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Abstract: Determination of mineral texture and diagenetic features in mudstones is crucial to reveal the history of their pore systems and provides key information to predict their future sealing ability, reactivity and storage capacity for sequestered CO2, hydrogen storage or nuclear waste disposal. To understand the spatial transport and storage of fluids, it is necessary to map the distribution of minerals and fractures in three dimensions (3D). This study proposes a novel, multi-scale three-dimensional (3D) imaging method, i.e., a combination of synchrotron- sourced micro- x-ray tomography and lab- sourced nano-tomography, to investigate the sedimentology and diagenetic features of the Bowland Shale, one of the most volumetrically important mudstone-dominated systems in the UK. Diagenetic minerals have been identified and characterised, including pyrite, calcite, kaolinite, illite, chlorite, dolomite, ankerite and authigenic quartz (micro-sized quartz and quartz overgrowths). Multi-scale 3D images provide detailed information about dolomite-ankerite zonation and carbonate dissolution pores. These features cannot be observed or quantified by conventional 2D methods, and they have not been reported in this subject area before. Using these results, potential reactions during carbon storage and other subsurface storage applications are predicted.

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Abstract: Injection of CO2 into shale reservoirs to enhance gas recovery and simultaneously sequester greenhouse gases is a potential contributor towards the carbon-neutral target. It offers a low-carbon, low-cost, low-waste and large-scale solution during the energy transition period. A precondition to efficient gas storage and flow is a sound understanding of how the shale's micro-scale impacts on these phenomena. However, the heterogeneous and complex nature of shales limits the understanding of microstructure and pore systems, making feasibility analysis challenging. This study qualitatively and quantitatively investigates the Bowland shale microstructure in 3D at five length scales: artificial fractures at 10–100 μm scale, matrix fabric at 1–10 μm-scale, individual mineral grains and organic matter particles at 100 nm–1 μm scale, macropores and micro-cracks at 10–100 nm scale and organic matter and mineral pores at 1–10 nm-scale. For each feature, the volume fraction variations along the bedding normal orientation, the fractal dimensions and the degrees of anisotropy were analysed at all corresponding scales for a multi-scale heterogeneity analysis. The results are combined with other bulk laboratory measurements, including supercritical CO2 and CH4 adsorption at reservoir conditions, pressure-dependent permeability and nitrogen adsorption pore size distribution, to perform a comprehensive analysis on the storage space and flow pathways. A cross-scale pore size distribution, ranging from 2 nm to 3 μm, was calculated with quantified microstructure. The cumulative porosity is calculated to be 8%. The cumulative surface area is 17.6 m2 g−1. A model of CH4 and CO2 flow pathways and storage with quantified microstructure is presented and discussed. The feasibility of simultaneously enhanced gas recovery and subsurface CO2 storage in Bowland shale, the largest shale gas potential formation in the UK, was assessed based using multi-scale microstructure analysis. The potential is estimated to store 19.0–21.2 Gt CO2 as free molecules, together with 18.3–28.5 Gt CO2 adsorbed onto pore surfaces, implying a theoretical maximum of 47.5–49.5 Gt carbon storage in the current estimate of 38 trillion cubic metres (∼1300 trillion cubic feet) of Bowland shale. Simple estimates suggest 6.0–15.8 Gt CO2 may be stored in practice.

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Abstract: Understanding the speciation and fate of radium during operational discharge from the offshore oil and gas industry into the marine environment is important in assessing its long term environmental impact. In the current work, 226Ra concentrations in marine sediments contaminated by produced water discharge from a site in the UK were analysed using gamma spectroscopy. Radium was present in field samples (0.1 - 0.3 Bq g-1) within International Atomic Energy Agency activity thresholds and was found to be primarily associated with micron sized radiobarite particles (≤2 μm). Experimental studies of synthetic/field produced water and seawater mixing under laboratory conditions showed that a significant proportion of radium (up to 97%) co-precipitated with barite confirming the radiobarite fate pathway. The results showed that produced water discharge into the marine environment results in the formation of radiobarite particles which incorporate a significant portion of radium and can be deposited in marine sediments.

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Abstract: Microstructures and pore systems in shales are key to understanding the role of shale in many energy applications. This study proposes a novel multi-stage upscaling procedure to comprehensively investigate the heterogeneous and complex microstructures and pore systems in a laminated and microfractured shale, utilizing 3D multi-scale imaging data. Five imaging techniques were used for characterisation from sub-nanoscale to macroscale (core-scale), spanning four orders of magnitude. Image data collected using X-ray tomography, Focused Ion Beam, and Electron Tomography techniques range in voxel size from 0.6 nm to 13 μm. Prior to upscaling, a novel two-step analysis was performed to ensure sub-samples were representative. Following this, a three-step procedure, based on homogenising descriptors and computed volume coefficients, was used to upscale the quantified microstructure and pore system. At the highest resolution (nanoscale), four distinct pore types were identified. At the sub-micron scale equations were derived for three pore-associated phases. At the microscale, the volume coefficients were recalculated to upscale the pore system to the macroscale (millimetre). The accuracy of the upscaling methodology was verified, predicting the total porosity within 7.2% discrepancy. The results provide a unique perspective to understand heterogeneous rock types, breaking though prior scale limitations in the pore system.

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Abstract: Understanding the distribution of pores and organic matter with varying organic matter concentrations and maturity is essential to understanding fluid flow in shale systems. Analysis of samples with low, medium, and high total organic carbon (TOC) and varying maturities (gas-mature and oil-mature) enables the impact of both organic matter concentrations and thermal maturation on organic matter porosity to be examined. Three gas-mature samples of varying TOC (Lublin Basin) and one oil-mature sample (Baltic Basin), both with similar mineral compositions, were selected from the same formation. Samples were imaged in 3D over four orders of magnitudes (pixel sizes from 44 μm to 5 nm). A combination of X-ray computed tomography (XCT) and Focus Ion Beam Scanning Electron Microscopy (FIB-SEM) enabled the morphologic and topological characteristics of minerals, organic matter and pores to be imaged and quantified. In the studied samples, organic matter primarily has two geometries: lamellar masses (length: 1–100 μm, thickness: 0.5–2.0 μm) and discrete spheroidal particles (0.5–20.0 μm). Organic matter forms an inter-connected network where it exceeds a concentration between 6 and 18 wt%. Different pore types have different diameters and total pore volumes: inter-mineral pores (0.2 μm, 10–94%), organic interface pores (0.2 μm, 2–77%), intra-organic pores (0.05 μm, 1–40%) and intra-mineral pores (0.05 μm diameter, 1–2% of total porosity). The major pore system in the studied shales is composed of inter-mineral pores which occur between clay mineral grains. TOC concentration influences the total volume of organic matter-related pores while maturity controls the presence of intra-organic pores. The study improves the understanding of the relationship of organic matter concentrations, maturity and pore systems in shales. This study characterises porosity and organic matter distributions in 3D; it also improves the understanding of the relationship of organic matter concentrations, maturity and pore systems in shales.