Vertical migration of CO_2 in layered sandstone reservoirs is controlled by thin shale barriers whose properties are often poorly known. We develop a Bayesian framework that uses time-lapse seismic plume observations to estimate effective parameters governing lateral and vertical CO_2 migration. The forward model is a fast graph-based invasion-percolation model that extends conventional IP by representing both capillary-controlled filling of structural traps beneath shale barriers and finite-rate transfer through them. Parameters are inferred with approximate Bayesian computation and sequential Monte Carlo sampling, and the posterior samples are propagated into forecasts. Applied to real data from Sleipner, posterior simulations reproduce the broad distribution of CO_2 across nine sand units and its redistribution between 2010 and 2023. In contrast, the quasi-static model fails to reproduce this temporal evolution. Complementary synthetic experiments assess parameter recovery, forecasting, monitoring duration, and model misspecification. These experiments show that the information gained from monitoring depends on the migration events captured, with breakthrough and post-breach redistribution providing particularly strong constraints. This combination of fast simulation, probabilistic updating, and interpretable effective parameters makes the framework well suited to repeated forecast revision during active injection, especially when full-physics inference is too computationally demanding.

Long-term geological storage of CO2 in subsurface formations necessitates understanding the potential for migration and leakage. To assess these risks and to help optimize monitoring systems, we develop a framework for assessing migration probability in a multi-layer stratigraphic system using invasion-percolation concepts. For each layer, we estimate the height of the CO2 column and the associated pressure, assuming the capillary threshold pressure as the limiting factor. When the capillary threshold pressure is exceeded, migration to the next layer occurs, and so on to the subsequent layers of the stratigraphic system. By modeling the heights in different layers explicitly, we facilitate the linkage to monitoring, assuming that seismic data can be used to estimate the heights of the CO2 columns that build up in various layers over time. We conduct Value of Information (VOI) analyses to understand when it is optimal to gather seismic data to monitor the CO2 plume. The VOI analyses reveal strong links between the migration times and valuable times for monitoring. We present two illustrative case studies - a simple synthetic case and a case inspired by the Sleipner CO2 storage project, the latter of which considers migration both through invasion-percolation and through chimneys bypassing sealing units. In our case studies, the VOI seems to be high at intermediate times, when the CO2 has migrated to the immediate overlying layers. There is less value in monitoring at very early times, when the CO2 is contained in the lower layers.

Iceland represents one of the few emerged portions of the Mid-Atlantic-Ridge, a 16,000 km-long tectonic boundary separating the North American and Eurasian plates. This unique position gives rise to Iceland’s characteristic geothermal manifestations and volcanic complexes (Figure 1). Here, CO2 from volcanic sources is released alongside other natural and anthropogenic emissions, as part of the global carbon cycle.Figure 1. Map of Iceland with distribution of averaged NDVI from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) for June 2018. The red triangles show locations of volcanoes and fumaroles and the grey squares represent pixels where OCO-2 XCO2 retrievals are available.The quantification of these fluxes in relation to the plate tectonic system provides valuable insights into the nature of long-term CO2 migration and retention, which, in turn, can help assess the potential for leakage and migration pathways in the context of geological storage of CO2. However, volcanogenic sources of CO2 remain poorly quantified, partly because such studies rely on ground-based measurements or airborne remote sensing, which can be challenging and hazardous during periods of volcanic unrest.NASA’s Orbiting Carbon Observatory-2 (OCO-2), launched in 2014, has been proven to be capable of observing localised point source signals thanks to its unprecedented instrument precision and resolution. In this study, we explore the use of OCO-2’s column-averaged dry-air mole fractions of CO2 (XCO2) retrievals to assess the onshore CO2 fluxes in Iceland.Because volcanogenic sources of CO2 typically exhibit small enhancements above background concentrations, a robust quantification of the influence of non-volcanic CO2 contributors on OCO-2 retrievals is needed. Therefore, we analysed nearly a decade of OCO-2 data to reveal the long-term anthropogenic emissions trends and the seasonal variations due to biogenic fluxes. We observe a steady increase of ~2 ppm per year in average atmospheric CO2 concentrations (Figure 2a), attributed to anthropogenic emissions, and an inverse trend between monthly XCO2 averages and the Normalised Difference Vegetation Index (NDVI) (Figure 2b), reflecting seasonal vegetation growth as a carbon sink. However, due to severe weather conditions and prolonged winter darkness at high latitudes, no data was available from October to March, limiting our window of observation.Figure 2. a) Evolution of OCO-2 XCO2 observations over Iceland since 2015 with yearly means in red squares. b) Evolution of mean OCO-2 XCO2 observations and NDVI values over Iceland in 2018.The observed trends provide the reference framework required for isolating volcanogenic CO2 contributions and are informative for understanding the carbon cycle in this region. Despite observational challenges posed by Iceland’s location in the high North, our analysis of these OCO-2 retrievals brings important insights into resolving spatiotemporal CO2 patterns from space over volcanically active regions. The ensuing step will be to quantify the relative contribution of known volcanogenic CO2 sources (e.g., volcanos, fumaroles, and diffuse soil degassing, etc.) to OCO-2 retrievals.

Seismic data at the Sleipner CO2 storage project are used to gain insights into the spatial distribution of CO2, revealing the fraction of the pore volume occupied by CO2 at different scales. The CO2 storage capacity coefficient (Cc) is determined and then scaled by estimates of the fractional area and fractional volume occupied by CO2 to determine a volume storage coefficient, Vc. Estimates of Vc are close to the independently determined maximum layer saturations for the case of CO2 columns under conditions of gravity equilibrium. The analysis is useful for understanding the nature of CO2 storage efficiency measures and for storage capacity estimates in general. The methods were applied to a dataset derived from time-lapse RMS amplitude difference maps for 9 repeat surveys (1999-2020) and then compared to an FWI velocity model of the 2010 survey, which allowed a layer-bylayer analysis. The macroscopic storage efficiency at Sleipner is found be in the range of 1-6% depending on the reference pore volume used in the analysis, while the fraction of the within-plume pore space occupied by CO2 within each layer is found to lie in range of 30% and 60%. The methods proposed here have the potential to significantly improve the quantitative analysis of seismic data used for monitoring other CO2 storage sites.

In this study, we demonstrate the benefits of applying full-waveform inversion (FWI), for the imaging of multi-layer CO2 plumes. We apply 3D FWI, up to 42 Hz, using the 2010 towed-streamer data from the Sleipner storage site offshore Norway. While the 3D FWI method does not aim to replace 4D FWI for CO2 migration monitoring, the properties of the Utsira aquifer at Sleipner, with low stiffness and shallow burial depth, enable effective mapping of the CO2 without the need of a repeated FWI application. Our FWI model and associated images offer a significant imaging improvement in the lower half of the plume compared with the legacy seismic data, revealing vertical CO2 migration routes that have not been observed previously at the site. We show the limitations of reflection-based migration methods for imaging CO2 migration pathways and demonstrate that FWI can improve their detection. The FWI model can support conventional time-lapse analyses by improving the interpretation of known CO2 migration routes, by highlighting CO2 layers with low reflectivity, and by attenuating multiples better. Our analysis suggests that CO2 migration at Sleipner is likely controlled by several vertical communication routes, including chimneys, and linear structures, several hundreds of meters long, that connect multiple accumulations of CO2. Within each layer, our analysis suggests buoyancy-driven, fill-to-spill migration, constrained by the topography of the sealing units. Finally, we conclude that, while simple CO2 accumulations can be monitored successfully using reflection-based migration methods and analyses, multi-layered CO2 plumes will greatly benefit from complementary analyses using FWI.

Potential CO2 storage sites need to perform risk assessments on the likelihood of anomalous events such as leakage. The intrinsic heterogeneity of the rock system with uncertain values for the capillary threshold pressures of the various rock elements is the most likely reason for unexpected vertical migration of CO2 within a storage complex. This study shows how the Invasion Percolation Markov Chain approach can be used to address this concern. We tested the approach using detailed 3D models of the multi-layer plume at Sleipner showing that even small variations in the threshold pressures of the shales can impact the flow of CO2 into multiple accumulations. Models with and without shale breaks reveal the importance of vertical feeders and/or faults, and the geometry of the shale layers is also crucial as the CO2 strongly conforms to topography. We demonstrate that the vertical migration of CO2 at Sleipner follows a Markovian model in which the probability of later migration events is highly dependent of the probability of preceding events. This case study illustrates how the initial migration events, which have the highest probability of occurring, should be the focus of CO2 storage risk assessments.

Disclaimer Text and Data Mining. Any automated analytical technique aimed at analysing text and data in digital form in order to generate information including - but not limited to - patterns, trends and correlations (so-called Text and Data Mining) may only be applied if it is done by a research organisation or cultural heritage institution solely for the purpose of (non-commercial) scientific research as mentioned in Clause 3 of Directive (EU) 2019/790 (the Digital Single Market Directive) and Clause 15n of the Dutch Copyright Act [Auteurswet]. Text and Data Mining for commercial purposes is not allowed; all rights in this respect are reserved pursuant to Clause 4 paragraph 3 of the Digital Single Market Directive and Clause 15o paragraph 1 of the Dutch Copyright Act.

SUMMARY We have derived an analytical approximate expression to estimate the delay in diving seismic waves due to thin layers of CO2. The expression is valid for high frequencies and can be used to estimate the delay in diving waves at seismic frequencies for large separations between the source and receiver (offset). The approximation may be used to assess CO2 detection limits using diving waves and to support survey planning for CO2 monitoring and full-waveform inversion (FWI) cycle skipping analysis. In this study, we analyse the diving-wave response to a thin layer of CO2 for band-limited data using acoustic finite-difference modelling, and compare the results against the analytical calculations. We find that the responses are offset-dependent and related to double- and single-leg interactions between the diving waves and the CO2. To test the methods, we created a synthetic representation of the 2010 subsurface conditions for the top CO2 layer at the Sleipner storage complex in the North Sea, by combining base and monitor post-stack seismic data with field velocity trends. Using the acoustic finite-difference method, we model pre-stack data that captures the complexity of field data and demonstrate the use of the diving-wave delay for CO2 migration monitoring and CO2 thin layer detection.

Prior to planned CO2 injection startup in the Horda platform offshore western Norway, in 2024, the Horda Network project has taken several measures to assess the potential of seismic hazard in the area. A study of the fault-plane solutions in the Horda platform region confirms that the direction of maximum horizontal stress is dominantly northwest–southeast to east–west over the entire area. The relative stress ratio is higher in the southeast near the Norwegian craton and lower in the northwest. Analysis of the catalog of seismicity (in the period of 2001–2021) in the Horda platform region suggests a moderate rate of seismicity with a b-value of ∼1. The magnitude of completeness is 1.5 (ML). One of the main challenges in monitoring offshore earthquakes in the Norwegian continental shelf (NCS) is the lack of azimuthal coverage when using the onshore permanent seismic stations from the Norwegian National Seismic Network (NNSN), located to the east of offshore events. To improve the azimuthal coverage, we integrated a limited number of offshore geophones from permanent reservoir monitoring systems of selected oil and gas fields (Grane and Oseberg on NCS) with the onshore NNSN seismic stations. This integration is challenging because of the level of ambient noise in the offshore geophones. To further improve the detection and location capability, we deployed a nine-element onshore array of broadband seismometers (HNAR) on Holnsnøy island to the east of the Horda platform. By incorporating array processing methods on HNAR, the signal-to-noise ratio is improved, and several previously uncataloged earthquakes could be detected. Offshore sensors are often subject to correlated noise from seismic interferences and platform or shipping noise sources, so we also incorporated array processing for selected geophones from offshore deployments, which greatly reduced such noise and hence improved the event detection.

Disclaimer Text and Data Mining. Any automated analytical technique aimed at analysing text and data in digital form in order to generate information including - but not limited to - patterns, trends and correlations (so-called Text and Data Mining) may only be applied if it is done by a research organisation or cultural heritage institution solely for the purpose of (non-commercial) scientific research as mentioned in Clause 3 of Directive (EU) 2019/790 (the Digital Single Market Directive) and Clause 15n of the Dutch Copyright Act [Auteurswet]. Text and Data Mining for commercial purposes is not allowed; all rights in this respect are reserved pursuant to Clause 4 paragraph 3 of the Digital Single Market Directive and Clause 15o paragraph 1 of the Dutch Copyright Act.

Mudrock compaction trends from the Rovuma Basin offshore Mozambique are compared with those of the Norwegian North Sea, the Gulf of Mexico and the Kutai Basin offshore Indonesia. The comparison reveals that burial rates and timing of rifting are the dominant causes for the differences observed. The compaction trend for the Rovuma Basin is broadly similar to the trends for the Kutai Basin and the Gulf of Mexico, but very different from those for the Norwegian North Sea data, which show higher porosity and shallower onset of overpressure than those from the other three basins. The relationships for seismic velocities as a function of depth show strong similarities between the Rovuma and Gulf of Mexico basins. These comparisons are used to make a general assessment of the capillary sealing potential of Cretaceous mudrocks in the Rovuma Basin, using a mudstone permeability-prediction function and a method for mapping permeability to threshold pressure, allowing estimation of maximum column heights for CO 2 and CH 4 , with uncertainty ranges. Predicted CO 2 column heights are slightly less than the equivalent CH 4 column heights. The observed CH 4 column height at one of the wells is significantly lower than that predicted from mudstone permeability, which is probably due to other factors such as fracturing or gas migration out of the structure. The comparison indicates generally good capillary sealing potential for the Rovuma Basin Cretaceous shales and offers a general approach for assessing CO 2 storage potential from hydrocarbon sealing datasets from multiple offshore basins. This article is part of the Energy Geoscience Series available at https://www.lyellcollection.org/cc/energy-geoscience-series

The Carbon Capture and Storage (CCS) industry is developing rapidly as part of initiatives to reduce CO2 emissions and to mitigate climate change. Monitoring CO2 storage sites is an essential element to ensure safe storage in the subsurface. In addition, monitoring will be an important tool to optimize injection operations and verify storage volumes. To design an optimal monitoring program, storage site characterization and leakage risk assessments need to be conducted to make sure that the monitoring solution covers legal conformance and containment assurance needs. The currently challenging business case for CCS can also be improved by optimizing monitoring solutions. In preparation for a large portfolio of future CCS sites, we are developing an optimized geophysical monitoring toolbox. The toolbox contains measurements and analysis tools for conventional 4D seismic and 4D gravity. Additional new technologies, including cheaper, versatile versions of 4D seismic, Distributed Acoustic Sensing (DAS), Full Waveform Inversion (FWI) and passive seismic detection are investigated for their use within the toolbox. Preliminary results indicate that the geophysical toolbox for offshore CCS monitoring can be extended beyond conventional use of 4D seismic and 4D gravity surveys by using targeted surveys and sparse acquisition layouts, including DAS data and applying FWI. This should enable more optimized and cost-effective monitoring solutions for different stages of storage operations.

The Norwegian State recently awarded several exploitation licenses for CO2 storage to support development of the world’s first full-scale CCS value chain, especially in the Horda platform region offshore the west coast of Norway. Prior to selecting a storage site, understanding of the natural seismicity can provide an insight into the state of tectonic stress field in the area of interest. By analyzing the national catalog of seismicity, we characterize a moderate level of seismicity in the Horda platform. Historical records show that events up to Mw 5.7(ML~5.3) have occurred in this region. The instrumental seismicity reveals an average b-value of ~1.0, indicating a normal seismo-tectonic behavior. The current magnitude of completeness is ML=1.5-1.6 in the near shore area but it increases with distance from shore. With the objective of improving detection capabilities, we have since 2017 tested and demonstrated the value of adding a dedicated onshore array (HNAR) and selected offshore geophones from oil and gas fields and integrating them with the Norwegian National Seismic Network (NNSN). Deployment of the HNAR array has had significant improvement in the detection threshold of seismicity in the Horda platform. About 132 un-cataloged (by NNSN) events were detected by HNAR during a period of 2 years, confirming how array processing can enhance signal to noise ratio and impact the detection threshold of earthquakes in the Horda platform. Furthermore, combined array processing of the HNAR together with offshore geophones from the PRM fields on Grane, Snorre and Oseberg hydrocarbon fields can further enrich the catalog of seismicity in this area by lowering detection thresholds and giving better location accuracy.

Carbon Capture and Storage is now widely accepted as a key technology for reducing the increase in green-house gasses (GHGs) in the Earth’s atmosphere and thus slowing the rate of global warming. Ultimately as capture technology improves and industrial capture along with Direct Air Capture becomes more cost effective, the acceptance of geological storage for the captured green-house gasses will become the key limiting factor in the CCS process. CCS as a technology has arisen during a time when there is increased scrutiny and opportunity for amplification of criticism (e.g. via social media). Much of this is not well informed, yet it still has the power to influence decision makers into risk-averse solutions. Regulation of geological storage could therefore end up being either a key enabler or disabler into the future. Overly prescriptive regulation based on avoidance of all risk has the potential to either stop projects in their approval phase or make them so costly that projects will be delayed to the point where they have significantly reduced benefit (i.e. they are too late). A conversation is therefore required on the level of risk acceptance for CO2 storage sites. The requirement for “perfect” solutions will mean that suitable storage sites may not be used. The risk of doing something (i.e. developing a storage site) needs to be weighed up against the risk of not doing something (i.e. venting injectable CO2 into the atmosphere). The number of large-scale CCS projects dedicated for storage, not enhanced oil recovery, is still relatively small, so they are not yet seen as common place. This novelty acts as an impediment to acceptance. There is a natural reluctance to accept a new process which is complex and difficult to grasp, hence regulators may feel (in response to community/political pressure) the need to ensure the project approval process removes ‘all possible risks’. Whilst this may seem reasonable, it needs to be balanced against the alternative of continuing to vent that GHG. Furthermore, the consequences of possible leakage risks need to be assessed. If a possible event has a low probability of occurring and a low consequence, it can be generally treated as acceptable. This is not to say that project proponents should be allowed to develop projects without scrutiny, that is patently not appropriate. However, the balance between the benefit of preventing industrial CO2 from entering the atmosphere should be part of the risk assessment and project approval process. New projects should be encouraged rather than stymied.This paper will discuss what is meant by risk, both as a technical term but also in terms of perception. This leads to a better understanding risk aversion and risk acceptance. We will review several different regulatory settings for comparison, and discuss risk-sharing, so that the greater common good is better understood. The aim is that all participants in the CO2 geological storage process including project proponents, regulators, financers, local and broader communities understand the how a better understanding of “risk” will lead to better environmental outcomes.

Dissolution of CO2 is a critical chemical trapping mechanism for the secure storage of CO2 in the subsurface. However, there is considerable uncertainty over the rate and cumulative mass of trapping via CO2 dissolution, particularly over longer timescales. Naturally occurring CO2 reservoirs are valuable analogues that can help to predict the long-term performance of anthropogenic CO2 storage. The 3He and δ13C content within the CO2 in natural analogues can be used to quantify the fraction of gaseous CO2 removed by dissolution. Previous studies show that dissolution is the dominant form of chemical trapping. In the Bravo Dome natural CO2 reservoir, dissolution has likely removed between 20-30% of all gaseous CO2. Here, we present a meta-analysis of dissolution rates constrained from a natural analogue of long-term storage (Bravo Dome), an operating CO2 reservoir (Sleipner) and a sample of published modelling studies. Our analysis tests end-member models for the change in CO2 dissolution rate over time, in order to better constrain the wide range of outcomes that current modelling studies indicate. Results from Bravo Dome are on the low end of the range from modelling studies. Analogue evidence supports a model of the majority of CO2 dissolution occurring during or shortly after CO2 injection into the reservoir, followed by a rapid decline in dissolution rate over time. As natural analogues are the only means to directly assess dissolution rates through geologic time, the analogue database needs to be expanded and here we identify sites that will be prioritized.

Thematic collection: This article is part of the Geoscience for CO2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage

An understanding of fault seal is crucial for assessing the storage capacity and containment risks of CO 2 storage sites, as it can significantly affect the projects on across-fault and along-fault migration/leakage risk, as well as reservoir pressure predictions. We present a study from the Smeaheia area in the northern Horda Platform offshore Norway, focusing on two fault-bounded structural closures, namely the Alpha and Beta structures. We aim to use this study to improve the geological understanding of the northern Horda Platform for CO 2 storage scale-up potentials and illustrate the importance of fault seal analysis in containment risk assessment and storage capacity evaluation of a CO 2 storage project. Our containment risk assessment shows that the Alpha structure has low fault-related containment risks; thus, it has a potential value to be an additional storage target. The Beta structure shows larger fault-related containment risks due to juxtaposition of the prospective storage aquifer with the basement across the Øygarden Fault System. The storage capacity of Smeaheia will be determined by the long-term dynamic interplay between pressure depletion and recharging. Our study shows that across-fault pressure communication between Smeaheia and the depleting Troll reservoir is likely to be through several relay ramps of the Vette Fault System. However, Smeaheia also shows pressure-recharging potentials, such as through the subcropping areas at the Base Nordland Unconformity. The depletion observed in the newly drilled well 32/4-3S gives a good validation point for our fault seal predictions and provides valuable insights for future dynamic simulations. Thematic collection: This article is part of the Geoscience for CO 2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage

ABSTRACTCretaceous shales from the emerging gas province of the Rovuma basin are examined using well‐log data from two exploration wells. The P‐wave acoustic impedance (AIp) data were estimated, and then brittleness and ductility were assessed within the framework of the Reuss–Voigt limits for acoustic impedance. We then predicted the shale consolidation using a weighting function (Wc) which varies between 0 for the case of grains in suspension (the Reuss limit) and 1 for the case of consolidated rock or cemented shale (the Voigt limit). At the Reuss limit, the formation AIp is highly sensitive to pressure while at the Voigt limit the formation AIp is insensitive to pressure. For the wells in this study, most data plot close to the lower bound with Wc < 0.5, hence showing a significant sensitivity to pressure. Although the Cretaceous shales in the Rovuma basin are dominated by a mechanical compaction regime, some onset of chemical compaction is observed. The rock‐physics analysis showed that the same shale in these two exploration wells had very distinctive and contrasting elastic properties. The deeper well interval clearly shows a ductile shale while the shallower well shows a range of ductile to brittle behaviour matching with the onset of chemical diagenesis at temperatures >58°C. These differences in the development and rate of chemical diagenesis in the same formation are likely due to contrasts in the rate of burial and have important implications for future seismic exploration studies.