Elin Skurtveit

This study presents the fluid flow testing results of 32 lava flow samples recovered from IODP Sites U1571/U1572 (145–315 mbsf) on the Skoll High, Vøring Margin. Few past studies have investigated the mid-Norwegian margin’s basalt reservoir and flow properties. Even fewer implement subsurface-like conditions to explore in-situ fluid flow and the possible response to changing pore fluids during injection. These aspects are critical for large-scale carbon sequestration. In this study, hydraulic properties including porosity and permeability were measured at ambient conditions for all samples along with multi-stress, multi-fluid core flooding experiments on a representative sample enclosed in an X-ray µCT scanner. Petrographic observations reveal varying degrees of alteration with secondary clays, including smectite, commonly lining pores and vesicles. He-porosities (4.2–45.6%) fall mostly below the theoretical percolation threshold of ~ 30% for randomly distributed spherical pores. Klinkenberg-corrected nitrogen-gas permeability (0.0038–361.9 mD) shows weak correlation to porosity and no apparent depth dependence. Further, brine permeability is four orders of magnitude lower than gas permeability, with no notable flow reduction during injection of CO ₂ . Rather than being stress-dependent, we argue that brine-induced swelling significantly reduced the accessibility of microstructural elements to fluids. The findings urge caution when using literature-reported, ambient gas measurements for basalts with similar levels of alteration as they may not accurately reflect in-situ fluid flow properties. Low matrix permeability of the moderately to highly altered vesicular basalts and prevalence of swelling clays may, without major contributions from fractures, limit the injectivity potential into the basaltic lava flows on the Skoll High, Vøring Margin.

The North Sea plays a strategic role in the transition towards the green economy, as it hosts many offshore wind farms and carbon capture and storage (CCS) sites. The North Sea is characterized by moderate seismicity. For example, the 2023 Mw 5.1 North Sea earthquake was the largest earthquake in the region in 33 years, and led to a temporary shutdown of production of Equinor’s oil platform Snorre B.The last comprehensive seismic hazard assessment of the North Sea was performed about twenty years ago (Bungum et al. 2000). Since then, there have been many new ground motions recorded from both onshore and offshore seismic stations in the North Sea region, as well as major advancements in probabilistic seismic hazard assessment (PSHA) methodology. As a result, this study aims to develop the first region-specific ground-motion model for the North Sea, which will be used in an updated PSHA to ensure the safe design of new offshore wind farms and CCS sites.This research forms a part of the SHARP-Storage project, an interdisciplinary Accelerating CCS Technologies project developing improved methods for quantitative assessment of subsurface CO2 storage containment risks. The SHARP project compiled a dataset of North Sea ground-motion records, which comprises data on natural seismicity from broadband seismometers, both onshore and offshore. Moreover, synthetic ground motions are generated to address the paucity of recordings (especially strong motions) in the North Sea region. The associated flat-file, including the metadata and intensity measures (e.g., spectral acceleration and Fourier amplitude) of manually processed waveforms is constructed for the analysis of the ground motion characteristics in the North Sea. An empirical ground-motion model is developed based on the recorded and synthetic data, which is validated and compared with the available observations and models for the North Sea.The ground-motion model is defined for a reference rock condition. Amplification factors to estimate shaking at the soil surface are derived based on a database of one-dimensional site response analyses. The sediments encountered in the North Sea have been deposited in dynamically changing environments ranging from arctic to temperate and reworked by the movement of ice sheets. As a result, a wide range of soil types and properties are found. To capture this variability, representative profiles are developed based on site investigations from locations with different soil conditions encountered in the North Sea. One dimensional non-linear site response analyses are then performed using the recorded and synthetic ground motion database to generate a database of response spectra amplification values.This study presents the preliminary results of the North Sea ground-motion model and highlights the challenges in conducting PSHA for the North Sea region. These results improve the assessment of seismic hazard and risks to CO2 storage security in the North Sea.

Fault zones contain a mixture of lithologies, various fault structures, and evi...

The mechanical integrity of caprocks overlying injection formations is one of the key factors for safe storage of carbon dioxide in geological formations. Undrained effects caused by CO₂ injection on strength and elastic parameters should be properly considered in the operational design to avoid fracture creation, fault reactivation and unwanted surface uplift. This study presents results from eleven undrained triaxial compression tests and one oedometer test on the Draupne shale, which is the main caprock of the Smeaheia site in the North Sea, to extract parameters relevant for seal integrity. Tests have been performed on samples oriented perpendicular to and parallel with the horizontal layering of the rock to study the effects of sample orientation relative to the loading direction. Results from undrained triaxial tests showed only minor effects of sample orientation on friction and cohesion. However, when loading during undrained shearing was parallel with layering (horizontal samples), measured Young’s modulus was roughly 1.4 times higher than for the vertical samples. Undrained shearing of vertical samples generated 30–50% more excess pore pressure than for horizontal samples with similar consolidation stress owing to more volume compaction of vertical samples. With apparent pre-consolidation stress determined from a high-stress oedometer test, the normalized undrained shear strength was found to correlate well with the overconsolidation ratio following the SHANSEP (Stress History and Normalized Soil Engineering Properties) procedure.