Flexible Funding 2023: Dr Emma Michie, University of Liverpool

Faulting in siliciclastic – carbonate sequences; an improved characterization for de-risking faulted CCS sites

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Key facts about this Flexible Funding research project

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Institution: University of Liverpool
Department: Earth, Ocean and Ecological Sciences
Start date: 1 October 2023
Principal investigator: Dr Emma Michie
Co-Investigators: Professor Quentin Fisher
Amount awarded by UKCCSRC: £29,356

Why is this research needed?

This project will reduce uncertainty when analysing fault seal behaviour within siliciclastic – carbonate (mixed) sequences within the subsurface, benefiting CO2 storage site assessment, allowing for improved identification of site suitability. To increase worldwide stored CO2 from 40Mt per year to 3Gt per year by 2050 to achieve net zero, we must utilise sites influenced, or bound, by faults, cutting mixed sequences and where we currently have no tools to accurately predict their sealing potential. The influence of carbonate interbeds on fault seal is currently unknown, with few examples of the variability of macro- and micro-structures within the fault core of these scenarios.

Further, we have limited data regarding the permeability of fault rocks within faulted mixed sequences. We will use outcrop analogues relevant to CO2 storage sites to characterise the overall structure and permeability of faults within mixed sequences. Detailed mapping of relevant outcropping faults will provide information regarding variability of fault core macrostructure associated with the influence of carbonate interbeds on fault development. Fault rock samples will be collected and used for microstructural analysis and permeability measurements in order to assess the likely seal potential and processes influencing it. Results will progress towards the modification of existing, or creation of new, algorithms to help stakeholders more accurately predict whether the faults will act as barriers or conduits to fluid flow within mixed sequences.

What is this research investigating?

There are currently only 30 CCS projects in operation worldwide, 11 under construction, and 153 in development. To increase the amount of CO2 we can store in the subsurface to achieve net zero, we must utilise more structurally complex sites, containing or bound by faults, within vertically heterogeneous sequences containing both siliciclastic and carbonate rocks. While there are well established techniques to assess a fault’s hydraulic behaviour within sandstone-shale (siliciclastic) sequences, and advancing research of fault seal behaviour within carbonate sequences, there are limited data available within vertically heterogeneous (i.e. siliciclastic-carbonate) sequences, hence we currently have no tools to accurately predict their sealing potential.

Objectives and Anticipated Outputs:

We will use outcrop analogues relevant to CO2 storage sites to characterise the overall structure of faults within mixed sequences. This study has the principal aim of identifying key relationships on fault rock behaviour to use predictively for energy transition industries, to fill the aforementioned knowledge gap. Improving our ability to assess fault behaviour in the subsurface is crucial to assess the longevity or any leakage potential of any storage site. The aims will be addressed by 2 work packages.

WP1) Macro-scale characterisation (EM, FA), using detailed mapping of relevant outcropping faults providing information regarding variability of fault core structure associated with the influence of carbonate interbeds on fault development. Storage sites such as Hewett Gas Field, Southern North Sea, the Captain Sandstone in the Moray Firth and Smeaheia, Norwegian North Sea, contain mixed sequences in the overburden; Haisborough Group (mudstone, sandstone, dolomitic mudstones, evaporites) and Cromer Knoll Group (clays, marls, chalks, limestones) often acting as a top seal to the CO2 reservoir units. When faulted, there is a high degree of uncertainty regarding the likely lateral seal potential associated with the stratigraphic heterogeneity. We have chosen two analogous field localities within the Central and Northern Apennines, Italy, where faults are documented as cutting mixed sequences containing carbonates, marls, clays and sandstones. Specifically, along-strike thickness, fracture, and observable and mappable fault rock variation will be recorded. We will collect roughly 20 oriented samples from the two field localities to be used within WP2. Field data collection will commence in September 2023, for roughly eight days.

WP2) Micro-scale characterisation (EM, QF) using laboratory analysis. Microstructural analysis and petrophysical measurements will be performed on the collected samples. Specifically, porosity, permeability and capillary entry pressure will be measured, to assess the degree of variation within and along these fault zones. Controls on the petrophysical properties will be assessed by examining thin sections taken from the core plugs. Microstructural analysis (optical and SEM) will be performed to indicate processes that have occurred on the micro-scale, that act to increase or decrease permeability.

This work package will also integrate previously collected subsurface data. Twelve subsurface samples from caprock formations in the Norwegian North Sea, adjacent to the potential CO2 storage site Smeaheia, have been collected, with permeability measurements taken, supported by Equinor. We will combine both datasets to assess likely fault behaviour in these types of sequences.

What does the research hope to achieve?

Not only would this benefit those working within the general structural geology topic, but more importantly this work will progress towards increasing our knowledge and confidence in assessing the validity of a CO2 storage site. This will benefit academics working within this topic, developers of CO2 storage sites, and also policy makers with increasing confidence of the general public.

Research outputs

This research is ongoing. Outputs will be shared below as they become available.