TGS Articles & Insights

Part 2: Gigatonne CCS - Defining the Structure of the Open Aquifer

Written by TGS | Jul 1, 2024 5:00:00 AM

Here in Part 2 of the series, we will define the basic structural geometry of the open aquifer opportunity identified within the PGS19MO2NWS or "The Elephant" dataset. Part Three will dive deeper into the nature and characteristics of the sandstone aquifers within the proposed storage complex.

This area has been highlighted in the Norwegian Sea CO2 Atlas, where the NPD has identified two closed structures previously targeted for hydrocarbon exploration and highlighted the value of the formations for CO2 Storage. As we discussed in part one, there are potential limitations to such closed structures. We are exploring the TGS data library and the regional knowledge TGS possesses to reach necessary storage targets and explore the potential for open aquifers and migration-assisted storage in this area.

The Elephant Concept: Storing CO2 in an Open Aquifer

The Elephant lies south of the Helgeland Basin on the Trøndelag Platform and is bound to the north by the Vega High and the Ylvingen Fault Zone. The seismic section crosses the Elephant with the Sør High to the west, dividing the Trøndelag Platform from the Halten Terrace. The target formations are shallow to the east, sub-cropping the Tertiary cover, a risk factor we evaluate during modeling and simulation.

The Elephant consists of a large slab that dips to the SW and is relatively undeformed and unfaulted. The up-dip direction is a fault-bound closure, which would trap any remaining mobile CO2 that hasn’t been trapped by capillary/residual or solution mechanisms. The overall configuration of the open aquifer store is illustrated in the cartoon below, showing the concept of a migration-assisted store developed in a regional gently dipping monocline structure.

GeoStreamer seismic section from PGS19MO2NWS from the Sør High across the Elephant structure on the Trøndelag Platform.

As the cartoon below illustrates, injected CO2 would initially develop as a plume around the injection well, and over time this plume would migrate up dip through the aquifers, interacting with multiscale geological heterogeneities within the aquifer sandstones as it migrates. Utilizing the favorable characteristics of the aquifers (particularly heterogeneity within the sandstones) and formation water chemistry, a combination of residual trapping and dissolution could achieve the goal of securing the CO2, without the need for buoyant trapping within a closed structure. The favorable characteristics of these aquifers will be discussed in more detail in the next article in this series.

Schematic showing the key concepts for an open aquifer store from injection, plume development, and plume migration and containment via residual trapping and dissolution.

Testing the Concept: Structure-Based Modeling of the Elephant

To validate the storage concept, initial modeling was conducted using the detailed horizons mapped on the seismic data to assess the potential catchment of the aquifers and enabling us to define the basic boundary and geometry of the store. This analysis is key for understanding the potential migration and storage area early in the screening phase, before moving to more detailed characterization, and it allows the boundaries of the reservoir model to be considered. The map below shows the structure of the base top seal (Base Spekk Formation) with a flow accumulation attribute overlying the top structure. This gives a sense of where migration will be directed should any CO2 reach the shallowest accessible sandstones within the complex (in this case the Garn Formation, immediately beneath the Spekk Formation) and assuming buoyancy is driving the flow. 

Using the top aquifer interpretation we can use horizon-based flow modeling to determine the likely open aquifer 'fairway' to outline areas for further investigation.

Mapping the catchment of the dipping region that ultimately reaches the structural closure at the pinnacle can provide an indication of the potential area available for migration-assisted CO2 storage and an initial basis for storage calculations using simple analytical volumetric methods. In this case, the highlighted (black dotted line) catchment in the image above is an area of just over 2 000 sq. km of rock that could be utilized for storage.

Having mapped out the gross regional structure, which defines the overall storage complex, detailed mapping of the Top Garn/Base Melke Formation revealed important geological complexity in the form of a rugose structuration that would not have been readily imaged on lower-quality data. This is an important advantage of prioritizing the use of high-quality data, such as GeoStreamer, early in the evaluation. It reveals additional beneficial geological characteristics that help improve the definition of subsurface risk at the candidate storage site and provides earlier line of sight on potential subsurface risks to injection, storage and containment.

The map below shows the characteristics of this structural fabric at the top of the complex, and we theorize that any CO2 reaching the shallowest accessible sandstones within the storage complex can then be trapped within these micro-closures before reaching the regional structural culmination of the dipping aquifer complex to the northeast of the site. We are still considering the origins of these structures, but they are real features of the geology and are one of the reasons why accessing quality data early in an evaluation can pay dividends when assessing how the store might behave.

Map of the base seal showing the varying degrees of small-scale trapping which is typically below the scale of standard simulation approaches.

In fact, we think these features may improve the concept by providing an additional subsurface mitigation to CO2 moving out of the complex. Initial simulations indicate that the features can capture any CO2 that has migrated to the top of the aquifer sequence from the Ile Formation and into the upper candidate storage unit, the Garn Formation. Of course, before migrating CO2 reaches the top of the aquifer complex, there is some complex stratigraphic ‘plumbing’ between these two formations to navigate, along with significant geological heterogeneity within the aquifer sandstones. Having completed the mapping and interpretation phase to define the overall envelope for the migration-assisted store, we were sufficiently encouraged to move to the next phase of the evaluation, where we would evaluate the reservoir characteristics through a blend of advanced geophysical methods and more traditional regional exploration techniques.

Introducing the Stratigraphy for the Aquifer Analysis To Come

To dive deeper, we focused on extracting as much insight out of the GeoStreamer seismic data as possible, and utilized well data within the RockAVO library to build a rounded understanding of the sedimentology and potential reservoir properties. Offset well data and regional analogs were also considered.

The stratigraphy of the aquifer systems is introduced below. We start with the Melke and Spekk Formation seals which blanket the Elephant and beneath are four potential aquifers which have been identified with potentially suitable characteristics for CO2 storage. These formations below are well known to the hydrocarbon industry as proven reservoirs.  

In Part Three we will share some of the insights we have found from our analysis of the stratigraphy and aquifers, focusing on the Garn and Ile Formations.

Simplified stratigraphy showing the distribution of aquifers and sealing lithologies we will discuss further in Part Three.
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