The crucial role of carbon capture and storage (CCS) in decarbonisation efforts is clear – but the path to scale up and delivery remains a complex one.

According to International Energy Agency estimates in 2019, approximately 100 billion tonnes of CO2 must be stored by the year 2060 to restrict global temperature rise to 2°C above pre-industrial levels.

Industrial-scale storage of CO2 is already progressing in the USA, Canada, Australia, and the UAE, but rolling out CCS capacity globally will also come with an increased need for assurance that climate-warming gases are indeed captured. With that comes growing interest and demand for safe, reliable leak detection and technologies capable of monitoring storage reservoirs over years.

For its part the UK North Sea has the potential for as much as 78 billion tonnes of theoretical storage capacity making it one of the most favourable opportunities in Europe.

The picture around the sharp end of emissions monitoring continues to develop. In June 2023, the UK government updated the business model for CCUS transport and storage through the Supplementary Compensation Agreement (SCA) to manage the risk of CO2 leakage and ensure operational recovery.

Transport and storage companies (T&S) will be obligated to have appropriate insurance available to cover the risk of CO2 leakage. The government’s previous update on the T&S business model notes that “certain types of commercial insurance cover likely to be needed to be able to access GSP support and the arrangements under the SCA,” and the SCA may be able to assist with compensation in some situations. This would also provides some safety net for T&S operators in the event of CO2 leakage incidents.

Yet any potential leakage not only sets companies – even countries – back on carbon-cutting targets, but can also pose a risk to human health and the environment, all of which are cause for stringent monitoring.

Technician on the Nini platform, Danish North Sea, where CO2 is injected as part of Project Greensand. Source: Ineos

From health and safety to emissions

CO2 gas is considered a health risk at levels above 50,000 parts per million (around 5% concentration), being toxic when inhaled and displacing oxygen in the air. According to the Health and Safety Executive, the gas can cause dizziness, confusion or loss of consciousness.

From an HSE perspective, the first line of defence is likely to be fixed detectors, already used to identify harmful gas leaks. Such detectors have been established for decades, making for a strong foundation based on proven technologies.

Looking more widely across installations and assets other solutions, such as optical detection cameras, can help make gas leaks visible.

One firm, German safety equipment manufacturer Draeger, already offers various solutions for gas detection. UK business development manager Megan Hine says the company is already using existing products, but is now reviewing its wider portfolio of offshore gas monitoring solutions in light of rising interest in CCS.

One solution – Metcam, an infrared camera system – can already help identify methane leaks. Compared to many previous technologies, it can scan a relatively large area, identifying sources and the intensity of visible leaks. With CO2 also visible through infrared, similar systems could offer long-range detection solutions.

Yet Ms Hine said delivery of such a system could require further work: “We have researched pivoting this on to CO2 and it is possible but requires bigger tweaks to the IR filter than what we have previously achieved for some other gasses, such as refrigerant gasses.”

And while it could be suitable for detecting small-scale leaks, its suitability for monitoring larger assets remains to be seen.

“We need to understand market demand for such monitoring to shore up the business case for the R&D on moving this device on to CO2, especially as there are no live, large-scale CO2 capture plants in the UK currently,” she added.

“All applications we have seen thus far have only asked for safety related detection.”

Other point-based detectors are also widely used, though again tend to be designed more specifically towards safety than emissions monitoring.

Ms Hine and the team at Draeger are looking to hear from oil and gas professionals about their CO2 detection needs and challenges, with feedback used to help to inform next steps and continued innovations in emissions monitoring.

The St Fergus Gas Terminal, site of the proposed Acorn CCS project. Supplied by Shell

Pipeline requirements

Safety is also crucial in transporting CO2 via pipelines, particularly in a high-pressured environment, and could have an impact on not only the nearby marine life but companies – even national – carbon budgets.

CO2 is corrosive if contaminated with water (forming carbonic acid), so the material used for pipelines must be suitable. Any pipelines transporting CO2 must be made and maintained to a high specification, and monitoring the purity of the gas will also be key.

Whether pipelines are onshore or offshore, leak detection systems must also be reliable. Again the nature of oil and gas operations mean sophisticated products are already on offer, such as Atmos Pipe which uses a sequential probability ratio test (SPRT), combining flow and pressure data from control room systems, to identify leaks.

As Atmos warns, pipelines handling CO2 as a gas or as a supercritical phase fluid at extremely high pressure means an explosive decompression of a CCS pipeline would release “more gas, much faster” than an equivalent explosion in a natural gas pipeline.

All these factors reinforce the need for robust leak detection systems to ensure CO2 pipelines are being operated safely – and indeed at transit points where supplies may also move to trucks, trains and ships.

Advanced sensors and AUVs

Accessing hard-to-reach zones like the seabed also presents a challenge for CO2 detection, though recent studies conducted in the North Sea show some hope.

A world-first 2022 study pioneered by the National Oceanography Centre (NOC) sought to see if storing CO2 under the North Sea is both possible and safe.

Their team tested whether they could identify tiny releases of CO2 using sensors they developed, coupled with digital modelling and their observations. Over 12 days at 120 metres depth, 3 metres below the seabed, they successfully measured where the gas was released from and how much was present.

Lead author Douglas Connelly of the NOC, commented: “The artificial CO₂ release in our study enabled us to detect any emissions to the marine environment using acoustic, chemical and physical approaches as well as identifying the location of any leaks – something not previously achieved.”

Professor Jonathan Bull from the University of Southampton was the acoustics lead in the project, who noted the team’s demonstration “that very small quantities of CO₂ can be detected and quantified using passive and active acoustic techniques, including systems mounted on autonomous underwater vehicles (AUVs)”.

This research, published in ScienceDirect, shows that cost-efficient monitoring is possible for offshore CCS operations, giving a clear example of how rapidly developing technologies like improved sensors and AUVs can assist.

A bird’s eye view: Sonar systems

As in other areas of oil and gas, there are a host of subsea leak detection solutions on the market.

Sonardyne provides subsea leak detection and warning systems, with its Sentry integrity monitoring system (IMS) labelled as the only commercially available wide-area system for identifying leaks. While most sonar-based techniques have been reliant on sensors attached to subsea infrastructure at specific locations (sometimes within a few metres), systems like these offer 360-degree coverage across a 1,200m radius from a single sonar unit.

“Around an injection well, Sentry can monitor an area of over 2.3 million square metres,” the firm describes on its website. This can be attached to seafloor infrastructure, or to a buoy or a USV, again offering flexibility.

A case study suggests Sentry can also help de-risk CCS projects, offering a swift response and alert system when there is a CO2 leak. The project proved that subsea leak detection works for CCS sites, and was completed in collaboration with Energy Technologies Institute, Fugro, National Oceanography Centre, British Geological Survey and Plymouth Marine Laboratory.

Long-term subsurface storage

The possibility of CO2 leakage is one of the main concerns around long-term CO2 storage in the subsurface, whether in saline aquifers or depleted hydrocarbon reservoirs. This lack of confidence in geological storage security throws in another obstacle for mass CCS uptake. It’s not only about storage capacity but reliability over the long term, based on the geological context.

Reliable storage sites reduce the risk of gas seepages reaching the atmosphere. Some studies have been reported concerns that escaping CO2 from storage reservoirs could eventually contaminate drinking water. Even if this is less of a concern in the offshore environment, any escape to the sea or atmosphere considerably undermines faith in the process.

Detecting CO2 leaks early helps to ensure the long-term safety of geological carbon storage. Collecting environmental data gives guidance to monitor CO2 in these hard-to-reach areas. Various detection methods spanning seismic monitoring, pressure monitoring, soil gas sampling, remote sensing, atmospheric monitoring, tracer gas injection and groundwater chemistry monitoring all have different pros and cons depending on project requirements.

Spatial competition and access may prove to be among the most significant challenges. The NSTA said in a recent report that full co-location of carbon storage sites within wind farms is in many cases “impossible”, given spatial requirements and the need for ongoing seismic monitoring once CO2 is injected.

While monitoring methods are helpful, it is important to remember that if operations are run to industry standards, chances of leakage from the geological reservoir are minimal. A study published in Nature, suggested that over 98% of injected CO₂ will remain stored for over 10,000 years. However, caution should still be taken to avoid risks.

Groundwater monitoring for onshore sites

Since a leak might occur slowly, it is vital to collect regular data over time, where continuous monitoring may offer a rapid response. A study by South Korean scientists published in October 2023 emphasised why continuous monitoring data for the fast early detection of CO2 leaks in the environment is important, having sampled groundwater periodically during a controlled CO2 release test at the K-COSEM research site.

For onshore CCS sites, collecting periodic groundwater and gas samples help scientists spot important changes. Other than the CO2 itself, related environmental changes such as pH can be indicators, as well as noble gases which allow for sensitive detection early on, according to research.

However, frequent sampling has often been inefficient regarding labor and cost.

The right tools for the job

Rolling out CCS at scale will come with an increased need for fast, safe, and reliable leak detection at a number of stages from onshore facilities, to gas transport infrastructure and across geological reservoirs.

For this, an array of technologies could be relied upon – some of which are readily available, and some of which still require further development.

Ultimately, applying the right methods can ensure safety standards are met, environmental risks are avoided, and net emissions can be depleted. Within a well-planned energy transition strategy, these measures will also help garner public trust in geological CCS.

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