Faulting in prospective CO2 storage sites in the UK Southern North Sea

Post-depositional folding of Triassic strata, formed largely by the development of salt domes and pillows in the underlying Zechstein Group, led to the formation of numerous large anticlinal structures at the level of the Triassic aged Bunter Sandstone Formation (BSF). These structural closures, some of which have formed effective traps to natural gas, have been mapped across the UK Southern North Sea (SNS), and are currently of interest as potential prospects for the storage of anthropogenic CO2

State of stress across UK regions

Knowledge of the in- situ stress field is a key constraint for a variety of sub surface activities and crucial for the safe and sustainable use of the sub surface. However is a lack of available stress magnitude data across the UK. This report assesses legacy stress magnitude data along with new analysis to characterise the UK onshore stress field. To investigate the UK onshore in-situ stress field, three regions were studied. The regions were selected based on the potential availability of information to characterise the stress field and their resource potential for unconventional shale resources, highlighted by Andrews et al. (2013). The study focused on: East Yorkshire and North Nottinghamshire, Cheshire and Lancashire and the Weald. The vertical stress across the UK varies between 23 and 26 MPakm-1 with higher values recorded in Cheshire and Scotland compared to East Yorkshire, North Nottinghamshire and the Weald. Pore pressure measurements from Cheshire, Lancashire, East Yorkshire and North Nottinghamshire are hydrostatic with a gradient of 10.19 MPakm-1. Leak off test and formation integrity test data has been used to estimate the gradient of minimum horizontal stress in Cheshire, Lancashire East Yorkshire and North Nottinghamshire. This estimates show that the minimum horizontal stress gradient is two MPakm-1 higher in Cheshire and Lancashire than East Yorkshire and North Nottinghamshire, which is similar to the differences in vertical stress gradients. Legacy maximum horizontal stress data has been compiled from a variety of techniques from the Coal Authority and peer review publications. This data shows that the maximum horizontal stress > vertical stress, When combined with the leak off test and formation integrity test data (which shows vertical stress > minimum horizontal stress) this indicates that the UK is predominately a strike slip faulting environment. Above 1200 m there are indications of reverse faulting though these are largely confined to igneous rocks in Cornwall, Leicestershire and Cumbria. The available information shows that there are similarities in the stress field across the UK though due to the geographic and stratigraphic constraints on the data more information would help to better characterise the stress field

In-situ stress orientations in the UK Southern North Sea: regional trends, deviations and detachment of the post-Zechstein stress field

The orientation of the maximum horizontal compressive stress (SHmax) 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 SHmax. This is interpreted as being a result of plate boundary convergence. Local deviations in the SHmax 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. Analyses of borehole resistivity image logs have been used to verify the SHmax orientations in some wells. These image logs validate some of the stress indicators 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 SHmax orientations observed. Further analyses of image log data over greater depth-ranges are therefore required in order to investigate more fully the effects of stress rotations near faults and apparent stress detachment above salt-cored anticlinal structures

Fault seal controls on security of CO2 storage in aquifers

Structural traps for engineered storage of CO2 usually rely on a component of fault seal. In assessing the performance risk of storage sites, the conditions under which natural CO2 and CO2/hydrocarbon mixtures are retained by faults is poorly known. Mechanical failure can occur by flow along the fault plane due to extension, compression or shear. Geometric juxtaposition of aquifers or lack of low permeability fault gouge can enable flow across the fault plane. It is well established that faults which are close to being critically stressed have markedly different properties with respect to both their fluid flow and geomechanical characteristics. Here we examine three case studies. In the first two, the Rotliegend Sandstone reservoirs of the Oak and Fizzy Fields in the Southern North Sea, both of which are natural fault-bound gas fields with high CO2 content, we modify standard fault seal approaches to account for the different physical and chemical properties of CO2 to oil and CH4. In particular the impact of IFT and contact angle on threshold capillary pressure is investigated. Faults of both the Oak and Fizzy fields are analysed for fracture stability and slip tendency and are found to be stable (relative to present-day stresses) in all modelled scenarios and could withstand CO2 column heights in excess of trap height. However, under detailed assessment of fault seal potential for CO2-CH4 mixtures, both fields appear to be limited in column height by cross-fault leakage through carbonate layers of the overlying Zechstein Group. The third case study assessed the Captain Sandstone saline aquifer of the Inner Moray Firth. The in situ stress field was characterised using data available from hydrocarbon exploration wells. A range of potential stress fields were identified, and regional 3D geometric mapping of the major faults was then used to assess fault stability under the different potential stress regimes. Additionally, stereographic plots of fault dip angle and strike were used to deduce the pore pressure perturbation that could cause the mechanical reactivation of faults of any orientation. This accounted for unmapped faults that might truncate the storage reservoir and its overburden. In the stress scenario with the highest differential stress magnitudes low overpressures in the region of ~1.5 MPa could cause the reactivation of preferentially oriented faults, whereas higher induced pressures may be supported in lower differential stress regimes. Higher overpressure would also be required to cause the reactivation of the non-optimally oriented faults

Estimating available salt volume for potential CAES development: a case study using the Northwich Halite of the Cheshire Basin

The massively bedded rock salts forming the Northwich Halite Member of the Cheshire Basin represent a huge mineral resource, which historically, have been worked by dry mining for rock salt and brine production from both the area of wet rockhead and also from solution-mined caverns. More recently, the halite beds have also provided the host storage horizon for natural gas storage in specifically designed and constructed solution-mined salt caverns. Increasingly, compressed air energy storage (CAES) is being viewed as a viable bulk storage option for surplus electrical energy, which may be through the use of off-peak electricity from both conventional and renewable sources. We describe a novel technique using Esri’s ArcGIS® Geographic Information System software, to derive potential storage cavern locations and an estimate of the physical volumes that might be available for storage purposes, including for CAES. The process involves defining the spatial distribution, thickness and insoluble content of the halite beds is described, together with an estimate of the potential physical volumes of solution-mined caverns. Cavern volumes compare favourably with those of current gas storage facilities, which are illustrated in terms of their surface footprints and use of resource

Impact of in situ stress and fault reactivation on seal integrity in the East Irish Sea Basin, UK

Despite having been affected by several stages of exhumation during the Cretaceous and Cenozoic, the contemporary stress state of the East Irish Sea Basin (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

Stochastic modelling of hydraulic conductivity derived from geotechnical data: an example applied to central Glasgow

Characterising the three-dimensional (3D) distribution of hydraulic conductivity and its variability in the shallow subsurface is fundamental to understanding groundwater behaviour and to developing conceptual and numerical groundwater models to manage the subsurface. However, directly measuring in situ hydraulic conductivity can be difficult and expensive and is rarely carried out with sufficient density in urban environments. In this study we model hydraulic conductivity for 603 sites in the unconsolidated Quaternary deposits underlying Glasgow using particle size distribution and density description widely available from geotechnical investigations. Six different models were applied and the MacDonald formula was found to be most applicable in this heterogeneous environment, comparing well with the few available in situ hydraulic conductivity data. The range of the calculated hydraulic conductivity values between the 5th and 95th percentile was 1.56×10–2–4.38mday–1 with a median of 2.26×10–1 mday–1. These modelled hydraulic conductivity data were used to develop a suite of stochastic 3D simulations conditioned to existing 3D representations of lithology. Ten per cent of the input data were excluded from the modelling process for use in a split-sample validation test, which demonstrated the effectiveness of this approach compared with non-spatial or lithologically unconstrained models. Our spatial model reduces the mean squared error between the estimated and observed values at the excluded data locations over those predicted using a simple homogeneous model by 73 %. The resulting 3D hydraulic conductivity model is of a much higher resolution than would have been possible from using only direct measurements, and will improve understanding of groundwater flow in Glasgow and reduce the spatial uncertainty of hydraulic parameters in groundwater process models. The methodology employed could be replicated in other regions where significant volumes of suitable geotechnical and site investigation data are available to predict ground conditions in areas with complex superficial deposits

Development of a city-wide physical property model of the greater Glasgow area using a voxel-based representation of lithology

The BGS PropBase project has sought to develop a physical property model of a major UK city and to identify potential aquifer sequences in shallow heterogeneous lithostratigraphic units. As a precursor to this, new methodologies have been applied to develop a city-scale lithology model of shallow, unconsolidated sediments in Glasgow, Scotland. The model has been created by developing a stochastic voxel representation of clastic geology, based on upscaling of observed borehole lithology, independent of lithostratigraphy. Assessments of uncertainty are made using multiple realisations of lithology and using these to compute the probability of one of a limited set of lithologies existing in any voxel. Although similar to work previously undertaken in the Netherlands by TNO, this model uses a borehole dataset which has not been cleaned and refined, nor has it been substantially reinterpreted to improve resolution. Results have been compared with existing lithostratigraphic models of the Greater Glasgow area created using the BGS GSI3D framework modelling methodology. In a dense borehole field, this demonstrated that voxel modelling may readily respect a larger proportion of the boreholes within the modelling area. Comparison of the voxel models with lithostratigraphic surfaces from the previous GSI3D models has resulted in a quality assurance process that allows assessment of the quality of the framework model in terms of representing subsurface lithologies. Subsequently, physical property data were upscaled into the voxel model on a per-lithology basis. These parameterised grids may be integrated into BGS ZOOM groundwater models; this project is thereby at the confluence of four different modelling approaches: (i) geological framework modelling, (ii) voxel modelling, (iii) property modelling, and (iv)input grids for groundwater modelling. The techniques applied in the course of the project have the potential to be regularly used in developing future BGS model outputs and to be incorporated as standard tools of the BGS modelling workflow

Refraction microtremor (ReMi) to determine the shear-wave velocity structure of the near surface and its application to aid detection of a backfilled mineshaft

Passive refraction microtremor (ReMi) surveys utilize standard field seismic-refraction recording equipment and linear geophone arrays to record ambient background noise owing to microtremors caused by natural and anthropogenic activities. The technique relies upon the detection of coherent phases of Rayleigh waves that have propagated along the axis of the geophone array, which is the same mode of propagation that causes ground roll on standard refraction surveys. Rayleigh-wave propagation is confined within one wavelength of the surface, causing dispersion because waves with longer wavelengths (lower frequencies) are controlled by ground stiffness and density properties at greater depths. Field records that include coherent modes of dispersive Rayleigh-wave propagation along the field array are processed using slowness (reciprocal of the phase velocity)–frequency transformations to extract the phase velocity–frequency dispersion curves. A series of dispersion curves are extracted by processing the field records of sub-groups including 6–8 geophones, from which 1D shear-wave velocity–depth profiles are constructed and attributed to the centre of each array sub-group. In this survey, nine overlapping sub-groups of eight geophones were selected along the whole field array of 24 geophones equi-spaced over 69 m. A 2D shear-wave velocity section was created by infilling a grid between each of the velocity–depth profiles using an anisotropic inverse distance weighting algorithm. Interpretation of the 2D section included the identification of: (1) reworked ground comprising colliery spoil and clay to around 5 m below ground level associated with shear-wave velocities from 100 to 700 m s−1; (2) deeper strata within the host formation associated with higher velocities that increased with depth to above 1000 m s−1 at depths below 10 m; (3) a backfilled mineshaft and a backfilled sandstone quarry at depths below 7 m associated with low-velocity perturbations within the background host velocity structure. Key recommendations from this case study include the use of low-frequency geophones to increase the depth of investigation and recording of high frequencies at reduced geophone spacings to increase near-surface resolution