Seismic-led Exploration and Characterization of Carbon Storage Sites

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

Recent technological advances in seismic acquisition and processing have proved valuable for CCS site screening projects. TGS authors describe a number of case studies from offshore North-West Europe in the June issue of First Break.

First Break June 2024 | Seismic-led exploration and characterization of carbon storage sites

The case studies described in our First Break paper fall into four categories:

Rejuvenation of data for CCS purposes
Using advanced imaging technologies to illuminate shallow structures
Using fit-for-purpose modern data to image from shallow to deep
Acquiring new data for CCS-specific purposes

Reprocessing Legacy Data at a Regional Scale for Efficient CCS Screening | Southern North Sea

The SNS Vision project, initiated by TGS in late 2022, exemplifies how the seismic imaging quality from rejuvenated multi-survey vintage data, can be enhanced to provide new products fit for both O&G exploration and CCS screening.

The common final Kirchhoff pre-stack depth migrated (KPSDM) product covers 26 legacy seismic surveys acquired between 1988 and 2006. It consists of approximately 12 000 sq. km of 3D seismic data predominantly located in the UK sector, with parts in the Dutch sector and provides a regionally consistent dataset for both structural, stratigraphic and quantitative interpretation purposes. Both pre- and post-salt intervals, whose depths vary greatly, were the main objectives of the reprocessing including an extensive velocity model building (VMB) sequence.

Quality improvements were significant at all depths, notably in the modeling and imaging of intra-salt heterogeneities and base salt anhydrites/dolomites, as well as at the deeper pre-salt Rotliegend which is mainly a target for gas exploration. The step change is even more significant in the post-salt section where a significant focus for CCS ventures is currently put on the Triassic Bunter sandstones and the overburden. Indeed, those intervals were historically poorly focused and therefore insufficiently imaged. The latest improvements are further emphasized by greater clarity of faults, some of which extend from top salt to near surface, a component that is critical for fully assessing containment risks.

The main KPSDM results provide reliable amplitudes for quantitative interpretation which still proves to be a challenge in this post-salt environment due to the scarcity of available well data. Fortunately, a novel approach based on machine learning allowed the reconstruction of missing overburden well information to support the generation of reliable rock properties, in particular for the Triassic Bunter Sandstone Formation BC28, as illustrated below.

On the left: porosity prediction rendered in a 300 m interval from Top Bunter, co-rendered with the reprocessed final KPSDM stack (a) and legacy KPSTM stack (b) showing significant changes in the distribution and variation across the sand units and structures. The porosity is extracted over the same interval and rendered in 3D over the full 12 000 sq. km reprocessed area (c) with an outline of structures of interest, which are represented in a 3D model (d).

Using SWIM to Illuminate the Near-Surface | Southern North Sea

The geometries of the legacy surveys in the Southern North Sea did not allow proper imaging of the very shallow near surface (seabed and up to several hundred meters below) in the KPSDM products, due to the inherent lack of very near offset information. Accurate imaging of the overburden is essential for a complete evaluation of CCS sites. Advanced imaging techniques, such as Separated Wavefield Imaging (SWIM) have demonstrated value for exploration and operation derisking. Similar applications have been adapted and proven for shallow-water ocean-bottom seismic datasets, even though the technology was not originally designed for this purpose.

The technology has been adapted to legacy conventional streamer data and the methodology was validated on a subset of the data, the results are illustrated in the figure below. We can observe an improved resolution compared to the Kirchhoff image and illumination is greatly recovered, revealing structures and amplitude changes of critical value for a more complete derisking of the CCS concepts in this area. Some of the assumptions in the wavefield separation, such as free surface characteristics, have an impact on the recoverable bandwidth using multiple reflection signals if those characteristics are subject to high variations caused by poor weather conditions that occurred during the acquisition.

Cross-section and time slices (112 ms) of the KPSDM stack (a and c) and SWIM image (b and d), highlighting the complexity of the structures to the seabed. The clearest sub-horizontal event in the KPSDM section (a) with the white-black-white sequence is not the seabed but a residual imprint of its first surface-related multiple reflection. This artifact has a major impact on the time slice (c). The larger folding is better outlined in the SWIM images (b, d) as well as smaller-scale folding, terminations, and possible faults within, especially on time slices.

Developing New CCS Concepts from Modern High-quality Seismic Data | Norwegian Sea

The Elephant project, a metaphoric name associated with the actual outline of the survey which may now as well represent its size and ambitions, utilizes a very extensive (>10 000 sq. km) high-quality TGS broadband GeoStreamer 3D seismic dataset. It is an excellent example of frontier CCS exploration in practice, where the area in question has not seen significant success for petroleum exploration and is relatively sparse in well data.

However, the quality of this seismic data is exceptional and ideal for assessing critical subsurface risks associated with storage site definition. The site itself is focused on up to four lower and middle Jurassic aquifer units and invokes multiple storage mechanisms (solution, residual/capillary, and local structural/stratigraphic trapping) to target a large gigatonne-scale potential storage site.

The figure below illustrates both the high-quality and the resolution of this data, which is sufficient to complete accurate and detailed mapping of the seal and overburden units above the top Garn, the uppermost store unit in the Elephant area. Toesets, onlaps and clinoform geometries can all be clearly identified in the overburden and assessed for their capacity to create configurations that might compromise integrity or create future bypass zones for any CO2 that might migrate out of the complex into the overburden. Similarly, faults are extremely well imaged, and these can be mapped with confidence and their impact on top seal and overburden integrity confidently assessed.

This data also demonstrates how attributes can be generated to create critical insights into other aspects of the store definition. Particularly relevant in data-lean saline aquifers is the ability to obtain reliable aquifer characterization insights, which can then be used to constrain geological and simulation models.

In the picture is also possible to observe the presence of a clear strand plain morphology within the Ile Formation, the next aquifer down from the Garn. Reliable ties to cored offset wells mean that integrated seismic and core data provide a robust understanding of reservoir characteristics that can then be applied with high confidence to geological modeling concepts, demonstrating that state-of-the art 3D seismic data and imaging have a crucial role to play in carbon storage site evaluation alongside more traditional methods.

New Acquisition Designs and Baseline Considerations for Future Monitoring

Reprocessing of legacy seismic data can play a major role in CCS site screening and development. However, repurposing existing seismic data may not always be sufficient, especially if the legacy data was acquired with very different imaging objectives in mind, is poorly sampled, and does consequently not lead to a more detailed characterization of the potential CO2 storage site.

Acquisition geometries for tailored new CCS marine seismic projects are constrained by the necessity to image the overburden including the near-surface and, in some cases, also by a shallow water environment. Advanced towing configurations that combine wide-tow multi-source configurations with multisensor streamers enable high-resolution imaging from the very shallow subsurface to deeper geological structures in a cost-effective manner. These modern seismic acquisition solutions, originally designed for hydrocarbon exploration with a focus on shallow reservoirs, have been quickly adapted for CCS development surveys and on a smaller scale for ultra-high resolution 3D site surveys for offshore wind.

The relatively shallow targets in typical CCS projects make it possible for new seismic surveys to routinely record the refracted wavefield needed for FWI velocity model building in addition to seismic reflections for reflection imaging. This can be achieved by using longer uniform streamer spreads or high-density spreads in combination with sparse streamer tails. Such solutions are feasible with neither significantly increasing survey cost nor compromising turnaround and have been deployed in recent CCS-related seismic surveys.

In the longer term, CCS exploration and development surveys will likely serve as baseline surveys for seismic monitoring of the CO2 storage site. Seismic monitoring is key to detecting the migration of the CO2 in the reservoir and studying the integrity of the seal over time. Densely sampled baseline surveys that provide an optimal sampling from near to far offsets can derisk future monitoring objectives and enable seismic monitoring methods based on high-resolution imaging, full waveform inversion, as well as quantitative interpretation. Traditional baseline surveys for monitoring of hydrocarbon reservoirs were often acquired with standard single or dual source arrays and short offsets only. Thus, direct experience transfer from hydrocarbon reservoir to CO2 storage site monitoring for wide-tow multi-source and longer offset streamer acquisition solutions are not necessarily possible. 4D repeatability and detectability requirements in the context of CCS are currently the subject of a major research effort in the industry with rapid progress in technology development and improved understanding.

Read more about acquisition for carbon storage in another recent First Break paper | Advanced 3D seismic crossover technologies between hydrocarbon exploration, CCS development, and offshore wind.

CCS Site Characterization Toolbox | Reprocessing or New Acquisition

Significant information can be extracted from existing data for comprehensive CCS site screening studies and adaptive thinking is required to address the challenges of both deep, overburden, and near-surface interpretation.

The resolution of the near-surface image can be inferior to more modern fit-for-purpose high-resolution seismic solutions (such as ultra-high resolution or site survey) and the age of a dataset introduces uncertainties for accurately quantifying shallow hazards.

Reprocessing still provides critical and early insights into the containment risks and provides a more relevant basis for future work, which may include new seismic acquisition and processing programs.

There is yet to commence a large dedicated CCS screening program, where the industry combines technology and resources from both traditional hydrocarbon exploration with the CCS effort for cost-effective screening of large and/or multiple sites.

There is no doubt that CCS will play a critical role in achieving net-zero targets, albeit the negative public perception associated with the industry’s own CO2 emissions needs to be addressed with more focused decarbonisation efforts. As such repurposing legacy data from traditional hydrocarbon exploration for CCS is less about shaping a positive legacy of the O&G industry and more about diversification and evolving the industry.

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