CCS refers to a suite of technologies that capture CO₂ and store it safely in deep geological formations. The principle is straightforward. Rather than CO₂ being released into the atmosphere where it contributes to global warming, it is captured and injected underground, where it is permanently trapped beneath impermeable rocks.

Firstly, CCS is necessary to stop CO₂ entering the atmosphere at the point of emission. This is essential to decarbonise hard-to-abate industrial sectors that provide products essential to modern life, like cement, steel, petrochemicals and fertilisers. It can also support low-carbon electricity generation using fossil fuels, which will remain a critical part of the energy system for decades to come.

Importantly, the technology to deliver CCS is already available. The focus now is scaling up, by improving efficiencies, reducing costs and building the infrastructure for mainstream adoption.

CCS may also be used to remove CO₂ from the atmosphere. Technologies such as direct air capture (DAC) and bioenergy with CCS (BECCS) aim to reduce atmospheric CO₂ concentrations. As we move through the second half of this century, these techniques will become increasingly important to correct any overshoot in global emissions and keep temperature increase within agreed limits.

The Global CCS Institute reports that around 60mn tonnes of CO₂ are captured and stored globally each year. While that sounds significant, it is only a fraction of what is required. To meet the net zero emissions by 2050 target, CCS must operate at the gigatonne scale; two orders of magnitude greater than today’s capacity.

According to DNV’s 2025 Energy Transition Outlook, CCS could grow to 1,300mn t/y by 2050, accounting for 6% of global CO₂ emissions reductions. Whilst encouraging, it falls far short of what is needed to reach net zero. Despite a growing number of projects, we are not on track to deliver these volumes, and action is required across all parts of the value chain to accelerate progress.

When viewed in the broader context, with global CCS deployment lagging behind climate targets, the importance of accurate, efficient well testing becomes clear. It is the foundation upon which safe and scalable carbon storage is built.

Journey into the subsurface 
A fundamental part of CCS is carbon storage, where CO₂ is injected into porous sedimentary rocks more than 800 metres below ground level, for permanent storage on geological timescales. A good store is defined by a combination of suitable storage volumes, high reservoir connectivity and an impermeable layer called a cap rock, which acts as a natural seal to prevent the CO₂ from migrating upwards.

Two main geological formations can be used for storage:

• Depleted oil and gas fields: These have the advantage of existing wells, pipelines and surface infrastructure that could potentially be repurposed. They also come with decades of geological data, so the reservoir’s characteristics are understood and the cap rock has safely contained hydrocarbons for millennia, proving the effectiveness of the seal. However, legacy wells in the field must be assessed and mitigations put in place if integrity risks are identified, like poor cement or corrosion of the metal casings.
• Deep saline aquifers: These are porous rocks filled with highly saline water. They haven’t been used for hydrocarbon production, which means there are fewer penetrations through the cap rock, reducing potential leakage pathways. Due to limited exploration in these formations, relatively little is known about the geology and fluid properties. Detailed testing is therefore essential to confirm that they are suitable for long-term CO₂ storage.

Both options can provide effective and permanent storage, but saline aquifers are far more abundant and offer significantly greater storage volumes.

Globally, CCS is advancing fastest in North America and north-west Europe. The US largely focuses on onshore projects, while Europe is developing offshore storage in the North Sea. Where a project is located depends on government policies, store availability and cost.

Exploring the unknown  
Carbon storage prospects are identified in the same way as hydrocarbon fields. Firstly, regional screening is conducted to find suitable geological basins, then detailed models of the subsurface are generated using magnetic, seismic and gravity survey data. Once potential sites are found, companies move into exploration and appraisal (E&A) and wells are drilled to collect rock cores, geophysical data and fluid samples.

Until a well is drilled, the prospect remains theoretical, although the chances of a positive result are significantly higher for CCS given that aquifers are easily identifiable and it is the presence and quality of hydrocarbons that present the greatest uncertainties.

Well tests are performed to evaluate a formation’s suitability to store CO₂ and aim to characterise the reservoir by performing dynamic flow tests and collecting formation fluid samples. These tests provide insight on permeability (how easily fluids flow through the rock), porosity (how much space is available to store CO₂), and connectivity (how different parts of the reservoir are linked).

The data is used to build reservoir simulation models that predict how injected CO₂ will disperse and become trapped over time. These models shape field development plans, including well count, well placement, well design and injection rates.

There are two main types of well test:

• Production tests, where formation fluids are flowed to surface. These provide comprehensive data but are technically demanding, requiring well flow management equipment on the surface and downhole pumps to lift brine that may not flow naturally.
• Injection tests, where treated seawater or brine are pumped into the formation. These are simpler; however, success is dependent on the well and near wellbore formation not being contaminated by drilling mud or debris, which can significantly reduce the permeability and test effectiveness.

The choice of test depends on the project objectives, budget and how much is already known about the reservoir. In all cases, data quality is paramount.

E&A campaigns can take years and cost tens, or even hundreds, of millions of dollars. Maximising the value of each well by obtaining reliable data is therefore essential. When viewed in the broader context, with global CCS deployment lagging behind climate targets, the importance of accurate, efficient well testing becomes clear. It is the foundation upon which safe and scalable carbon storage is built.

Revitalising the North Sea 
The North Sea, synonymous with oil and gas production, is becoming a focal point for clean energy projects, including CCS. The same expertise that produces hydrocarbons can now be used to return CO₂ to the subsurface, safely and permanently.

In 2020, Expro performed appraisal testing operations that provided critical data for the geology and reservoir models for the flagship Northern Lights project in Norway – the first open access CO₂ transportation and storage network in Europe. This paved the way for Northern Lights reaching a positive investment decision and injecting its first CO₂ in the summer of 2025, marking a major milestone for global CCS.

At the start of 2025 Expro delivered consultancy and operations to support new appraisal wells drilled by Equinor and Harbour Energy.

The next major step will take place in the UK North Sea, where the Northern Endurance Partnership (NEP) plans to drill and test additional wells in the Bunter Sandstone as part of the East Coast Cluster expansion. Expro will again play a key role, delivering integrated project management and well testing expertise (see also Box).

As our energy systems continue to evolve, CCS will play a crucial role in achieving net zero and become critical global infrastructure. It can decarbonise hard-to-abate industrial sectors, support low-emissions energy production, and will eventually remove CO₂ directly from the air.

There are challenges ahead with policy, investment, and public acceptance. But when it comes to the subsurface, the actual storage of CO₂, the science is sound and the expertise already exists.

Well testing is central to that assurance. It transforms geological uncertainty into valuable data, ensuring that every tonne of CO₂ injected is permanently stored. Every successful well test is another step towards a future where we can lock carbon underground and keep the planet in balance.

• Further reading: ‘What’s the latest outlook for the energy transition? BP and DNV forecasts compared’. New Energy World compares two recently published authoritative energy transition outlook reports. Although BP’s 2025 energy outlook favours a scenario approach, considering the ‘current trajectory’ and a greener approach keeping ‘below 2°C’ by 2050, DNV issued a more specific forecast. Common findings include delays in progress towards global net zero, increasing electricity demand and more emphasis on energy security.
• Discover more about the Longship CCS project and the planned expansion of Northern Lights’ carbon storage capacity.