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.