The growing importance of subsurface carbon storage for tackling anthropogenic carbon emissions requires new ideas to improve the rate and cost of carbon capture and storage (CCS) project development and implementation. We assess sandstones from the UK Geoenergy Observatories (UKGEOS) site in Glasgow, UK and the Wilmslow Sandstone Formation (WSF) in Cumbria, UK at the pore scale to indicate suitability for further assessment as CCS reservoirs. We measure porosity, permeability and other pore geometry characteristics using digital rock physics techniques on micro computed tomographic images of core material from each site. We find the Glasgow material to be unsuitable for CCS due to very little porosity—up to 1.65%—whereas the WSF material showed connected porosity up to 26.3% and permeabilities up to 6040 mD. Our results support the presence of a percolation threshold at 10% total porosity, introducing near full connectivity. We find total porosity varies with permeability with an exponent of 3.19. This provides reason to assume near full connectivity in sedimentary samples showing porosities above this threshold without the need for expensive and time consuming analyses.Supplementary material: Information about the boreholes sampled in this study, additional well logs of both boreholes and a summary of the supporting data plotted throughout this article from literature is available at https://doi.org/10.6084/m9.figshare.c.5260074.Thematic collection: This article is part of the Geoscience for CO2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage

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Despite having been affected by several stages of exhumation during the Cretaceous and Cenozoic, the contemporary stress state of the East Irish Sea (EISB) is poorly characterised. As the basin is mature in terms of exploitation of hydrocarbons, future exploration beyond the conventional Sherwood Sandstone Group reservoir (Triassic) necessitates a greater understanding of the in situ stress field, while proposed natural gas storage and carbon sequestration schemes also require detailed stress field information. Using petroleum well data, the in situ stress field of the EISB has been characterised to assess the mechanical seal integrity. A strike-slip stress regime most-likely prevails in the basin, meaning the Maximum Horizontal Stress (SHmax) is the greatest of the principal stresses. Interpretation of stress orientation data suggests that SHmax is oriented 152˚ ± 12˚, consistent with mean stress orientations across the wider region associated with plate boundary forces. Some degree of structural control appears to influence the orientation of SHmax, with orientations locally aligned sub-parallel to major Permo-Triassic basin-bounding faults. Fault reactivation risk is evaluated through modelling the pore pressure increase required to induce failure on pre-existing faults. Vertical faults striking 30˚ from SHmax are optimally-oriented to become reactivated under elevated pore pressure conditions. For any project relying on an element of fault seal for the containment of buoyant fluids at the average reservoir depth of 800 m, pore pressure increase should be less than 3.3 MPa to avoid reactivating pre-existing optimally-oriented faults. Higher pressure increases would be required to initiate reactivation of faults with other orientations. Vertical faults striking perpendicular to SHmax are least likely to become reactivated, and in the absence of halite, seal integrity would instead be limited by caprock strength and capillary-entry pressure. Major faults affecting the basin have been analysed for their slip tendency (ratio of shear to normal stress), which provides an indication of their susceptibility to become reactivated. Although the analysis is limited due to lack of an accurate 3D representation of the fault network, the results suggest that many of the fault orientations observed in the EISB exhibit high slip tendencies, including N-S striking faults to the north and west of the East Deemster Fault, where the SHmax orientation is NW-SE. Faults striking perpendicular to SHmax, such as the Lagman Fault, are least likely to become reactivated due to higher normal stresses that inhibit frictional sliding, while faults striking parallel or very close to SHmax also exhibit low slip tendency as they are not subjected to significant shear stresses.

The Lower Cretaceous Captain Sandstone Member of the Inner Moray Firth has significant potential for the injection and storage of anthropogenic CO 2 in saline aquifer parts of the formation. Pre-existing faults constitute a potential risk to storage security owing to the elevated pore pressures likely to result from large-scale fluid injection. Determination of the regional in situ stresses permits mapping of the stress tensor affecting these faults. Either normal or strike-slip faulting conditions are suggested to be prevalent, with the maximum horizontal stress orientated 33°-213°. Slip-tendency analysis indicates that some fault segments are close to being critically stressed under strike-slip stress conditions, with small pore-pressure perturbations of approximately 1.5 MPa potentially causing reactivation of those faults. Greater pore-pressure increases of approximately 5 MPa would be required to reactivate optimally orientated faults under normal faulting or transitional normal/strike-slip faulting conditions at average reservoir depths. The results provide a useful indication of the fault geometries most susceptible to reactivation under current stress conditions. To account for uncertainty in principal stress magnitudes, high differential stresses have been assumed, providing conservative fault-stability estimates. Detailed geological models and data pertaining to pore pressure, rock mechanics and stress will be required to more accurately investigate fault stability.

The orientation of the maximum horizontal compressive stress (S Hmax ) in the UK Southern North Sea has been determined using data derived from borehole breakout analysis of four-arm caliper logs.The results agree with existing stress models for NW Europe, confirming that horizontal stresses in the region have an approximately NW-SE orientation of S Hmax . This is interpreted as being a result of plate boundary convergence. Local deviations in the horizontal stress orientations are observed spatially and also vertically within some wells. Some of these deviations are attributed to rotations of the stress field adjacent to faults or between different fault blocks. The data also suggest detachment of the stress regime in the post-Permian cover rocks, caused by the presence of a thick underlying Permian-aged evaporite sequence and associated halokinesis. Despite the generally low scoring of the stress measurements when compared with the quality ranking scheme proposed by the World Stress Map project, the consistent orientation of the stress trajectories suggests that these data are reliably indicative of regional stress orientations. Analyses of borehole resistivity image logs have been used to verify the calculated stress orientations in some wells. These image logs validate some measurements whilst highlighting a number of deficiencies in the use of four-arm caliper data to characterise borehole breakouts. From the available data it is difficult to unambiguously define the nature of variations from the mean stress orientations observed. Further analysis of image log data over greater depth intervals is therefore required in order to investigate more fully the effects of stress rotations near faults and apparent stress detachment above saltcored anticlinal structures.