Carbon capture and storage (CCS) is often seen as putting CO2 back into exhausted oilfields. Such thinking is unnecessarily restrictive, though. Saline aquifers offer one such option and more are under consideration.

New thinking suggests a series of mini seals, described as “stacks of pancakes” and defined as a composite confining system, are worthy of consideration. When it comes to CO2 storage, researchers proposing the concept encourage thinking beyond the neat “crepe” of the conventional petroleum seal.

Petroleum thinking

CCS projects today all share one common goal: to secure CO2 emissions in the subsurface for the long term. Many professionals are taking what they know from petroleum industries to tackle this challenge. But CO2 and hydrocarbons are different, so is “petroleum style thinking” helpful or a hindrance?

Conventional petroleum seals are well known, so it is not surprising these are preferred bets for CO2. They are thick, impermeable and laterally extensive, like many mudstones and shales. But these may not always be available in areas where CCS is desirable.

One research team has proposed another option for geologically stored CO2, using a series of “stacked lids” in a composite confining system. Alexander Bump and Hailun Ni at the University of Texas at Austin co-authored a recent study to demonstrate and test this system.

Bump, who also worked with colleagues at the Gulf Coast Carbon Centre to test it out, explained it was another possible way of securely containing CO2 in the subsurface. He told E-FWD about the concept.

Carbon maze

Traditional petroleum seals “rely on creating a single impermeable barrier to vertical flow”, he said. “Composite confining systems use many, many layered barriers to create a maze that soaks up migrating CO2 and ultimately stops it in its tracks.”

Essentially, this composite confining system immobilises the CO2 under stacks of discontinuous barriers, which act like seals. This spreads CO2 horizontally, rather than vertically.

While both are valid strategies, they have different pros and cons. “Conventional petroleum seals are well understood and proven to securely retain oil, gas and CO2 for tens of millions of years,” Bump said. “But those seals are always present and they retain fluids in a mobile state—if there is an escape path through the seal (such as a poorly constructed well), there is a lot of fluid ready to move through it.”

Above: Fig 1 from Bump et al (2023): https://www.sciencedirect.com/science/article/pii/S1750583623000786 Under Creative Commons license due to open-access publication.

“By contrast, composite confining systems disperse and immobilise fluids in the reservoir – even if there is an escape path, there is little or no mobile left to flow out. The disadvantage is that the concept is unfamiliar (at least in this context) and so takes extra modelling and assurance,” Bump said. The scientist recently co-authored a paper modelling these systems.

It showed how a small change in grain size between rock layers can be enough to work as a seal, because of the effects of capillary entry pressure.

Validating and digging for insights

Composite confining systems were investigated and demonstrated by the researchers’ sand tank experiments, numerical modelling, field scale simulations and seismic calibration.

Hailun Ni explained the advantage of these sand tank experiments. “The main advantage of sand tank experiments is that they capture real plume physics, whereas simulations only approximate real physics. Therefore, experiments can be used to validate simulation results.” They do not only represent a variety of scenarios but also support reservoir modelling.

An example was given at SPE11, inspired by the Norwegian FluidFlower experiments. These illustrated CO2 plume migration and trapping mechanisms for safe containment.

“Once the simulator is sufficiently validated, we can be more confident about the simulation results and predictions,” Ni added.

Combining the sand tank experiments with numerical modelling helped Ni and colleagues “crash test” the theory. This also matched simulation results for validation, even projecting CO2 saturation 1000 years after injection.

Frequency and detail

Their studies show the barrier frequency remains important. Even minor changes in sand grain size can be effective barriers for upward migration of the buoyant CO2 plume.

After the composite confining systems concept was introduced in the 2023 publication, Bump collaborated in the May 2024 study led by Ni. This further analysed flow scenarios and details, such as grain size. Small reductions, even from a coarse to a medium sand, appeared enough to deflect CO2 horizontally in the same way a seal does, the team confirmed. They also observed longer barriers retained more CO2.

Ni, lead author of the 2024 paper, explained to E-FWD that the 2024 study provided more detail. It focuses on the “laboratory experimental aspect of the 2023 study and adds quantitative experimental evidence for how much CO2 can be trapped by barriers with different grain sizes, lengths, shapes, and graduation”.

Ultimately, the scientist said, the “effectiveness of a composite confining system relies on barrier quantities and properties”. These “barriers can be formed even with small grain size contrasts, as demonstrated by the experimental results”.

Capillary pressure

Barriers, which can be small changes in grain size, don’t need to be perfect. They only need to offset enough capillary entry pressure contrast to divert CO2. This means the substantial size of conventional regional seals are not the only suitable sites for CCS. In composite confining systems, it is capillary pressure that locks in the gas.

In the system there needs to be enough discontinuous barriers to trap the CO2 as a system, even if they are imperfect seals. The more of these barriers, the better, as they work in aggregate.

A computer simulation of a composite reservoir tank experiment described in the study. The experiment is run until the fluid escapes from the confining system. The grey figures are barriers. The blue is a fluid plume meant to simulate carbon dioxide. The graphic is cropped from Figure 2 in the 2024 study. Credit: Hailun Ni, et al.

Region first

The “stacked geologies” of composite confining systems are found across the world, including the North Sea. That does not mean these are a replacement for the wealth of regional seals present in the area.

Bump explained the “North Sea has a large number of very high-quality, regionally extensive seals that provide very effective containment for all of the potential storage reservoirs – there is no need for composite confining systems as primary containment.”

However, the scientist explained, “composite confining systems could support CCS efforts in other ways. The concept may be useful for minimising the footprint of a storage project and reducing or eliminating pressure on the primary seal.”

The composite confining systems concept may offer opportunities for CCS across the world, where regional seals are not present.

Sleipner

A real-world example behind the concept can be seen in Sleipner, which is the first and longest running CCS project in the world. The project involved injecting supercritical CO2 into the Utsira sands, featuring 1 metre-thick silts.

Initially incentivised by the Norwegian state tax on offshore petroleum industry emissions, field operator Statoil injected CO2 in 1996. Within three years, the buoyant CO2 plume had ascended through the eight thin shale barriers to the top seal.

Sleipner’s thin seals in the Utsira sand helped distribute the CO2, rather than it being solely trapped under the top seal. These “local mini traps” of imperfect seals were noticed by researchers Andrew Cavanagh and Stuart Haszeldine, who speculated they kept two thirds of the injected CO2 from the top of the Utsira formation.

“The Sleipner project injects [CO2] into a reservoir that has at least eight low-permeability silts, interlayered with high-quality reservoir sands. They are thoroughly inadequate from confinement on their own, but each layer traps some of the rising CO2,” Bump told EFWD.

“The aggregate effect is a CO2 plume that looks like a stack of small pancakes, rather than a single plate-filling crepe. The result is less pressure on the top seal and a smaller project footprint to monitor.”

In tandem, the underlying confined system appears to support Sleipner’s top seal, the Nordland shale, by reducing pressure on it.

Advantages and risks

Sleipner’s Utsira sand injection zone is considered an extremely high-quality reservoir. There are vertical chimneys, shown in 4D seismic.

“In the case of Sleipner, it’s not fractures that provide the vertical conduits but ancient volcanic fluid pipes – basically ancient springs that would have brought hot water to the surface,” said Bump.

Such springs can be seen in Yellowstone or Iceland today. However, at Sleipner, these features are covered by the Nordland shale. This top seal removes the risk seen from the conduits.

“The pipe of interest is about 30 metres wide and goes right up through the silts. So they have a hole 30 metres wide, going straight through them and they still retain a lot of CO2!”

These features are usually considered for subsurface projects. In this instance, though, they show the benefits of a composite confining system.

“Sleipner demonstrates the capacity of imperfect barriers, but also the sort of failure scenario we have to guard against if we are going to rely on composite confinement (or any other seal, for that matter),” Bump cautioned.

“The silts are interesting because they illustrate the ability of truly low-quality barriers to arrest a bunch of CO2. In that sense, it is very much a real-world demonstration of what we see in the sand tank models.”

Trap competition

Bump and colleagues wrote in 2023 that the greatest risks to the performance of composite confining systems are the same with conventional seals. These include fluid escape pipes, legacy wells and permeable faults that focus flow.

When producing hydrocarbons, operators favour large-volume, concentrated and mobile accumulations, backed by large traps with high-permeability reservoirs and impermeable seals.

One pitfall of conventional petroleum traps is that they keep CO2 in a more concentrated, yet mobile, state. There may then be the risk of a small leak allowing a large concentration of CO2 to leak through a breach, such as in a poorly decommissioned well. Composite confining systems instead spread the gas out laterally, immobilising and securing it in place.

“You could put out a bucket to catch the drips, but that relies on finding a bucket, making sure that it has no holes and then keeping pets and kids from kicking it over,” Bump said. “Alternatively, you could put out a stack of towels, which are readily available and impervious to spillage if they get kicked.”

The bucket is like the conventional O&G reservoir, and the stack of towels represent composite confining systems. Both do work in their own way, but with different advantages.

Bump said neither had infinite capacity. “But if you want to capture the water for future use, the bucket is clearly the better option. If you just want to keep the water from staining the floor, towels will do the job with no risk of spillage when you carry them outside.”

Beyond petroleum

The team conclude from their “crash test” models that composite confining systems are robust under many scenarios. But they are fundamentally different to regional seals, which are ideal for hydrocarbons. Each individual seal is less important than the whole system in aggregate.

Bump and Ni’s teams do not propose rejecting regional seals or petroleum geology knowledge. Instead, they call for thinking beyond industry norms, since CO2 has fundamental differences to hydrocarbons.

Composite confining systems could offer more sites for CCS expansion in the future. It is, though, still very early days for the concept. More needs to be done to further test, model and understand these systems.

Bump is developing some best practices to help. “We’re figuring out how to take a good idea and apply it. There are already commercial projects moving forward with this on the Gulf Coast. We want to make it a standard part of the global toolkit for carbon storage.”

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