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High-resolution X-ray computed tomography in geosciences: A review of the current technology and applications

[1] While geological carbon dioxide (CO2) storage could contribute to reducing global emissions, it must be designed such that the CO2 cannot escape from the porous rock into which it is injected An important mechanism to immobilize the CO2, preventing escape, is capillary trapping, where CO2 is stranded as disconnected pore-scale droplets (ganglia) in the rock, surrounded by water We used X-Ray microtomography to image, at a resolution of 64 µm, the pore-scale arrangement and distribution of trapped CO2 clusters in a limestone We applied high pressures and temperatures typical of a storage formation, while maintaining chemical equilibrium between the CO2, brine, and rock Substantial amounts of CO2 were trapped, with an average saturation of 018 The cluster sizes obeyed a power law distribution, with an exponent of approximately −21, consistent with predictions from percolation theory This work confirms that residual trapping could aid storage security in carbonate aquifers

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Pore-scale imaging of geological carbon dioxide storage under in situ conditions

https://spiral.imperial.ac.uk/bitstream/10044/1/49429/4/61_Saif_Lin_Bijeljic_Blunt.pdf

Microstructural imaging and characterization of oil shale before and after pyrolysis

[1] A new equation for the collector efficiency (η) of the colloid filtration theory (CFT) is developed via nonlinear regression on the numerical data generated by a large number of Lagrangian simulations conducted in Happel's sphere-in-cell porous media model over a wide range of environmentally relevant conditions. The new equation expands the range of CFT's applicability in the natural subsurface primarily by accommodating departures from power law dependence of η on the Peclet and gravity numbers, a necessary but as of yet unavailable feature for applying CFT to large-scale field transport (e.g., of nanoparticles, radionuclides, or genetically modified organisms) under low groundwater velocity conditions. The new equation also departs from prior equations for colloids in the nanoparticle size range at all fluid velocities. These departures are particularly relevant to subsurface colloid and colloid-facilitated transport where low permeabilities and/or hydraulic gradients lead to low groundwater velocities and/or to nanoparticle fate and transport in porous media in general. We also note the importance of consistency in the conceptualization of particle flux through the single collector model on which most η equations are based for the purpose of attaining a mechanistic understanding of the transport and attachment steps of deposition. A lack of sufficient data for small particles and low velocities warrants further experiments to draw more definitive and comprehensive conclusions regarding the most significant discrepancies between the available equations.

New collector efficiency equation for colloid filtration in both natural and engineered flow conditions

Dissolution and precipitation reactions are the primary mechanisms that occur when a rock (i.e., a mineral assemblage) is in contact with a fluid out of equilibrium. They play a critical role in natural processes (e.g., weathering, compaction, meteoric and marine diagenesis) and anthropogenic processes (e.g., reservoir acidizing, CO2 sequestration, acid mine drainage, contaminant mobility, bioremediation). Such fluid–rock interactions result in complex changes in pore structure and mineral composition, leading in turn to changes in flow, mechanical, and transport properties, such as permeability, dispersivity, strength, and pore accessibility. Consequently, geochemical disequilibrium can lead to complex modifications of hydrodynamic and transport properties in porous and fractured rocks.

Porous rocks are often characterized by complex textures and mineral compositions that are derived from their depositional and diagenetic environments. They typically have heterogeneous structures, the macroscopic physical properties of which depend on microscopic characteristics. Permeability, for example, is closely related to the microstructure, in particular the size and the spatial distribution of pore throats, pore roughness, and presence of fine clogging particles. The coupled hydrological, mechanical, and chemical (HMC) processes are highly non-linear and minor changes at the pore scale in one property can result in large modifications of the others properties. Prediction of system response to chemical conditions requires understanding how individual processes that occur at the microscopic scale contribute to the observed large-scale flow and transport distribution patterns.

Predictive modeling remains challenging for the time and spatial scales involved in geological processes and because of the lack of information about how the physical properties of the porous medium evolve as a result of chemical reactions. In particular, the role of microstructures and their possible effects on flow and transport have long been neglected. Consequently, upscaling the flow and transport properties remains poorly constrained by pore-scale observations despite a multitude of experiments, …

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Resolving Time-dependent Evolution of Pore-Scale Structure, Permeability and Reactivity using X-ray Microtomography

Understanding the physical and hydraulic properties of the vadose zone is important for modeling land use effects on groundwater quality. This study used a variety of characterization methods to derive conceptual understanding and estimates of hydraulic properties of a coarse alluvial gravel vadose zone in New Zealand. Sandy gravel (SG) material constituted approximately 90% of the vadose zone, with the remainder comprising sand lenses and open-framework gravels. The gravel content of the SG material was approximately 70% (v/v) (range 68–73%). The water content of the bulk SG material (43 samples) ranged from 3.5 to 13.9%. The average bulk density of the SG material was 2.20 g/cm 3 (range 2.00–2.33 g/cm 3 ) giving an average calculated porosity of 17%. The average porosity of the open-framework gravels was 34% and these gravels were often coated with 2- to 3-mm-thick deposits of amorphous Fe and Mn oxides. Neutron probe (NP) depth profiles indicated unsteady conditions, with variable water contents with depth and time reflecting the vertical heterogeneity and the variably saturated state of the vadose zone. Time series NP data to 3 m indicated water content in the alluvial gravels responded quickly to soil drainage events, and saturation variability was greater in the sand lenses and the SG material immediately underlying. When compared with derived water retention curves, variability in the water content equated to significant fluctuation in unsaturated hydraulic conductivity ( K unsat ). Tension infiltrometer measurements were variable but were within the range of the K unsat estimates from site-average particle size distribution data. The gravel-transformed, texture-based models used to estimate saturated water content values in this study appeared to underestimate the measured values.

Characterization and Estimation of Hydraulic Properties in an Alluvial Gravel Vadose Zone

The combined use of pumping and tracer test data enabled the derivation of equivalent average hydraulic conductivities (K
avg) for each test in a heterogeneous channelized alluvial aquifer, whereas K values of the preferential flow paths were two orders of magnitude higher. Greater and earlier drawdown was generally observed along preferential flow lines in a pumping test, within an array of 21 wells. The study aim was to characterize hydraulic properties of a channelized aquifer system in New Zealand by combining tracer and pumping test data. Estimates were able to be made of the percentage of highly permeable channels within the profile (∼1.2%), effective porosity that reflected the maximum fraction of highly permeable channels within the aquifer (ϕ
eff
–pc ∼0.0038), and flows through highly permeable channels (∼98%) and the sandy gravel matrix material (∼2%). Using ϕ
eff
–pc, a tracer test K
avg value (∼93 m/day) was estimated that was equivalent to pumping test values (∼100 m/day), but two orders of magnitude smaller than K calculated solely from transport through permeable channels (K
pc ∼8,400 m/day). Derived K values of permeable and matrix material were similar to values derived from grain size distribution using the Kozeny-Carman equation.

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Complementary use of tracer and pumping tests to characterize a heterogeneous channelized aquifer system in New Zealand

To be able to predict contaminant transport in groundwater, an accurate conceptual and physical understanding of aquifer properties at multiple scales is required. In this study, physical and hydraulic properties of a coastal sand aquifer were derived using micro and macro X-ray computed tomography (XCT) techniques. Qualitative and quantitative data improved conceptualisation of the aquifer structure at micro and macro scale. At the macro scale (50-mm diameter by 1,500-mm long core) XCT images of undisturbed drill core identified coarse grained laminae (not obvious to the naked eye) of increased porosity and permeability, variations in mineral assemblage and particulate organic matter distribution within the core. Micro-XCT analysis (16-mm diameter cores) of the three main aquifer layers provided quantitative micro-scale data on permeability, porosity, grain, pore and throat size distribution statistics, and grain sphericity. Ratios of mean pore to grain diameter were ~0.65–0.75 and ratios of mean throat to mean grain diameter were ~0.2. Multiple permeability and porosity values were derived from micro domains (~4.35 mm3) within each micro-XCT core. Permeability values varied between and within micro-XCT core samples reflecting the heterogeneity at the millimetre core scale in these sediments. Sphericity values were similar for all three layers (average ~0.4) which reflected SEM observations of the semi-spherical nature of the dominant quartz and feldspar grains. The results of this study indicate that models based on the assumptions of homogeneity in depositional structure below centimetre scale may not suitably address factors affecting flow and transport of contaminants.

Multi-scale characterisation of coastal sand aquifer media for contaminant transport using X-ray computed tomography

Understanding solute transport and flux through the vadose zone is important for predicting potential contaminant loading to groundwater systems. Two dual-permeability models: mixing cell (MC) and HYDRUS 1D (H1D DP ) were used to simulate long-term nonequilibrium Br − leaching using data derived from suction cup samples at two field sites (sampled to 7- and 4-m depths) on the Canterbury Plains, New Zealand. Effective model parameters were derived by inverse methods to represent average transport processes through the heterogeneous profiles. Suction cup samples indicated rapid initial movement of solute through the profiles followed by a long tail. The “management type” MC model results were comparable to the more complex HYDRUS model, providing similar overall fits to the observed data (similar RMSE ∼ 0.05–0.1). Modeling indicated that macropore flux was significant in transporting solute through the profile (MC range 5–59%; H1D DP range 15–54% of total fluxes) and simulations of cumulative fluxes from both models were similar. The MC model results indicated macropore transporting water contents of ∼0.003 (v/v) and matrix domain transporting water contents of around 0.1 to 0.15 (v/v) for the two field sites. These estimates suggest that only 3 mm of bulk drainage is required to transport solutes 1 m through the macropore domain, whereas approximately 100 to 150 mm of drainage is required to transport solutes the same distance through the matrix domain. More accurate representation of boundary conditions, texture spatial distributions, and hydraulic interactions is important for obtaining a better understanding of flux dynamics in future studies.

Modeling of Nonequilibrium Bromide Transport through Alluvial Gravel Vadose Zones

Understanding nitrogen (N) processes within the vadose zone is important to estimate N losses to groundwater systems. A field trial was undertaken to examine the dynamics of nitrate (NO 3 ) and nitrous oxide (N 2 O) in an alluvial gravel vadose zone underlain by shallow groundwater. Synthetic urine (980 kg N ha −1 ) with a bromide (400 kg Br ha −1 ) tracer was applied to the surface of a 10- by 20-m plot, and changes in subsurface NO 3 , Br, and N 2 O concentrations were compared with those from an adjacent plot, with just the Br tracer applied. Soil solution and air were monitored at multiple depths (from 0.2 to 5 m) over an 18-mo period. Transport of solution was rapid in the gravel material with some Br transported to 3-m depth immediately after the urine application. N 2 O was produced within the soil after urine application. NO 3 to Br ratios indicated denitrification in the soil above the gravels, but none within the alluvial gravel vadose zone. In the “–urine” plot N 2 O concentrations increased with depth, with upward fluxes above the water table to the soil, suggesting N 2 O production near the water table. A different pattern was observed in the “+urine” plot where N 2 O was produced both at the soil zone above the gravels and near the water table leading to both upward and downward N 2 O fluxes. Overall N 2 O fluxes in the soil zone were greater than at the water table.

Rodney Dann

Papers

Characterization and Estimation of Hydraulic Properties in an Alluvial Gravel Vadose Zone

Complementary use of tracer and pumping tests to characterize a heterogeneous channelized aquifer system in New Zealand

Multi-scale characterisation of coastal sand aquifer media for contaminant transport using X-ray computed tomography

Modeling of Nonequilibrium Bromide Transport through Alluvial Gravel Vadose Zones

Nitrate and Nitrous Oxide Dynamics Under Urine Application in an Alluvial Gravel Vadose Zone