I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[26232]
Abstract: Neutron and X-ray tomography (NCT and XCT, respectively) are imaging techniques increasingly being applied in geomechanics research. They are used to non-destructively reveal different microstructural aspects of geomaterials: XCT is often used to observe/quantify differences in density or porosity, while NCT reveals the presence and distribution of hydrogenous materials such as water. The correlated use of NCT and XCT for geomechanics and geotechnics research is in its infancy. To this date, very few experiments have been carried out that combine both techniques, and none have been used to investigate geomaterial-structure interaction. This paper presents the first correlative NCT-XCT imaging study of pile installation. A scaled model pile was installed in an unsaturated intact chalk cylinder and in-situ NCT and ex-situ XCT synchrotron-based imaging was applied consecutively. Chalk was used because the behaviour of displacement piles installed in this material is still subject to considerable uncertainty. Results reveal for the first time the interaction between installation-induced changes in chalk density and water distribution variations, with evidence of water displacement into the densified material in the vicinity of the installed pile. A straightforward method for correlative bulk density-moisture content determination from NCT-XCT images of geomaterials are presented and their limitations discussed.
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Jul 2022
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I13-2-Diamond Manchester Imaging
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Diamond Proposal Number(s):
[16205]
Open Access
Abstract: Gas bubble in aquatic sediments has a significant effect on its geophysical and geomechanical properties. Recent studies have shown that methane gas and hydrate can coexist in gas hydrate–bearing sediments. Accurate calibration and understanding of the fundamental processes regarding such coexisting gas bubble dynamics is essential for geophysical characterization and hazard mitigation. We conducted high-resolution synchrotron imaging of methane hydrate formation from methane gas in water-saturated sand. While previous hydrate synchrotron imaging has focused on hydrate evolution, here we focus on the gas bubble dynamics. We used a novel semantic segmentation technique based on convolutional neural networks to observe bubble dynamics before and during hydrate formation. Our results show that bubbles change shape and size even before hydrate formation. Hydrate forms on the outer surface of the bubbles, leading to reduction in bubble size, connectivity of bubbles, and the development of nano-to micro-sized bubbles. Interestingly, methane gas bubble size does not monotonously decrease with hydrate formation; rather, we observe some bubbles being completely used up during hydrate formation, while bubbles originate from hydrates in other parts. This indicates the dynamic nature of gas and hydrate formation. We also used an effective medium model including gas bubble resonance effects to study how these bubble sizes affect the geophysical properties. Gas bubble resonance modeling for field or experimental data generally considers an average or equivalent bubble size. We use synchrotron imaging data to extract individual gas bubble volumes and equivalent spherical radii from the segmented images and implement this into the rock physics model. Our modeling results show that using actual bubble size distribution has a different effect on the geophysical properties compared to the using mean and median bubble size distributions. Our imaging and modeling studies show that the existence of these small gas bubbles of a specific size range, compared to a bigger bubble of equivalent volume, may give rise to significant uncertainties in the geophysical inversion of gas quantification.
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Jun 2022
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[26307]
Open Access
Abstract: X-ray computed tomography (XCT) is regularly employed in geomechanics to non-destructively measure the solid and pore fractions of soil and rock from reconstructed 3D images. With the increasing availability of high-resolution XCT imaging systems, researchers now seek to measure microfabric parameters such as the number and area of interparticle contacts, which can then be used to inform soil behaviour modelling techniques. However, recent research has evidenced that conventional image processing methods consistently overestimate the number and area of interparticle contacts, mainly due to acquisition-driven image artefacts. The present study seeks to address this issue by systematically assessing the role of XCT acquisition parameters in the accurate detection of interparticle contacts. To this end, synchrotron XCT has been applied to a hexagonal close-packed arrangement of glass pellets with and without a prescribed separation between lattice layers. Different values for the number of projections, exposure time, and rotation range have been evaluated. Conventional global grey value thresholding and novel U-Net segmentation methods have been assessed, followed by local refinements at the presumptive contacts, as per recently proposed contact detection routines. The effect of the different acquisition set-ups and segmentation techniques on contact detection performance is presented and discussed, and optimised workflows are proposed.
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May 2022
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I13-2-Diamond Manchester Imaging
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Diamond Proposal Number(s):
[18758]
Abstract: Natural surface gas seeps provide a significant input of greenhouse gas emissions into the Earth’s atmosphere and hydrosphere. The gas flux is controlled by the properties of underlying fluid-escape conduits, which are present within sedimentary basins globally. These conduits permit pressure-driven fluid flow, hydraulically connecting deeper strata with the Earth’s surface; however they can only be fully resolved at sub-seismic scale. Here, a novel ‘minus cement and matrix permeability’ method using three-dimensional X-ray micro-computed tomography imaging enables the improved petrophysical linkage of outcrop and sub-surface data. The methodology is applied to the largest known outcrop of an inactive fluid-escape system, the Panoche Giant Intrusion Complex in Central California, where samples were collected along transects of the 600 to 800 m stratigraphic depth range to constrain porosity and permeability spatial heterogeneity. The presence of silica cement and clay matrix within the intergranular pores of sand intrusions are the primary control of porosity (17 to 27%) and permeability (≤1 to ca 500 mD) spatial heterogeneity within the outcrop analogue system. Following the digital removal of clay matrix and silica (opal-CT and quartz) cement derived from the mudstone host strata, the sand intrusions have porosity-permeability ranges of ca 30 to 40% and 103 to 104 mD. These calculations are closely comparable to active sub-surface systems in sedimentary basins. Field observations revealed at decreasing depth, the connected sand intrusion network reduces in thickness and becomes carbonate cemented, terminating at carbonate mounds formed from methane escape at the seafloor. A new conceptual model integrates the pore-scale calculations and field-scale observations to highlight the key processes that control sand intrusion permeability, spatially and temporally. The study demonstrates the control of matrix and cement addition on the physical properties of fluid-escape conduits, which has significance for hydrocarbon reservoir characterization and modelling, as well as subsurface CO2 and energy storage containment assessment.
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Apr 2022
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I12-JEEP: Joint Engineering, Environmental and Processing
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Abstract: Zero carbon energy generation from renewable sources can reduce climate change by mitigating carbon emissions. A major challenge of renewable energy generation is the imbalance between supply and demand. To overcome the energy imbalances, subsurface storage of hydrogen in porous mediais suggested as a large-scale and economic solution, yet its mechanisms are not fully understood. Important unknowns are the effect of the high migration potential of the small and mobile hydrogen molecule and the volume of recoverable hydrogen.
We conducted non-steady state, cyclic hydrogen and brine injection experiments at 2-7 MPa and flow rates of 2-80 µl min-1 using water-wet Clashach sandstone cylinders of 4.7 mm diameter and 53-57 mm length (Clashach composition: ~96 wt.% quartz, 2% K-feldspar, 1% calcite, 1% ankerite). Two sets of experiments were performed using our new transparent flow-cell designed for x-ray computed microtomography: 1) Experiments using a laboratory x-ray source (University of Edinburgh) imaged the flow, displacement and capillary trapping of hydrogen by brine as a function of saturation after primary drainage and secondary imbibition. 2) Experiments using synchrotron radiation (Diamond Light Source, I12-JEEP tomography beamline) captured time-resolved hydrogen and brine flow and displacement processes. Pressure and mass flow measurements across the experimental apparatus complemented the microtomography volumes in both sets of experiments.
Results from a water-wet rock show that hydrogen behaves as a non-wetting phase and sits in the centre of the pore bodies, while residual brine sits in corners and pore throats. Hydrogen saturation in the pore volume is independent of the injection pressure and increases with increasing hydrogen/brine injection ratio up to ~50% saturation at 100 % hydrogen. Capillary trapping of hydrogen during brine imbibition occurs via snap off and is greatest at higher brine injection pressures, with 10 %, 12% and 21% hydrogen trapped at 2, 5 and 7 MPa, respectively. Higher brine flow rates reduce capillary trapping and increase hydrogen recovery at any given injection pressure. Based on these results, future hydrogen storage operations should inject 100% hydrogen and manage the reservoir pressure to avoid high pressures and minimize capillary trapping of hydrogen during brine reinjection.
Ongoing analysis of time-resolved experimental data will provide further insight into the critical pore-scale processes that ultimately influence the potential for geological hydrogen storage and recovery.
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Mar 2022
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I13-2-Diamond Manchester Imaging
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Diamond Proposal Number(s):
[11587]
Abstract: Hypothesis: Imbibition of a fluid into a porous material involves the invasion of a wetting fluid in the pore space through piston-like displacement, film and corner flow, snap-off and pore bypassing. These processes have been studied extensively in two-dimensional (2D) porous systems; however, their relevance to three-dimensional (3D) natural porous media is poorly understood. Here, we investigate these pore-scale processes in a natural rock sample using time-resolved 3D (i.e., four-dimensional or 4D) X-ray imaging. Experiments: We performed a capillary-controlled drainage-imbibition experiment on an initially brine-saturated carbonate rock sample. The sample was imaged continuously during imbibition using 4D X-ray imaging to visualize and analyze fluid displacement and snap-off processes at the pore-scale. Findings: We discover a new type of snap-off that occurs in pores, resulting in the entrapment of a small portion of the non-wetting phase in pore corners. This contrasts with previously-observed snap-off in throats which traps the non-wetting phase in pore centers. We relate the new type of pore-snap-off to the pinning of fluid-fluid interfaces at rough surfaces, creating contact angles close to 90°. Subsequently, we provide correlations for displacement events as a function of pore-throat geometry. Our findings indicate that having a small throat does not necessarily favor snap-off: the key criterion is the throat radius in relation to the pore radius involved in a displacement event, captured by the aspect ratio.
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Nov 2021
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Abstract: Seismic studies are essential for accurate characterisation of planetary interior structures, but are dependent on modelling for interpretation, requiring data on the elastic properties of likely constituent minerals. With the potential deployment of seismic stations on icy worlds such as Europa and Titan envisioned for the near future, a campaign of study into the elasticity of potential icy ocean world minerals is of paramount importance.
In the paper we assess the role of first-principles computer simulations to this problem, in particular focussing on the application of recent advances in simulating dispersion forces in loosely-bonded molecular solids, likely to be the main constituents of icy ocean worlds. This is of particular interest for these kinds of materials, since the complex sample handling, phase transitions and the difficulty of obtaining single crystals often greatly complicates the experimental determination of the full elastic tensor.
We focus on CO2, C6H6, MgSO4·7H2O and CaSO4·2H2O as they allow us to benchmark the performance over a wide range of chemical space, structural topologies, crystal symmetries and bonding types, and moreover have accurate experimentally determined unit-cell dimensions, bulk moduli and full elastic tensors for benchmarking purposes.
We demonstrate that the dispersion corrected approaches indeed perform superior in modelling the experimental density profiles (mean unsigned differences of merely 0.04 g/cm3 (CO2), 0.02 g/cm3 (C6H6), 0.003 g/cm3 (MgSO4·7H2O) and 0.013 g/cm3 (CaSO4·2H2O)) and may find application in exploring the compressive parameters of candidate materials, which could then be used in rheological models of icy ocean worlds.
Moreover, we have assessed if the elastic constants computed by dispersion corrected density functional theory are accurate enough to be used in a reference data base for the seismic exploration of icy ocean worlds. Despite one approach having demonstrated good accuracy compared with the experimental values in modelling the elasticity of CO2, we instead find average differences from expected P and S wave velocities of around 10 to 25% for the elastically more complex title compounds. In part these differences are due to the large temperature difference between the experimental elasticity data (typically near 300 K) and our calculations, which were performed in the athermal limit.
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Jul 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[21972]
Open Access
Abstract: High pressure–temperature experiments provide information on the phase diagrams and physical characteristics of matter at extreme conditions and offer a synthesis pathway for novel materials with useful properties. Experiments recreating the conditions of planetary interiors provide important constraints on the physical properties of constituent phases and are key to developing models of planetary processes and interpreting geophysical observations. The laser-heated diamond anvil cell (DAC) is currently the only technique capable of routinely accessing the Earth’s lower-mantle geotherm for experiments on non-metallic samples, but large temperature uncertainties and poor temperature stability limit the accuracy of measured data and prohibits analyses requiring long acquisition times. We have developed a novel internal resistive heating (IRH) technique for the DAC and demonstrate stable heating of non-metallic samples up to 3000 K and 64 GPa, as confirmed by in situ synchrotron x-ray diffraction and simultaneous spectroradiometric temperature measurement. The temperature generated in our IRH-DAC can be precisely controlled and is extremely stable, with less than 20 K variation over several hours without any user intervention, resulting in temperature uncertainties an order of magnitude smaller than those in typical laser-heating experiments. Our IRH-DAC design, with its simple geometry, provides a new and highly accessible tool for investigating materials at extreme conditions. It is well suited for the rapid collection of high-resolution P–V–T data, precise demarcation of phase boundaries, and experiments requiring long acquisition times at high temperature. Our IRH technique is ideally placed to exploit the move toward coherent nano-focused x-ray beams at next-generation synchrotron sources.
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Jun 2021
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I13-2-Diamond Manchester Imaging
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Diamond Proposal Number(s):
[19904]
Open Access
Abstract: The development of pore and fracture networks at the nano-scale as a response to heating can reveal coupled physical relationships relevant to several energy applications. A combination of time-lapse 3D imaging and finite-element modelling (FEM) was performed on two typical thermally immature shale samples, Kimmeridge Clay and Akrabou shale, to investigate thermal response at the nm-scale for the first time. Samples were imaged using Transmission X-ray Microscopy (TXM) with a voxel resolution of 34 nm at the I13–2 beamline at Diamond Light source, UK. Images were taken after heating to temperatures of 20 °C, 300 °C, 350 °C and 400 °C. The initiation of nano-pores within individual minerals and organic matter particles were observed and quantified alongside the evolution from nano-pores to micro-fractures. The major expansion of pore-volume occurred between 300 and 350 °C in both samples, with the pores elongating rapidly along the organic-rich bedding. The internal pressures induced by organic matter transformation influenced the development of microfractures. Mechanical properties and strain distributions within these two samples were modelled under a range of axial stresses using FEM. The results show that the overall stiffness of the shale reduced during heating, despite organic matter becoming stiffer. The varied roles of ductile (e.g., clay minerals, organic matter) and brittle materials (e.g., calcite, pyrite) within the rock matrix are also modelled and discussed. The configurations of organic matter, mineral components, porosity and connectivity impact elastic deformation during shale pyrolysis. This work extends our understanding of dynamic coupled processes of microstructure and elastic deformation in shales to the nm-scale, which also has applications to other subsurface energy systems such as carbon sequestration, geothermal and nuclear waste disposal.
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Jun 2021
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I12-JEEP: Joint Engineering, Environmental and Processing
I13-2-Diamond Manchester Imaging
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Diamond Proposal Number(s):
[13824, 9866, 17606]
Open Access
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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Jun 2021
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