Understanding geochemical dissolution in porous materials is crucial, especially in applications such as geological CO
storage. Accurate estimation of reaction rates enhances predictive modeling in ...geochemical-flow simulations. Fractured porous media, with distinct transport time scales in fractures and the matrix, raise questions about fracture-matrix interface dissolution rates compared to bulk dissolution rate and the scale-dependency of reaction rate averaging. Our investigation delves into these factors, studying the impact of flow rate and mineralogy on interface dissolution patterns. By injecting carbonated water into carbonate rock samples containing a central channel (mimicking fracture hydrodynamics), our study utilized μCT X-ray imaging at 3.3 μm spatial resolution to estimate the reaction rate and capture the change in pore morphology. Results revealed dissolution rates significantly lower (up to 4 orders of magnitude) than batch experiments. Flow rate notably influenced fracture profiles, causing uneven enlargement at low rates and uniform widening at higher ones. Ankerite presence led to a dissolution-altered layer on the fracture surface, showing high permeability and porosity without greatly affecting the dissolution rate, unlike clay-rich carbonates. This research sheds light on controlling factors influencing dissolution in subsurface environments, critical for accurate modeling in diverse applications.
Drought-induced xylem embolism is a serious threat to plant survival under future climate scenarios. Hence, accurate quantification of species-specific vulnerability to xylem embolism is a key to ...predict the impact of climate change on vegetation. Low-cost hydraulic measurements of embolism rate have been suggested to be prone to artefacts, thus requiring validation by direct visualization of the functional status of xylem conduits using nondestructive imaging techniques, such as X-ray microtomography (microCT).
We measured the percentage loss of conductance (PLC) of excised stems of Laurus nobilis (laurel) dehydrated to different xylem pressures, and compared results with direct observation of gas-filled vs water-filled conduits at a synchrotron-based microCT facility using a phase contrast imaging modality.
Theoretical PLC calculated on the basis of microCT observations in stems of laurel dehydrated to different xylem pressures overall were in agreement with hydraulic measurements, revealing that this species suffers a 50% loss of xylem hydraulic conductance at xylem pressures averaging −3.5 MPa.
Our data support the validity of estimates of xylem vulnerability to embolism based on classical hydraulic techniques. We discuss possible causes of discrepancies between data gathered in this study and those of recent independent reports on laurel hydraulics.
A unique sandwich structure is designed with pure sulfur between two graphene membranes, which are continuously produced over a large area, as a very simple but effective approach for the fabrication ...of Li–S batteries with ultrafast charge/discharge rates and long lifetimes.
There is growing interest in using advanced imaging techniques to describe the complex pore-space of natural rocks at resolutions that allow for quantitative assessment of the flow and transport ...behaviors in these complex media. Here, we focus on representations of the complex pore-space obtained from X-ray microtomography and the subsequent use of such ‘pore-scale’ representations to characterize the overall porosity and permeability of the rock sample. Specifically, we analyze the impact of sub-resolution porosity on the macroscopic (Darcy scale) flow properties of the rock. The pore structure of a rock sample is obtained using high-resolution X-ray microtomography
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. Image analysis of the Berea sandstone sample indicates that about 2 % of the connected porosity lies below the resolution of the instrument. We employ a Darcy–Brinkman approach, in which a Darcy model is used for the sub-resolution porosity, and the Stokes equation is used to describe the flow in the fully resolved pore-space. We compare the Darcy–Brinkman numerical simulations with core flooding experiments, and we show that proper interpretation of the sub-resolution porosity can be essential in characterizing the overall permeability of natural porous media.
ABSTRACT
In‐situ synchrotron X‐ray computed microtomography with sub‐micrometer voxel size was used to study the decomposition of gas hydrates in a sedimentary matrix. Xenon‐hydrate was used instead ...of methane hydrate to enhance the absorption contrast. The microstructural features of the decomposition process were elucidated indicating that the decomposition starts at the hydrate‐gas interface; it does not proceed at the contacts with quartz grains. Melt water accumulates at retreating hydrate surface. The decomposition is not homogeneous and the decomposition rates depend on the distance of the hydrate surface to the gas phase indicating a diffusion‐limitation of the gas transport through the water phase. Gas is found to be metastably enriched in the water phase with a concentration decreasing away from the hydrate‐water interface. The initial decomposition process facilitates redistribution of fluid phases in the pore space and local reformation of gas hydrates. The observations allow also rationalizing earlier conjectures from experiments with low spatial resolutions and suggest that the hydrate‐sediment assemblies remain intact until the hydrate spacers between sediment grains finally collapse; possible effects on mechanical stability and permeability are discussed. The resulting time resolved characteristics of gas hydrate decomposition and the influence of melt water on the reaction rate are of importance for a suggested gas recovery from marine sediments by depressurization.
Key Points
Time resolved, microstructural aspects of pressure‐induced gas hydrate dissolution in sediments followed with in situ synchrotron tomography
A diffusion‐limit on the liberated gas transport through the gas‐enriched water and a resulting gas concentration gradient is indicated
Possible effects of the microstructural behavior of hydrate decomposition on mechanical stability and permeability properties are discussed
Abstract
The ANATOMIX beamline at Synchrotron SOLEIL, operational since 2018, is dedicated to hard X-ray full-field tomography techniques. Operating in a range of photon energies from approximately 5 ...to 50 keV, it offers both parallel-beam projection microtomography, in absorption and phase contrast, and nanotomography using a zone-plate transmission X-ray microscope. With these methods, the beamline covers a range of spatial resolution from 20 nm to 20 µm, expressed in terms of useful pixel size. The variable beam size of up to 40 mm allows users to image large objects. Here we describe the microtomography instrumentation of the beamline.
Synchrotron X‐ray microtomography (XMT) was used to measure total air‐water interfacial areas (Aaw) as a function of water saturation (Sw) for several soils that comprise a range of physical and ...geochemical properties. Measurements were also conducted for glass beads and quartz sands for comparison. Apparent near‐linear Aaw‐Sw relationships are observed for the three sands and the three sandy soils. In contrast, the measured interfacial areas for two soils that contain greater proportions of silt and clay are strongly nonlinear functions of water saturation. The greater degree of nonlinearity observed for these two soils is due to their much greater particle‐size distributions (i.e., uniformity coefficients) and their concomitant greater range in pore sizes. Interfacial areas determined with the thermodynamic method were used to benchmark the XMT measurements. XMT‐measured interfacial areas compare well to the thermodynamic‐determined values for the sands and sandy soils. In contrast, the XMT‐measured interfacial areas for the two soils with larger particle‐size distributions are not fully congruent with the thermodynamic‐determined values. Both of these soils have large fractions of pore space comprising nominal pore diameters smaller than the resolution of the XMT imaging. These results suggest that air‐water interfacial area may not always be fully characterized by standard XMT for soils with large particle‐size distributions.
Plain Language Summary
High‐resolution pore‐scale imaging is conducted for several sands and soils to examine the relationship between the magnitude of air‐water interfacial area and water saturation as well as to evaluate the effectiveness of the imaging method.
Key Points
X‐ray microtomography (XMT) measurements of air‐water interfacial area are reported for soils
The relationship between air‐water interfacial area and water saturation is nonlinear
Benchmarking results indicate that XMT may not measure all interfacial area for soils with higher silt and clay contents
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