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

放射性废物地质处置设施（GDF）的要求之一是，它需要包含远离地表环境的放射性核素，同时它们仍然是有害的，对于某些放射性核素，这可能是许多到数万年。断层-断裂系统的存在影响了地质背景的包容功能，它可以为气体和水携带的放射性核素的迁移创造路径。麦西亚泥岩群（MMG）被认为是一个潜在的容矿岩石，其中的断层-裂缝系统是流体流动的潜在通道。GDF考虑的近岸、深部、含盐环境因MMG的断层-断裂系统结构和互层泥岩和泥岩的复杂力学地层学而进一步复杂化。这其中必不可少的一部分是展示对断裂托管的理解。（单相和两相）流体流动和溶质运移过程，这一广泛的研究范围包括i）从露头和岩心观察描述MMG断层系统，包括评估对裂缝网络几何形状的机械-地层控制;（二）单项指标裂缝中的（和多相）流动与岩石断裂性质、有效应力和裂缝/应力场方向的关系; iii）测量吸附溶质和流体特定的传输过程。所有这些信息都需要整合到模型框架中，重点关注裂缝中的流动和运输或裂缝与基质之间的交换。考虑到需要模型来描述各种长度尺度的流动，包括区域尺度，需要详细的数值放大工作流程来推导十米尺度放射性核素和气体输运的本构关系和有效性质。这特别需要在露头（断裂网络及其统计数据，断裂矿化与应力方向的理解等）和耦合水化学力学模型的发展之间进行强有力的相互作用。这种相互作用得到实验室数据的支持，将是本项目的一个关键成果，以促进对沿裂缝流动的理解，并突出多尺度、多方法方法的潜力，以评估MMG地层中放射性废物储存的潜在情况。虽然裂缝特征可以在传统的实验室研究中确定，通过研究断裂成矿作用，还可以从唯象上理解断裂泥岩-蒸发岩序列中的流体流动。矿化裂缝的分布、裂缝横切关系、裂缝填充厚度、地球化学和同位素可以提高我们对古流体成分、优先流动路径、时间尺度和流体流动速率的了解。化石流体流动系统的耦合水-化学-力学模型可以提供过程理解和数据集，以校准/验证用于103至106年时间尺度的前瞻性预测的模型。这些信息将直接为建模和实验室方法提供信息，以评估由于盐的矿化作用（例如，石膏、岩盐），例如，扰动（即重新激活）或挖掘区。

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.