Preferential Paths of Air-water Two-phase Flow in Porous Structures with Special Consideration of Channel Thickness Effects.

Accurate understanding and predicting the flow paths of immiscible two-phase flow in rocky porous structures are of critical importance for the evaluation of oil or gas recovery and prediction of rock slides caused by gas-liquid flow. A 2D phase field model was established for compressible air-water two-phase flow in heterogenous porous structures. The dynamic characteristics of air-water two-phase interface and preferential paths in porous structures were simulated. The factors affecting the path selection of two-phase flow in porous structures were analyzed. Transparent physical models of complex porous structures were prepared using 3D printing technology. Tracer dye was used to visually observe the flow characteristics and path selection in air-water two-phase displacement experiments. The experimental observations agree with the numerical results used to validate the accuracy of phase field model. The effects of channel thickness on the air-water two-phase flow behavior and paths in porous structures were also analyzed. The results indicate that thick channels can induce secondary air flow paths due to the increase in flow resistance; consequently, the flow distribution is different from that in narrow channels. This study provides a new reference for quantitatively analyzing multi-phase flow and predicting the preferential paths of immiscible fluids in porous structures

Influence of microstructural properties on geophysical measurements in sand-clay mixtures

We have performed a series of laboratory experiments on saturated sand-clay mixtures. Measurements include frequency-dependent electrical properties using the four-electrode technique (10 niHz to 1 MHz), permeability, porosity, and acoustic velocities. We mixed clean Ottawa (quartz) sand with Na-montmorillonite (Wyoming bentonite) in a number of different configurations containing 0 to 10% clay: as a dispersed mixture, as discrete clay clusters, and arranged in distinct layers. Solutions of CaCl{sub 2} ranging from 0.0005 N to 0.75 N (0.05 to 64 mS/cm) and deionized water were used as saturating fluids. We found the electrical properties to be dependent on clay content, fluid conductivity, and microstructure in a complex fashion. Increasing fluid conductivity and increasing clay content generally resulted in higher electrical conductivity. For an individual sample, two main regions of conduction exist: a region dominated by surface conduction and a region where the ionic strength of the saturating fluid controlled conduction. The sample geometry (dispersed, nondispersed, or layered clay configuration) was found to greatly affect the magnitude of the surface conductance in the range of low fluid conductivity

Flow Processes in the Dry Regime: The Effect on Capillary Barrier Performance

Engineered capillary barriers typically consist of two layers of granular materials designed so that the contrast in material hydraulic properties and sloping interface retain infiltrating water in the upper layer. We conducted two benchtop capillary barrier experiments, followed by interpretation and numerical modeling. The hydraulic parameters for two coarse materials were measured using standard methods, and we found that the materials had similar hydraulic properties despite being morphologically different (round vs. angular). The round sand provided a better functioning capillary barrier than the angular sand, but neither experiment could be characterized as a perfectly working capillary barrier. In both cases, >93% of the infiltrating water was successfully diverted from the lower layer; however, infiltration into the underlying layer was observed in both systems. Based on this work, we believe that noncontinuum processes such as vapor diffusion and film flow contribute to the observed phenomena and are important aspects to consider with respect to capillary barrier design as well as dry vadose zone processes in general. Using a theoretical film flow equation that incorporates the surface geometry of the porous material, we found that infiltration into the coarse underlying sand layer appeared to be dominated by water film flow. The NUFT (Nonisothermal Unsaturated-Saturated Flow and Transport) model was used for qualitative comparison simulations. We were able to reproduce the barrier breach observed in the experiments using targeted parameter adjustment, by which pseudo-film flow was successfully simulated.Keywords: Film flow, Adsorption, Porous media, Soil, Hydraulic conductivity, Enhancement, Water vapor diffusion, Fracture surfaces, Condensation, Potential

Effects of X-ray dose on rhizosphere studies using X-ray computed tomography

X-ray Computed Tomography (CT) is a non-destructive imaging technique originally designed for diagnostic medicine, which was adopted for rhizosphere and soil science applications in the early 1980s. X-ray CT enables researchers to simultaneously visualise and quantify the heterogeneous soil matrix of mineral grains, organic matter, air-filled pores and water-filled pores. Additionally, X-ray CT allows visualisation of plant roots in situ without the need for traditional invasive methods such as root washing. However, one routinely unreported aspect of X-ray CT is the potential effect of X-ray dose on the soil-borne microorganisms and plants in rhizosphere investigations. Here we aimed to i) highlight the need for more consistent reporting of X-ray CT parameters for dose to sample, ii) to provide an overview of previously reported impacts of X-rays on soil microorganisms and plant roots and iii) present new data investigating the response of plant roots and microbial communities to X-ray exposure. Fewer than 5% of the 126 publications included in the literature review contained sufficient information to calculate dose and only 2.4% of the publications explicitly state an estimate of dose received by each sample. We conducted a study involving rice roots growing in soil, observing no significant difference between the numbers of root tips, root volume and total root length in scanned versus unscanned samples. In parallel, a soil microbe experiment scanning samples over a total of 24 weeks observed no significant difference between the scanned and unscanned microbial biomass values. We conclude from the literature review and our own experiments that X-ray CT does not impact plant growth or soil microbial populations when employing a low level of dose (<30 Gy). However, the call for higher throughput X-ray CT means that doses that biological samples receive are likely to increase and thus should be closely monitored

Compressional and Shear Wave Velocities for Artificial Granular Media Under Simulated Near Surface Conditions

Laboratory ultrasonic experiments were made on artificial soil samples in order to observe the effects of slight overburden, sand/clay ratio and pore fluid saturation on compressional and shear wave velocities. Up to several meters of overburden were simulated by applying low uniaxial stress of about 0.1 MPa to a restrained sample. Samples were fabricated from Ottawa sand mixed with a swelling clay (Wyoming bentonite). The amount of clay added was 1 to 40 percent by mass. Most measurements were made under room-dry conditions, but some measurements were made for fully-saturated sand-clay mixtures and for partially-saturated sand samples. For the dry sand-clay samples, compressional (P) velocities were low, ranging from about 200 to 500 m/s for the mixtures at low stress. Shear (S) velocities were about half of the compressional velocity, about 70 to 250 m/s. Dramatic increases in all velocities occurred with small uniaxial loads, indicating strong nonlinearity. Composition and grain packing control the mechanical response at grain contacts and the resulting nonlinear response at low stresses. P and S velocities are sensitive to the amount of clay added, even at low concentrations. At these low equivalent overburden conditions, adhesion and capillarity at grain contacts affect wave amplitudes, velocities, and frequency content in the partial saturation case

Report on laboratory scale thermally-coupled processes experiments

Yucca Mountain Site Characterization Project (YMP) is studying Yucca Mountain, Nevada as a potential repository for high-level nuclear wastes. The studies include predictions of the quantity and composition of water in the repository near-field environment that will affect the release rate of radioactive nuclides from waste packages, and the transport of the nuclides through the rock mass adjacent to these packages. The radioactive decay heat from the high- level nuclear waste may increase the temperature in the rock mass to the extent that coupled thermal-mechanical-hydrological-chemical (TMHC) processes may exist in the originally -partially-saturated Topopah Spring tuff-the host rock for the potential repository in Yucca Mountain. Modeling the coupled TMHC processes is necessary to predict the quantity and quality of water in the near-field environment for the entire life span of a repository (tens of thousands of years). In situ thermal tests are required to build up the confidence level of the coupled TMHC models. The purposes of conducting the laboratory studies of the coupled TMHC processes are to enhance our understanding of those processes, and to assist the interpretation of the field test results. Laboratory experiments deal with controlled experimental and boundary conditions, smaller sample sizes, and simpler geometrical configurations (e.g., regular shape and single fracture). These characteristics make the laboratory results suitable for understanding the processes. This in turn will make incorporation of these processes in model calculations more manageable. However, it should be noted that small sample size and simple geometrical configuration make the results of the laboratory tests unsuitable for direct use in predicting behaviors of in situ rock mass. The laboratory tests included in this reporting period are summarized below, along with projection of future work. This report fulfills the level 4 Milestone ID: SPL7A5M4

Fracture/matrix flow experiments results

The impact of vapor diffusion and its potential enhancement are of concern with respect to the performance of the potential nuclear waste repository at Yucca Mountain. Under non-isothermal conditions, such as those prevailing in the near-field environment, gas-phase diffusion of water vapor (a condensable component) may be enhanced as compared to isothermal conditions. Two main phenomena are responsible for this enhancement (Philip and DeVries 1957, p. 226). Normally, diffusive transport of water vapor is obstructed by the presence of liquid islands in the pore throats, and diffusion is reduced at higher saturations. However, under a thermal gradient, a vapor-pressure gradient develops in the gas phase, causing water to evaporate from one side of the liquid island and to diffuse in the gas phase to a liquid island of lower temperature, where it condenses (Figure 1). Water flows through the liquid island as a result of differences in meniscus curvature between the two sides. This difference is caused by the temperature gradient between the liquid-vapor interfaces on the two ends of the liquid island. The evaporation-condensation process repeats itself on the other side of the liquid island; the result is an enhanced diffusive flux through the medium

Pore-scale observations of supercritical CO₂ drainage in Bentheimer sandstone by synchrotron x-ray imaging

This work utilizes synchrotron-based x-ray computed microtomography (x-ray CMT) imaging to quantify the volume and topology of supercritical carbon dioxide (scCO₂) on a pore-scale basis throughout the primary drainage process of a 6 mm diameter Bentheimer sandstone core. Experiments were performed with brine and scCO₂ at 8.3 MPa (1200 psi) and 37.5°C. Capillary pressure–saturation curves for the scCO₂-brine system are presented and compared to the ambient air-brine system, and are shown to overlay one another when pressure is normalized by interfacial tension. Results are analyzed from images with a voxel resolution of 4.65 μm; image-based evidence demonstrates that scCO₂ invades the pore space in a capillary fingering regime at a mobility ratio M = 0.03 and capillary number Ca = 10[superscript −8.6] to an end-of-drainage brine saturation of 9%. We provide evidence of the applicability of previous two-dimensional micromodel studies and ambient condition experiments in predicting flow regimes occurring during scCO₂ injection.Keywords: Carbon sequestration, Capillary fingering, Supercritical CO₂, Drainage, X-ray microtomographyKeywords: Carbon sequestration, Capillary fingering, Supercritical CO₂, Drainage, X-ray microtomograph

Trapping and hysteresis in two-phase flow in porous media: A pore-network study

Abstract: Several models for two-phase flow in porous media identify trapping and connectivity of fluids as an important contribution to macroscale hysteresis. This is especially true for hysteresis in relative permeabilities. The trapping models propose trajectories from the initial saturation to the end saturation in various ways and are often based on experiments or pore-network model results for the endpoints. However, experimental data or pore-scale model results are often not available for the trajectories, that is, the fate of the connectivity of the fluids while saturation changes. Here, using a quasi static pore-network model, supported by a set of pore-scale laboratory experiments, we study how the topology of the fluids changes during drainage and imbibition including first, main and scanning curves. We find a strong hysteretic behavior in the relationship between disconnected nonwetting fluid saturation and the wetting fluid saturation in a water-wet medium. The coalescence of the invading nonwetting phase with the existing disconnected nonwetting phase depends critically on the presence (or lack thereof) of connected nonwetting phase at the beginning of the drainage process as well as on the pore geometry. This dependence involves a mechanism we refer to as reversible corner filling. This mechanism can also be seen in laboratory experiments in volcanic tuff. The impact of these pore-network model results on existing macroscopic models is discussed.Keywords: hysteresis, trapping, two-phase flow, fluid topology, pore geometry, pore networ

Pore-scale displacement mechanisms as a source of hysteresis for two-phase flow in porous media

The macroscopic description of the hysteretic behavior of two-phase flow in porous media remains a challenge. It is not obvious how to represent the underlying pore-scale processes at the Darcy-scale in a consistent way. Darcy-scale thermodynamic models do not completely eliminate hysteresis and our findings indicate that the shape of displacement fronts is an additional source of hysteresis that has not been considered before. This is a shortcoming because effective process behavior such as trapping efficiency of CO₂ or oil production during water flooding are directly linked to pore-scale displacement mechanisms with very different front shape such as capillary fingering, flat frontal displacement, or cluster growth. Here we introduce fluid topology, expressed by the Euler characteristic of the nonwetting phase (χ[subscript]n), as a shape measure of displacement fronts. Using two high-quality data sets obtained by fast X-ray tomography, we show that χ[subscript]n is hysteretic between drainage and imbibition and characteristic for the underlying displacement pattern. In a more physical sense, the Euler characteristic can be interpreted as a parameter describing local fluid connectedness. It may provide the closing link between a topological characterization and macroscopic formulations of two-phase immiscible displacement in porous rock. Since fast X-ray tomography is currently becoming a mature technique, we expect a significant growth in high-quality data sets of real time fluid displacement processes in the future. The novel measures of fluid topology presented here have the potential to become standard metrics needed to fully explore them