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

One of the requirements for a Geological Disposal Facility (GDF) for radioactive waste is that it needs to contain radionuclides away from the surface environment whilst they are still harmful, for some radionuclides this can be many to tens of thousands of years. The containment function of a geological setting is affected by the presence of fault-fracture systems, which can create pathways

for the migration of radionuclides carried by gas and water. Mercia Mudrock Group (MMG) is considered as a potential host rock, and fault-fracture systems therein are potential conduits for fluid flow. The inshore, deep, saline setting considered for the GDF is further complicated by the fault-fracture system architecture and the complex mechanical stratigraphy of the interbedded

mudrock and evaporites of the MMG. Integral to this will be to demonstrate understanding of the fracture 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-fracture systems from outcrops and cores including assessing the mechanical-stratigraphic controls on

fracture network geometries; ii) the measurement of single (and multi-phase) flow in fractures as a function 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 needs

to be integrated in model frameworks focusing on flow and transport in fractures or the exchange between 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 constitutive relationships and effective properties for radionuclide and gas transport at decametre scales. This

specifically requires a strong interplay between observations done in outcrops (fracture network and statistics thereof, understanding of fracture mineralisation versus stress directions etc) and the development of coupled hydro-chemical-mechanical models. This interplay, supported by laboratory data, will be a key output of this project to advance the understanding of fluid flow along

fractures and to highlight the potential of multi-scale, multi-method approaches to evaluate the safety 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 sequences

can also be obtained by studying the fracture mineralization. The distribution of mineralised fractures, fracture cross cutting relationships, fracture fill thickness, geochemistry, and isotopic ages, can improve our understanding of paleofluid composition, preferential flow paths, time scales and rates of fluid flow. Coupled hydro-chemical-mechanical modelling of fossil fluid flow systems

can provide process understanding and data sets to calibrate / validate models used for forward predictions over 103 to 106-year timescales. This information will directly inform modelling and laboratory 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.