There are different ways to conceptualize different types of CO2 storage concepts. Here we consider that saline aquifer concepts can be considered as either closed, where the store utilizes structural or stratigraphic confinement, or open, where the store does not involve the accumulation of CO2 but instead relies on migration-assisted trapping (including via capillary/residual and solution mechanisms) as the injected CO2 moves through the storage system.
The difference between the two end-member main saline aquifer concepts is shown above. For a closed aquifer the CO2 is contained under a seal like a conventional trap. In an open aquifer, the CO2 is trapped by capillary/residual and solution mechanisms.
Can Closed Aquifer Solutions Offer Enough Capacity?
We recently explored the example of the BC28 structure, a candidate CO2 storage site located in the developing UK Southern North Sea CCS province, which also lies within the SNS (Southern North Sea) Vision dataset we are producing for completion in Q4 2023. The site typifies the ‘closed’ structural concept being sought in the basin. The BC28 structure is comprised of a storage reservoir within the well-known Bunter Sandstone Formation (BSF), where Triassic fluvial and aeolian sediments are deformed by salt movement into a series of simple structural closures. The SNS BSF storage play represents an end member of the saline aquifer storage concept, effectively relying on a classical trapping concept to initially store the CO2. CO2 is injected into the aquifer and begins to ascend several hundred meters away from the injection point, becoming trapped beneath a sealing claystone and evaporite unit as a CO2 cap. The trapping concept is extremely familiar to those of us used to exploring and producing oil and gas and is reliant on good trap definition and the presence of sealing lithologies across the structure.
However, these sites have some limitations. The volumes expected to be stored in the individual structures are relatively modest, being limited by the size of the structural closures available. The best known of these in the SNS, Endurance, is expected to store up to 100 million tonnes of CO2, although this could be expanded in the future with pressure management and brine production. Earlier estimates of BSF storage capacity within each structure were much higher, but were based upon unrealistically high storage efficiency assumptions.
While Endurance is a significant pioneer project for the UKCS, the interim 2030 Net Zero Emissions target (International Energy Agency) would require around 12 Endurance-size stores to be filled annually to meet those targets. To put the required storage numbers into stark perspective, the IEA forecast that cumulative world energy sector CO2 emissions from existing power and industrial facilities, 2019-2050, will exceed 600 Gt. A massive scale-up of capacity in the second half of this decade is needed to meet carbon storage targets. Large-scale storage, with room to expand, is clearly needed.
Aside from volumetric constraints, the relatively homogeneous nature of the BSF aquifer at certain site locations means that once the CO2 is injected it is expected to rapidly ascend to the top of the storage structure, accumulating as a free-gas cap at the crest. We have examined how this might happen in our dynamic reservoir simulation of CO2 injection into BC28 within UKCS Quadrant 49 and this is illustrated below.
Dynamic simulation time-steps illustrating the evolution of the injected CO2 plume at 4, 8, 16, and 40 years following the start of injection and storage into the BC-28 Bunter closure in Quadrant 49, UK Southern North Sea.
Open Aquifers Offer More Potential for Larger Scale Carbon Storage
This explains why concepts involving closed trapping styles are preferable where the expected storage reservoir characteristics are likely to favour more rapid vertical migration of the plume (i.e. high porosity/permeability, low heterogeneity, kV/kH c. 1), and direct pathways to the top seal when there are few significant capillary barriers to CO2 movement. This is also why there must be a particular emphasis on containment and leakage risk assessment associated with the top seal in such sites. This is the primary barrier to the migration of CO2 out of the store and into the overburden, and potentially back into the atmosphere.
Freed from the constraints of identifying closed structures, the alternative is to identify dipping, open saline aquifer systems. In this concept, storage volumes are not constrained by trap size and storage of larger-scale volumes may be considered.
How Do We Meet Larger Storage Goals?
It is this concept that motivated us to think about investigating the potential of areas with significant seismic data library coverage, but that have been found to be not prospective for oil and gas exploration. These areas have the potential for the right combination of relevant geological characteristics to make them a successful large-scale storage site. This is why we are exploring the Elephant.
As the CCS industry looks to identify projects to meet large-volume storage goals, an important question is: Can we locate large storage opportunities with geological characteristics that take advantage of different trapping mechanisms and mitigate some of the risks around containing the CO2 in the subsurface?
We think the answer is yes! Part of the answer to identifying large-scale storage to support European and even global CO2 storage needs lies beneath the waters of the Norwegian Sea. Welcome to the PGS19MO2NWS dataset or "The Elephant".
PGS19MO2NWS or "The Elephant" dataset acquired in 2020 is located on the Trøndelag Platform in the Norwegian Sea.
Learn More About the Elephant
The Elephant dataset was acquired in 2020 and is located on the Trøndelag Platform in the Norwegian Sea. Here, a suite of up to four store-seal pairs exist as a sequence, dipping west to east. The sequence is relatively un-faulted, comprising the latest Triassic to Middle Jurassic sediments. The storage targets are known formations containing proven hydrocarbon accumulations along the faulted terraces bounding the platform area to the west. Importantly, retention of naturally occurring CO2 has been proven within two of the target horizons in the Victoria gas discovery immediately to the east of The Elephant area in NCS quadrant 6506.
Coming in Part Two
Our next exploration phase will offer a comprehensive look at the Elephant's geology, its potential as a large-scale storage site, and further insights from this dataset.
Within the Elephant dataset and the surrounding area, we are conducting a deliberate search for a combination of geological characteristics that might prove suitable for the development of large-capacity, scalable storage.
The search includes the identification and characterization of geological features needed for a successful store, including reservoir quality and heterogeneity, structural geometry, and an absence of major structural complexity and faulting. We will be assessing how these geological elements in combination with favorable formation waters, can mitigate some of the risks associated with dipping open saline aquifer storage sites.
For example, we know that the dipping aquifers emerge close to the seabed and subcrop Quaternary sequences far to the east, at the terminal end of the migration route. Will this be a subsurface risk that condemns the concept as a potential site for the large-scale storage of CO2 or will reservoir heterogeneity supply a sufficient trapping mechanism?
We will be seeking to answer this question and more as we explore the potential of the Elephant for large-scale storage of CO2 in the Norwegian Sea.
Stay tuned for the next update, where we will describe in more detail the geology and potential storage complex within The Elephant.
Get in touch with the authors of this blog, Nick Lee or Bill Powell to find out how PGS can assist you with your CCS characterization challenges.
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