Field, laboratory and modelling constraints on fluid transport in fractured mudrocks with a focus on chemical self-healing

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One of the requirements for a Geological Disposal Facility (GDF) for radioactive waste is that itneeds to contain radionuclides away from the surface environment whilst they are still harmful, forsome radionuclides this can be many to tens of thousands of years. The containment function of ageological setting is affected by the presence of fault-fracture systems, which can create pathwaysfor the migration of radionuclides carried by gas and water. Mercia Mudrock Group (MMG) isconsidered as a potential host rock, and fault-fracture systems therein are potential conduits forfluid flow. The inshore, deep, saline setting considered for the GDF is further complicated by thefault-fracture system architecture and the complex mechanical stratigraphy of the interbeddedmudrock and evaporites of the MMG. Integral to this will be to demonstrate understanding of thefracture hosted (single and two phase) fluid flow and solute transport process.This broad scope of this research involves i) the observational description of MMG fault-fracturesystems from outcrops and cores including assessing the mechanical-stratigraphic controls onfracture network geometries; ii) the measurement of single (and multi-phase) flow in fractures as afunction of rock-fracture properties, effective stress and fracture / stress field orientations and; iii)measurement of adsorbing solute and fluid specific transport processes. All this information needsto be integrated in model frameworks focusing on flow and transport in fractures or the exchangebetween fractures and matrix. Given the need for models to describe flow at various length-scales,including regional scales, detailed numerical upscaling workflows are required to derive constitutiverelationships and effective properties for radionuclide and gas transport at decametre scales. Thisspecifically requires a strong interplay between observations done in outcrops (fracture networkand statistics thereof, understanding of fracture mineralisation versus stress directions etc) and thedevelopment of coupled hydro-chemical-mechanical models. This interplay, supported bylaboratory data, will be a key output of this project to advance the understanding of fluid flow alongfractures and to highlight the potential of multi-scale, multi-method approaches to evaluate thesafety case for radwaste storage in MMG formations.While the fracture characterisation can be determined in conventional laboratory studies,phenomenological understanding of fluid flow in faulted/fractured mudrock-evaporite sequencescan also be obtained by studying the fracture mineralization. The distribution of mineralisedfractures, fracture cross cutting relationships, fracture fill thickness, geochemistry, and isotopicages, can improve our understanding of paleofluid composition, preferential flow paths, time scalesand rates of fluid flow. Coupled hydro-chemical-mechanical modelling of fossil fluid flow systemscan provide process understanding and data sets to calibrate / validate models used for forwardpredictions over 103 to 106 year timescales. This information will directly inform modelling andlaboratory approaches to assess the potential for fracture self-healing due to mineralization by salts(e.g., gypsum, halite) in e.g., perturbed (i.e. reactivated) or excavated zones.