth’s tectonic plates. These fracture networks can create connected voids within the igneous rocks where we can store CO₂. Irrespective of rock type (i.e. sedimentary or igneous), we usually call the rocks where we store the CO₂ the ‘reservoir’.

The CO₂ is injected into the pore space of these reservoir rocks through circular shafts called boreholes or wells. We inject the CO₂ either in a pure free phase form, or dissolved in water:

· Free-phase CO₂: a concentrated form of CO₂ in a supercritical state where it has properties of both a liquid and a gas. Crucially, it has the low viscosity (a measure of a fluid’s resistance to flow) of a gas, meaning it can flow through the pore space in the rock layers more easily, but has the density of a liquid allowing more CO₂ to be stored per unit volume for more efficient storage. A supercritical fluid has exceeded its critical pressure and temperature; For CO₂ these are a temperature of 31°C (~88°F) and a pressure of 73-74 bar (1059-1073 psi). These conditions occur naturally at depths of 800-1000 metres beneath the Earth’s surface. Temperature and pressure both increase with depth beneath the Earth’s surface as we get closer to the planet’s core and due to the loading of overlying rocks, respectively.

· Aqueous CO₂: CO₂ can also be dissolved in water and injected as a solution. This can be done at shallower depths than those required for supercritical storage. Some of the dissolved CO₂ will react with water molecules to form carbonic acid (H₂CO₃), a weak acid which can partially dissolve the surrounding rock. This is an important part of the process for in-situ CO₂ mineralisation and is the preferred injection method for CO₂ mineralisation projects, but more on those in a later article…

Before we inject any CO₂, the pore space in the rock layers is usually occupied by fresh or saline water, commonly called formation fluids. How the CO₂ interacts with these fluids and the rocks around them is a critical part of the trapping mechanisms that keep the CO₂ in place underground. We will look at these trapping mechanisms in more detail in the next article.

Crucially, CO₂ is less dense than the surrounding rocks and formation fluids, which will cause it to rise buoyantly through the Earth’s crust back to the surface when injected underground. We therefore require a regional layer of rocks overlying our reservoir to act as a barrier to this vertical migration of CO₂, which we call seals or caprocks. These rocks should have low porosity, but they should also have very low permeability. Permeability is a measure of how easily a fluid can flow through medium, like CO₂ through a rock mass, with high permeabilities meaning the fluid flows more easily while low permeability means less fluid flow. We therefore want our reservoir rocks to have high permeability values and our seal rocks to have low permeability values. Some examples of seal rocks include mudstones, salt, and layers of unfractured basalt.

Therefore, our ideal geological storage sites consist of extensive layers of high-porosity and high-permeability reservoir rocks, overlain by equally extensive layers of low-porosity and low-permeability seal rocks. There are a variety of other technical, economic, and social aspects to consider, but keep these fundamental characteristics in mind as we delve deeper into geological storage.

Part 2: How does Geological Carbon Storage compare to other carbon sinks?

Geological storage is an artificial method to recharge one of several carbon sinks (natural systems which absorb more carbon than they release) which include the Earth’s oceans, atmosphere, and living biosphere. The Earth’s rocky crust (known as the Lithosphere) is the largest carbon sink with an estimated volume of 100,000,000 gigatonnes of carbon, but most of it is considered ‘inactive’ as there is no direct exchange of CO₂ with the atmosphere. The burning of fossil fuels represents a major disruption, returning large amounts of CO₂ stored in ancient biomatter to the atmosphere. Geological storage, when paired with CCS and CDR techniques, aims to return this carbon to the lithosphere. The atmosphere itself currently holds around 850 gigatonnes of carbon.

The three main natural carbon sinks are the Earth’s crust (the lithosphere), the oceans, and the terrestrial and oceanic biosphere. Photos by David Kovalenko, Mourad Saadi, Degleex Ganzorig, and Francesco Ungaro respectively on Unsplash.

The Earth’s oceans are the largest ‘active’ carbon sink with an estimated volume of 38,000-40,000 gigatonnes. Atmospheric CO₂ dissolves in the surface waters of the oceans and is carried down into the deep ocean over the course of centuries and millennia through oceanic circulation, ultimately being stored as dissolved inorganic carbon (i.e. not associated with living organisms) with residence times of 100s-1000s of years. The surface waters of the ocean can absorb CO₂ from the atmosphere relatively quickly, with this process influenced by atmospheric CO₂ concentrations, ocean temperature, and circulation patterns, but the slower rate of deep oceanic mixing means this is a slow sink for excess atmospheric carbon.

The biosphere is the next largest active carbon sink with an estimated volume of 2,000-3,000 gigatonnes of carbon. This is made up of all living organisms and associated organic matter on land and in the ocean. Some parts of the biosphere, like forests and soils, are some of the fastest natural absorbers of CO₂, taking up this atmospheric CO₂ through photosynthesis. However, the carbon stored in organism like trees, plants, and soils can be quickly released back into the atmosphere through forest fires, deforestation, or decomposition, meaning their security and residence times are much lower than oceanic and geological sinks, typically ranging from years to centuries.

Part 3: The benefits of Geological Carbon Storage

The most evident benefit of geological carbon storage lies in its potential to safely store CO₂ for geological timescales (thousands to millions of years). This stands in contrast to the dynamic and sometimes vulnerable carbon stored in the natural sinks discussed in Part 2. To have a tangible impact averting climate change, it is generally accepted that captured CO₂ must be isolated form the active environment for 1,000s of years. Geological carbon storage achieves this by storing carbon dioxide in the sedimentary and igneous rocks within the Earth’s crust. This makes it a key component of CCS projects, ensuring the CO₂ captured from point-source emitters do not enter our atmosphere. It also presents the most secure form of storage for CO₂ absorbed using CDR techniques and provides much more durable ‘removal’ than CO₂ stored in parts of the biosphere such as plant matter.

Geological storage, when paired with CO₂ capture as noted above, is also a key stepping stone in our societies drive towards deep and meaningful decarbonisation. Removing our dependence on fossils fuels is the most important societal change we can make to improve our climate. However, as our society has fundamental foundations in fossil fuel usage stretching back centuries, several industries are still struggling to find meaningful ways to decarbonise their processes. These industries include aviation, shipping, and steel, cement, and chemical manufacturing. Abating their on-going CO₂ emissions and storing it durably in geological reservoirs provides these sectors with a bridge through the energy transition while more sustainable and less carbon intensive methods are developed.

Industries which will be slower to decarbonise, including aviation, shipping, cement manufacture, and steel foundries. Photos from Mark Olsen, Elias, Yasin Hemmati, and Julian Schultz respectively on Unsplash.

Geological carbon storage projects can also provide economic and societal benefits to their local communities. First and foremost, these projects generate a variety of jobs in engineering, logistics and geosciences required to support their operation. These roles can include greenfield exploration for suitable storage sites, the operation of capture plants, transport of CO₂ through road, rail, ship or pipeline, and running and monitoring of injection infrastructure.

Geological storage projects are also important revenue generators; they generate this revenue through both compliance and voluntary carbon markets. In compliance markets, like the European Union’s Emissions Trading Scheme (the EU-ETS), emitters can monetise their captured emissions by selling surplus emissions allowances when geological storage of CO₂ reduces their emissions below their allocated limits. In voluntary carbon markets, revenue is generated through the sale of carbon removal credits, where verified storage projects issue credits based on how much CO₂ has been removed from the atmosphere and stored in a durable manner. These credits can be purchased by corporations looking to offset their own emissions. In addition to these revenue streams, additional sources of funding such as government subsidies and grants and mechanisms like Carbon Contracts for Difference (CCfDs) can provide financial incentive for the development and operation of CSS and CDR projects paired with geological storage sites. These mechanisms can help alleviate the high capital expenditure (CapEx) often associated with geological storage sites. We will explore the economic aspect of geological carbon storage and carbon markets in a future article.

Conclusion: the role of rocks in the race to net zero

By harnessing the natural carbon storage space afforded to us in the Earth’s crust, we can unlock a durable storage mechanism that is stable, scalable, and essential for industries which struggle the hardest to tackle their emissions. While the Earth’s forests and oceans play their part in the carbon cycle, the resilience of geological storage promises unmatched permanence and piece of mind when it comes to abating and removing CO₂. But how do we ensure the safety of this solution when it occurs out of sight and deep underground? Join us in the next article where we will explore the trapping, monitoring, and security of geological storage.

References used in writing this article:

Berner, R.A., 2003. The long-term carbon cycle, fossil fuels and atmospheric composition. Nature, 426(6964), pp.323-326.

Rickels, W., Proelß, A., Geden, O., Burhenne, J. and Fridahl, M. 2021. Integrating Carbon Dioxide Removal into European Emissions Trading. Frontiers in Climate, 3, 1–10.

Ringrose, P., 2020. How to store CO2 underground: Insights from early-mover CCS projects.