Seismic data de‑risk Elephant CO2 storage site offshore Norway

By Kristian B. Brandsegg, Sougata Halder, Allan McKay and Gunhild Myhr, TGS

As industry and governments intensify efforts to meet net-zero targets, carbon capture and storage (CCS) is becoming a cornerstone of long-term emissions management. The maturation of secure, high-capacity carbon storage sites that are demonstrably monitorable has become a defining requirement. Yet, fewer than 50 Mtpa of CO₂ are currently captured worldwide, far short of the multi-gigaton scale needed to curb atmospheric emissions. Therefore, developing robust geological storage at an industrial scale is no longer optional; it is an essential enabler of decarbonization.

Against this backdrop, TGS’ Elephant CO₂ storage project emerges as a strategically significant offshore carbon storage development in the Norwegian Sea off mid-Norway. This project site is developed on modern 3D broadband seismic data and strengthened by a comprehensive, risk-based monitoring measurement and verification (MMV) philosophy. The project demonstrates how integrated geoscience and optimized monitoring planning can accelerate CCS deployment across Northern Scandinavia.

A stable geological foundation

The Elephant storage complex lies on the Trøndelag Platform, a region characterized by long-term structural stability with relatively limited faulting compared with adjacent offshore provinces. Thick, laterally extensive Jurassic saline aquifers have potential of gigaton-scale storage, offering capacity sufficient to support multiple industrial CO₂ value chains over decades of operation. The primary reservoirs, the Garn and Ile formations, are overlain by laterally extensive seals of the Melke and Spekk formations. This reservoir-seal configuration, validated by oil and gas accumulations in the region, provides several trapping mechanisms, including structural, residual and solubility trapping, supporting permanent containment of CO₂ within mapped reservoir zones.

Further, proximity of the Elephant CO₂ storage site to emerging ship-based CO₂ transport hub, provides a cost-effective CO₂ transport model for dispersed northern European emitters or combined with a pipeline solution from an intermediate onshore terminal.

Seismic data driving subsurface certainty

More than 10,000 sq km of modern 3D broadband seismic data underpin characterization of the Elephant CO₂ storage site (Figure 1a). High fidelity imaging enables detailed mapping of reservoir connectivity, depositional architecture and seal continuity. These insights reduce uncertainty related to injectivity, storage capacity and containment performance, forming the basis for robust static and dynamic modeling.

Static and dynamic reservoir models were developed to test industrial-scale injection scenarios involving multiple wells and sustained injection rates. Simulation results showed low-pressure buildup and predictable plume migration, largely confined to the Ile Formation (Figure 1b). Vertical migration into overlying units was limited and transient, with long-term immobilization achieved through multiple trapping processes.

Subsurface risks: Designing for the unexpected

Even with thick regional seals and some of the highest quality seismic imaging available offshore Norway, the subsurface beneath Elephant CO₂ storage site does not come with guarantees. A detailed understanding of subsurface risks will play a central role in shaping the site’s monitoring and risk management strategy as the project moves toward Phase 1 development.

Despite the presence of regionally extensive seals and a high-capacity storage system, project assessments indicate that several geological and operational uncertainties must be actively managed to ensure long-term CO₂ containment (Table 1).

There are three ways of carbon leakage from the storage site influencing the environment:

1. CO₂ leaves the storage site vertically;
2. CO₂ leaves the storage site horizontally; and
3. Brine discharge via subcrop in the northeast part of the structure.

One of the primary considerations is the potential for unexpected plume migration driven by small-scale reservoir heterogeneity or previously unrecognized connectivity that could alter CO₂ flow paths from those predicted by models, underscoring the need for continuous subsurface surveillance once injection begins. Potentially reservoir pressure buildup and/or salinity variations may impact CO₂ solubility, plume migration and injectivity, necessitating ongoing calibration within dynamic models.

While the likelihood of leakage is considered low, the project team emphasized that containment must be demonstrated over time, not assumed. After first injection is initiated, the ongoing MMV will be used to validate plume behavior and provide early detection of any deviations from expected performance.

Further monitorability challenges could arise as multiple injection wells are phased in, requiring optimized well placement and seismic acquisition strategies to maintain full field visibility.

MMV: The cornerstone of storage confidence

In offshore CCS, monitorability is inseparable from storage viability. At the Elephant CO₂ storage site, MMV planning (and execution) is treated as an engineering control system rather than solely a regulatory requirement.

Designing a risk-based MMV program emphasizes requirements of a high-resolution baseline 3D seismic survey for identification and mapping of containment risks to define a scalable, fit-for-purpose monitoring program. Time-lapse (4D) seismic monitoring forms the backbone of the MMV strategy, supported by ocean-bottom sensing, well integrity surveillance and seabed monitoring. Together, these tools provide continuous feedback on plume behavior and pressure response throughout the injection lifecycle (Figure 2).

A blueprint for regional CCS deployment

With offshore CCS moving from pilot projects toward large-scale deployment, the Elephant CO₂ storage site is positioned as a regional storage hub linking offshore infrastructure with onshore industrial capture projects across Norway, Sweden and Finland. Its scale, geological robustness and monitoring framework support its role as a foundational asset within a broader Northern European CCS network.

The Elephant CO₂ storage site demonstrates how broadband seismic acquisition, advanced seismic imaging and risk-based monitoring can be integrated into a storage system that can transform a subsurface concept into a mature, investment-ready storage solution.

As offshore CCS becomes a material component of the energy transition, projects like the Elephant CO₂ storage site provide a practical blueprint for building confidence in the subsurface evaluation and modeling for developing an optimized monitoring strategy for secure, long-term permanent CO₂ storage.

Given the multi-decade lifecycle of offshore storage assets, the Elephant CO₂ storage site needs to rely on a digital integration platform designed to harmonize real-time operational data across the CO₂ injection infrastructure to ensure data continuity, unification, traceability and real-time monitoring of the operational workflows for performance tracking and efficient regulatory reporting. These digital workflows reduce handover risk between project phases and help support long-term stewardship and liability management.