Unexpected CO₂ movements at pioneering Norwegian CCS sites

The CO₂ in the pioneering Norwegian CCS projects has moved in an unexpected way, says a recent IEEFA study. This raises questions about the feasibility and economy of large-scale CO₂ storage.

If CCS – carbon capture and storage – is to be used at large scale, the proponents will have to demonstrate that the CO₂ will not leak back into the atmosphere from the storage site.

The longest experience of dedicated storage is at the Sleipner field in the North Sea off Norway, which has been in operation since 1996. Another Norwegian storage site, Snøhvit, far further north, has been in operation since 2008. CCS proponents have for decades pointed at the supposed successes of the two projects.

A new study1 by Grant Hauber of IEEFA, the Institute for Energy Economics and Financial Analysis2, concludes that “the two projects have been successful in sequestering their intended annual CO₂ deposit volumes”, i.e. there has been no leakage, but both have “also experienced unexpected subsurface storage behaviours that could have led to CO₂ leakage and, in the case of Snøhvit, potential subsurface geological failure.”

Potential geological failure was also the reason why the In Salah CO₂ storage operation in Algeria, which began in 2004, was shut down in 2011³. The build-up of pressure one kilometre down, caused a measurable uplift (more than 2 cm, an earthquake) on the surface. CO₂ injection never resumed. This was in effect the second Norwegian CCS project, as Statoil (now Equinor) and BP were partners.

At Sleipner, problems started in 1999, after three years of operation, according to the report. The CO₂ moved up 220 metres to a previously unknown “layer 9”. Fortunately, it has stayed there, just under the caprock, but the flow has accelerated over the years and the capacity of layer 9 to store more CO₂ remains unknown. There is no way to move the CO₂ back to where it was supposed to be.

“Once you have injected the CO₂, you lose control over it,” said Grant Huber in a presentation of his findings in September.

“Remedial actions are always a possibility and must be anticipated and budgeted for,” says the report. That includes plugging a hole.

Snøhvit encountered problems after just 18 months of operation. The storage pressure, 2,600 metres down, was building up. The reason was that the storage volume was far smaller than expected. The CO₂ was supposed to migrate into a porous structure (like a sponge), but the porosity was too low, so the intended storage site could only take 1.4 million tonnes instead of the expected 13–14 million tonnes. Instead of 18 years of expected injection, the storage was full after less than two years. A new, shallower, layer was found to take care of some of the CO₂, but not enough for the lifetime of Snøhvit.

If the problem had not been correctly diagnosed, a well failure or a big crack in the caprock might have followed.

While both projects encountered problems, the roots of these problems differed. The report finds that due to the unique geology of each site, field operators must make detailed plans that take contingencies into account.

The report furthermore highlights among its key findings that “Sleipner and Snøhvit cast doubt on whether the world has the technical prowess, strength of regulatory oversight, and unwavering multi-decade commitment of capital and resources needed to keep CO₂ sequestered below the sea – as the Earth needs – permanently”.

This is not what CCS proponents have been saying. For instance – according to the press release associated with the report – the Norwegian government is justifying larger-scale projects with the experiences from Sleipner and Snøhvit. But if every site is different, the economy of scale may not be there, especially with regards to costs for monitoring and remedial action. According to the report such measures and their associated resources are needed long after the end of injection,

Fredrik Lundberg

1. Institute of energy economics and financial analysis  14 June, 2023 https://ieefa.org/resources/norways-sleipner-and-snohvit-ccs-industry-mo...

2. EEEFA is a non-profit corporation with some 70 employees in several countries, mainly active in the Americas, South Asia and Australia. They publish large numbers of reports on energy choices.

3. https://sequestration.mit.edu/tools/projects/in_salah.html

CCS geology – how do they know where the CO2 is?

The underground geology consists of rock, soil and liquids with varying pressures, temperatures and chemical properties. A CCS storage site also contains supercritical CO2, which is something between a liquid and a gas.

The Sleipner and Snohvit storage sites have been closely monitored. They have been the subject of more than 150 academic papers and have used the best and most modern technology.

The main monitoring technology is reflection seismology, a method originally developed to find locations for oil drilling. A (computer-generated) 3D image of what is under the ground can be pieced together by sending a sound wave from explosions, airguns or large vibrating plates and recording the echo, and repeating this from several locations. Development over time can be seen by comparing one image with an earlier image.

Reflection seismology was developed to locate where to drill for oil. It has limited accuracy, because it is not feasible to fire an unlimited amount of sound waves, and because ocean-going seismic survey vessels are expensive.

Caprock is like the icing on a cake, a more solid (impermeable) layer on top of a softer layer. The caprock is where the oil company drills through to get to the oil or gas. In CCS the caprock is the main barrier that prevents the CO2 from escaping.

Source: Institute for Energy Economics and Financial Analysis, June 2023. Norway’s Sleipner and Snøhvit CCS: Industry models or cautionary tales? https://ieefa.org/resources/norways-sleipner-and-snohvit-ccs-industry-mo...