Multiple pixel-scale soil water retention curves quantified by neutron radiography

Kang, Misun; Perfect, Edmund; Cheng, Chu-Lin; Bilheux, Hassina Z.; Lee, Jaehoon; Horita, J.; Warren, Jeffrey M.

doi:10.1016/j.advwatres.2013.12.004

Cited by 23 publications

(18 citation statements)

References 32 publications

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“…The median value of the 10 images was chosen for each pixel since it is less likely that all 10 images have detected gamma ray contamination for the same pixel [ Kim et al ., ]. Then I (Wet) and I (Dry) were normalized with respect to I (OB) and I (DF) as described by equation to remove detector background noise, fluctuations in the neutron flux, and the heterogeneities in the beam and detector [ Kang et al ., ]. Then the area of interest was cropped.…”

Section: Methodsmentioning

confidence: 99%

Effects of microstructure on water imbibition in sandstones using X‐ray computed tomography and neutron radiography

et al. 2017

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Capillary imbibition in variably saturated porous media is important in defining displacement processes and transport in the vadose zone and in low‐permeability barriers and reservoirs. Nonintrusive imaging in real time offers the potential to examine critical impacts of heterogeneity and surface properties on imbibition dynamics. Neutron radiography is applied as a powerful imaging tool to observe temporal changes in the spatial distribution of water in porous materials. We analyze water imbibition in both homogeneous and heterogeneous low‐permeability sandstones. Dynamic observations of the advance of the imbibition front with time are compared with characterizations of microstructure (via high‐resolution X‐ray computed tomography (CT)), pore size distribution (Mercury Intrusion Porosimetry), and permeability of the contrasting samples. We use an automated method to detect the progress of wetting front with time and link this to square‐root‐of‐time progress. These data are used to estimate the effect of microstructure on water sorptivity from a modified Lucas‐Washburn equation. Moreover, a model is established to calculate the maximum capillary diameter by modifying the Hagen‐Poiseuille and Young‐Laplace equations based on fractal theory. Comparing the calculated maximum capillary diameter with the maximum pore diameter (from high‐resolution CT) shows congruence between the two independent methods for the homogeneous silty sandstone but less effectively for the heterogeneous sandstone. Finally, we use these data to link observed response with the physical characteristics of the contrasting media—homogeneous versus heterogeneous—and to demonstrate the sensitivity of sorptivity expressly to tortuosity rather than porosity in low‐permeability sandstones.

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Section: Methodsmentioning

confidence: 99%

Effects of microstructure on water imbibition in sandstones using X‐ray computed tomography and neutron radiography

et al. 2017

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“…13, U.S. Silica Co.), was selected for use due to its relatively rapid pressure equilibrium times across the range of column lengths studied. The material is mainly composed of quartz (99.8%), with a particle density of 2.65 × 10 3 kg m −3 (Kang et al, 2014). Its grain diameters range from 0.11 to 0.60 mm, with a median grain diameter of 0.56 mm (Kang et al, 2014).…”

Section: Methodsmentioning

confidence: 99%

“…The saturated hydraulic conductivity was measured by Kang et al (2014) on replicate columns of 5.6 (±0.1) cm height, packed to an average bulk density of 1740 (±30) kg m −3 , using the constant‐head method. The resulting average saturated hydraulic conductivity was 1.66 ± 0.32 × 10 −4 m s −1 (Kang et al, 2014). This value was assumed to be independent of column height.…”

Section: Methodsmentioning

confidence: 99%

“…The capillary pressure–saturation function can be measured directly at a physical point using methods such as time domain reflectometry (Sakaki and Illangasekare, 2007) and neutron imaging (Kang et al, 2014). It can also be determined by inverse modeling of water content (averaged across the column height) vs. capillary pressure data obtained using the hanging water column, pressure cell, and centrifuge methods (Jalbert et al, 1999; Dane and Hopmans, 2002; Cropper et al, 2011).…”

Section: Theorymentioning

confidence: 99%

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Upscaling Capillary Pressure–Saturation Functions Using Different Reference Pressure Elevations

Cheng

Cropper

Perfect

et al. 2017

Vadose Zone Journal

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Core Ideas The BC‐vG Upscaler predicts the drainage of porous media of different heights. The default reference pressure elevation (RPE) is the middle of the porous medium. Predicted drainage functions deviate from observed functions as height increases. A top RPE improves accuracy of predicted drainage functions regardless of height. Choice of reference pressure elevation (RPE) on average effective saturation–capillary pressure functions, 〈〉(ψ), was investigated for monotonic drainage of homogeneous porous media. Nine columns of Flint sand with heights ranging from 4.3 to 55.0 cm were prepared. Measured 〈〉(ψ) functions were determined gravimetrically using the hanging water column method. Predicted 〈〉(ψ) functions were obtained by upscaling point function parameters determined by neutron radiographic imaging of a single drainage experiment. Bottom and midpoint RPEs resulted in the inaccurate parameterization of 〈〉(ψ) functions for tall columns. A top RPE produced accurate upscaled functions for all column heights. To evaluate the overall performance of this RPE, observed effective saturations for the nine columns were linearly regressed against predicted values. The resulting best‐fit model (slope = 0.98; intercept = 0.03; R2 = 0.98) corresponded closely to a 1:1 line. The van Genuchten (vG) α and n parameters for the observed and predicted 〈〉(ψ) functions decreased with increasing column height. A power model explained >95% of the variance in the predicted vG parameters and between 69 and 78% of the variance in the observed vG parameters. The lower R2 values for the observed parameter models were attributed to experimental variation among the nine columns, whereas the predicted parameter models were upscaled from a single column. Despite these differences, the magnitudes and height dependencies of the observed and predicted average vG parameters were similar. For tall columns, the RPE should be established at the top for drainage experiments and at the bottom for wetting experiments.

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“…However, a detailed examination of the plots of tensiometer readings in published multistep outflow experiments (Rasmussen et al, 2000;Weller et al, 2011;Kang et al, 2014), and in multistep flux experiments (Kumahor et al, 2015) can reveal some pressure fluctuations. However, a detailed examination of the plots of tensiometer readings in published multistep outflow experiments (Rasmussen et al, 2000;Weller et al, 2011;Kang et al, 2014), and in multistep flux experiments (Kumahor et al, 2015) can reveal some pressure fluctuations.…”

Section: P 2 Of 12mentioning

confidence: 99%

Pressure Jumps during Drainage in Macroporous Soils

Soto

Paradelo

Corral

et al. 2017

Vadose Zone Journal

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Core Ideas Large pressure jumps occur in macroporous soils during drainage. Jumps result from the fast entrance of a fluid phase into macropores. The entrance occurs after a break in the capillary shield surrounding macropores. Scaling factors of pressure jump sizes may differ from the Haines jumps. Discontinuous air–water displacement at the pore scale (from 10−5 to 10−3 m) affects fluid invasion in porous media at the core scale (10−3 to 1 m). Understanding of this effect is essential to upscale flow processes. In this study we used the analysis of pressure jumps to propose an upscaling mechanism. Large pressure jumps occur during drainage in macroporous structured soils; we suggest a hypothesis for their occurrence. Drainage experiments in packed sand and structured soils enclosing large pores showed large jumps (∼5‐hPa peak pressure). Large jumps resulted from a pressure relaxation process that first initiates from pore‐scale air–water displacements and then expands to larger scales. We found that the power‐law exponents for the distribution of the size of large jumps found in structured soil are greater than the typical values reported for Haines jumps in packed granular porous media. The difference in the exponent suggests that the magnitude of displacements occurring in structured soil has different scaling factors than in simple media. A mechanism for this change of scale is proposed on the basis of the large contrast between pore throat and matrix in macroporous soil. The mechanism consists of a fast pressure relaxation in the macropores triggered by a break in the capillary shield at the pore throat. These findings contribute to an explanation of the scaling relations of air–water displacements in complex porous media and unveil links between the soil structure and flow of fluids.

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Multiple pixel-scale soil water retention curves quantified by neutron radiography

Cited by 23 publications

References 32 publications

Effects of microstructure on water imbibition in sandstones using X‐ray computed tomography and neutron radiography

Effects of microstructure on water imbibition in sandstones using X‐ray computed tomography and neutron radiography

Upscaling Capillary Pressure–Saturation Functions Using Different Reference Pressure Elevations

Pressure Jumps during Drainage in Macroporous Soils

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