Matrix Flow Research Articles

3D Multi-scale Reconstruction of Fractured Shale and Influence of Fracture Morphology on Shale Gas Flow

Quantitative characterization of shale gas flow in fractured shale is a key problem in shale gas extraction. In this study, 3D multi-scale structures in fractured shale were reconstructed, firstly, by X-ray micro-computerized tomography (CT), high-resolution scanning electron microscope (SEM), and fractal function. Then, a REV (representative elementary volume)-scale lattice Boltzmann (LB) model, considering Klinkenberg’s effect and gas absorption, was built, and the effects of fracture complexity and gas pressure on shale gas flow and shale permeability were analyzed and studied quantitatively. The simulation results indicate that the gas flow behaviors in the fractured shale are related strongly to fracture morphology and gas pressure. The decreased fracture fractal dimension or increased complexity of the fracture network leads to increase in permeability of the fractured shale. The gas velocity in the shale matrix decreases with increasing fracture roughness. Increased fracture network connectivity contributes to the formation of local high-intensity velocity fields and the improvement of gas flow in the shale matrix. The gas rarefaction effect has a significant influence on gas flow in the fractured shale, and permeability of the fractured shale must be regarded as a dynamic shale reservoir parameter. The gas flow behavior in the fractured shale is more sensitive at low-pressure condition whereas the gas rarefaction effect is more sensitive to the fractured shale with highly rough fracture or low fracture connectivity.

Numerical simulation of the feasibility of supercritical CO2 storage and enhanced shale gas recovery considering complex fracture networks

In recent years, the greenhouse gas control has been a hot issue. A solution of injecting supercritical CO 2 (scCO 2 ) in Longmaxi shale gas formation (in Southwest China) is proposed in this study. To evaluate the feasibility of this method that storing CO 2 and enhanced gas recovery (EGR) simultaneously, we first carried out isothermal adsorption experiments on samples from Longmaxi formation to describe the adsorption behavior of CH 4 and CO 2 , and a generalized Ono-Kondo Lattice (OK) model was applied to predict adsorption amount of pure CH 4 /CO 2 and their binary mixtures under supercritical, high-pressure conditions. In addition, discrete fracture network (DFN) model was adopted to characterize the complex hydraulic fracture networks constructed from micro-seismic monitoring (MSM) data and engineering analysis. The porous flow in shale matrix was modeled with the multiple interacting continua (MINC) and fractal theories. A multiscale compositional numerical model based on unstructured tri-prism grids was finally developed and solved by control volume finite element (CVFE) method. The simulation results of base and CO 2 injection cases presented that scCO 2 huff ‘n’ puff in Longmaxi formation might be an effective method for CO 2 storage and EGR, and the orthogonal experiments were designed to optimize scenarios in the field application and obtain a maximum balanced result of EUR and CO 2 storage capacity (CSC) for multistage fractured horizontal well (MFHW) with complex fracture networks in shale. • The OK model was applied to predict pure CH 4 /CO 2 and their mixtures adsorption amount under supercritical conditions. • The hybrid DFN and MINC model was established to describe the flow in fractal porous media and complex fracture system. • The CVFE simulator was successfully conducted for comprehensive analysis of mechanisms of scCO 2 storage and EGR in shale.

Verification of Coupled Hydraulic Fracturing Simulators Using Laboratory-Scale Experiments

In this work, we aim to verify the predictions of the numerical simulators, which are used for designing field-scale hydraulic stimulation experiments. Although a strong theoretical understanding of this process has been gained over the past few decades, numerical predictions of fracture propagation in low-permeability rocks still remains a challenge. Against this background, we performed controlled laboratory-scale hydraulic fracturing experiments in granite samples, which not only provides high-quality experimental data but also a well-characterized experimental set-up. Using the experimental pressure responses and the final fracture sizes as benchmark, we compared the numerical predictions of two coupled hydraulic fracturing simulators—CSMP and GEOS. Both the simulators reproduced the experimental pressure behavior by implementing the physics of Linear Elastic Fracture Mechanics (LEFM) and lubrication theory within a reasonable degree of accuracy. The simulation results indicate that even in the very low-porosity (1–2 %) and low-permeability ({10}^{-18} {mathrm{m}}^{2}- {10}^{-19} {mathrm{m}}^{2}) crystalline rocks, which are usually the target of EGS, fluid-loss into the matrix and unsaturated flow impacts the formation breakdown pressure and the post-breakdown pressure trends. Therefore, underestimation of such parameters in numerical modeling can lead to significant underestimation of breakdown pressure. The simulation results also indicate the importance of implementing wellbore solvers for considering the effect of system compressibility and pressure drop due to friction in the injection line. The varying injection rate as a result of decompression at the instant of fracture initiation affects the fracture size, while the entry friction at the connection between the well and the initial notch may cause an increase in the measured breakdown pressure.

A multi-scale approach for simulation of capillary absorption of cracked SHCC based on crack pattern and water status in micropores

Strain-hardening cement-based composites (SHCC) under cracked condition exhibit distinct behaviour of capillary absorption because of multiple fine cracks and complicated crack patterns. Past studies for estimating capillary absorption of cracked SHCC mainly focused on evaluating the results by total gravimetric weight. In addition, the average values of crack width and crack spacing were often used to characterize crack patterns. In this study, a multi-scale approach for numerical simulation of the capillary absorption of cracked SHCC is proposed by considering the detailed crack patterns such as the width and location of each crack. Water flows in bulk matrix and multiple cracks are simulated using two individual transport equations. Water absorbed from a crack to its adjacent matrix is treated as the mass exchange of the two equations, taking into account pore structure and water status in the bulk matrix as well as the damage level of the crack surfaces. Analysis is firstly carried out for SHCC with uniform crack width and crack spacing, showing consistent results with test data of previous studies in which the total absorption increased with decreasing crack spacing. Analysis for SHCC with non-uniform crack widths and locations, furthermore, exhibit significant water penetration at crack zones. Other factors, such as relative humidity for pre-drying, specimen height, water-to-binder ratio, and fibre content, are also discussed in the analysis. This approach enables the estimation of water profile of cracked SHCC, so can be used for further crack-linked process, for example, chloride penetration or self-healing.

Use of Vibrational Optical Coherence Tomography to Analyze the Mechanical Properties of Composite Materials.

Energy storage and dissipation by composite materials are important design parameters for sensors and other devices. While polymeric materials can reversibly store energy by decreased chain randomness (entropic loss) they fail to be able to dissipate energy effectively and ultimately fail due to fatigue and molecular chain breakage. In contrast, composite tissues, such as muscle and tendon complexes, store and dissipate energy through entropic changes in collagen (energy storage) and viscous losses (energy dissipation) by muscle fibers or through fluid flow of the interfibrillar matrix. In this paper we review the molecular basis for energy storage and dissipation by natural composite materials in an effort to aid in the development of improved substrates for sensors, implants and other commercial devices. In addition, we introduce vibrational optical coherence tomography, a new technique that can be used to follow energy storage and dissipation by composite materials without physically touching them.

Poroelastoplastic Modeling of Complex Hydraulic-Fracture Development in Deep Shale Formations

Summary In this paper, we propose a hydromechanical model to simulate hydraulic fracture propagation in deep shale formations. The Drucker-Prager plasticity theory, Darcy’s law, Reynolds lubrication theory, and Kirchoff’s laws are adopted to describe the plastic deformation, matrix flow, fracture flow, and wellbore flow, respectively. A global embedded cohesive zone model is constructed to achieve the free evolution of hydraulic fractures and the characterization of natural fractures. The finite element method (FEM) and finite volume method (FVM) are used for the spatial discretization of the stress field and pressure field. On the basis of Newton-Raphson iteration, fixed-stress iteration, and Picard iteration, a mixed numerical scheme is built up to solve the strong nonlinear coupling problem. The proposed model is verified against several reference cases and experimental results. Finally, some numerical cases are carried out to investigate the influences of rock properties, natural fracture distribution, and fracturing fluid properties on the complex hydraulic fracture development. The results show that rock plasticity leads to a decrease in stimulated fracture area, an increase in average fracture width, and an increase in propagation pressure. As the cluster number increases, the adverse effect of rock plasticity on multiple hydraulic fracturing in deep shale formations increases significantly. In addition, appropriate optimization of cluster spacing could weaken the adverse effect of rock plasticity on fracturing treatment to a certain extent by using the stress interference effect.

Optimization of the shale gas reservoir fracture parameters based on the fully coupled gas flow and effective stress model

AbstractFracture parameters are important for oil and gas production. In this paper, we establish a fully coupled gas flow and effective stress model for the single well multi‐stage hydraulic fracturing shale gas reservoir and analyze the optimal fracture parameters of this model. Firstly, the reservoir consists of single‐porosity medium zone, dual‐porosity medium zone, and hydraulic fracture zone. Then, on the basis of the effective stress mechanism, the theory of porous elastic medium, evolution of fractal dimensions, and the mechanism of fluid transportation, we establish an apparent permeability model of matrix and fracture system to couple the deformation of reservoir and gas flow in matrix. After that, the partial differential equation is solved by COMSOL and verified using the field shale gas production data of Barnett and Marcellus. Finally, the model is used to optimize the fracture parameters. The results show that by increasing the half‐length of hydraulic fracture, the gas production gets the maximum growth, followed by increasing the fracture diversion capacity and width of hydraulic fractures. For the shale gas reservoir with large‐fractured and relatively high permeability, increasing the length of hydraulic fractures can bring higher production, while for the micro‐fractured and low‐permeability, hydraulic fractures with complex shapes can contribute to production increase.

Optimising development and production of naturally fractured reservoirs using a large empirical dataset

Naturally fractured reservoirs are important contributors to global petroleum reserves and production. Existing classification schemes for fractured reservoirs do not adequately differentiate between certain types of fractured reservoirs, leading to difficulty in understanding fundamental controls on reservoir performance and recovery efficiency. Three hundred naturally fractured reservoirs were examined to define a new classification scheme that is independent of the type of fracturing and describes fundamentally different matrix types, rock properties, fluid storage and flow characteristics. This study categorises fractured reservoirs in three groups: (1) Type 1: characterized by a tight matrix where fractures and solution-enhanced fracture porosity provide both storage capacity and fluid-flow pathways; (2) Type 2: characterized by a macroporous matrix which provides the primary storage capacity where fractures and solution-enhanced fracture porosity provide essential fluid-flow pathways; and (3) Type 3: characterized by a microporous matrix which provides all storage capacity where fractures only provide essential fluid-flow pathways. Differentiation is made between controls imparted by inherent natural conditions, such as rock and fluid properties and natural drive mechanisms, and human controls, such as choice of development scheme and reservoir management practices. The classification scheme presented here is based on reservoir and production characteristics of naturally fractured reservoirs and represents a refinement of existing schemes. This refinement allows accurate comparisons to be made between analogous fractured reservoirs, and trends and outliers in reservoir performance to be identified. Case histories provided herein demonstrate the practical application of this new classification scheme and the benefits that arise when applying it to the understanding of naturally fractured reservoirs.

Achieving low wear in a μ-phase reinforced high-entropy alloy and associated subsurface microstructure evolution

Face-centered cubic (fcc) high-entropy alloys (HEAs) with dispersed hard intermetallic compounds have demonstrated a good combination of high strength-ductility. However, limited research effort has been devoted to exploring their siding wear behavior. Herein, we report the effects of load and sliding velocity on the dry sliding wear behavior Fe40Co20Mo20Ni20 HEA consisting of an ultrafine-grained (UFG) fcc matrix and a high density of dispersed hard μ-phase. Pin-on-disk dry sliding wear tests show that the Fe40Co20Mo20Ni20 HEA exhibits low wear rates of the order of 10−6–10−5 mm3/(N·m) at different test conditions. The wear rates and coefficients of friction generally increase with load and sliding velocity, but wear modes show no apparent change, all in the form of abrasive, oxidation and fatigue wear. Sliding-induced gradient microstructure was formed, consisting of a nanostructured glaze layer at the sliding surface and two typical plastically deformed regions further away from the surface. The presence of massive hard μ-phase particles is suggested to hinder the plastic flow of the fcc matrix during sliding, thereby achieving low wear.

Effects of Supercritical CO2 on Matrix Permeability of Unconventional Formations

We studied the effects of supercritical carbon dioxide (scCO2) on the matrix permeability of reservoir rocks from the Eagle Ford, Utica, and Wolfcamp formations. We measured permeability using argon before exposure of the samples to scCO2 over time periods ranging from days to weeks. We measured permeability (and the change of permeability with confining pressure) when both argon and scCO2 were the pore fluids. In all three formations, we generally observe a negative correlation between initial permeability and carbonate content—the higher the carbonate content, the lower the initial permeability. In clay- and organic-rich samples, swelling of the matrix resulting from adsorption decreased the permeability by about 50% when the pore fluid was scCO2 although this permeability change is largely reversible. In carbonate-rich samples, dissolution of carbonate minerals by carbonic acid irreversibly increased matrix permeability, in some cases by more than one order of magnitude. This dissolution also increases the pressure dependence of permeability apparently due to enhanced mechanical compaction. Despite these trends, we observed no general correlation between mineralogy and the magnitude of the change in permeability with argon before and after exposure to scCO2. Flow of scCO2 through μm-scale cracks appears to play an important role in determining matrix permeability and the pressure dependence of permeability. Extended permeability measurements show that while adsorption is nearly instantaneous and reversible, dissolution is time-dependent, probably owing to reaction kinetics. Our results indicate that the composition and microstructure of matrix flow pathways control both the initial permeability and how permeability changes after interaction with scCO2. Electron microscopy images with Back-Scattered Electron (BSE) and Energy Dispersive Spectroscopy (EDS) revealed dissolution and etching of calcite minerals and precipitation of calcium sulfide resulting from exposure to scCO2.

Optimization of Fracturing Parameters by Modified Variable-Length Particle-Swarm Optimization in Shale-Gas Reservoir

Summary Horizontal drilling and hydraulic fracturing are recognized as the most efficient techniques to enhance recovery in shale-gas reservoirs. Because of the exploitation difficulties and complex flow mechanism in shale gas, it is imperative to focus on the optimization of fracturing parameters. However, most of the current heuristic algorithms follow the principle that the variable dimension is constant during iteration, which leads to poor performance when dealing with dimension-varying problems. The optimization of fracturing parameters can be regarded as a typical dimension-varying problem when considering the difference among fracture properties such as half-length and conductivity. Thus an improved algorithm named modified variable-length particle-swarm optimization (PSO) (VPSO) (MVPSO) was proposed to automatically select the optimal fracturing parameters: the number of fractures as well as the corresponding fracture properties. Then, MVPSO was verified and compared with VPSO by several benchmarks. In addition, a gas/water two-phase model considering gas-adsorption and Knudsen-diffusion effects was used to describe the shale-gas flow in matrix and fracture domains. An embedded discrete-fracture model (EDFM) was applied to model the hydraulic-fracture geometries and fractal methods were adopted to generate the fracture networks. The results indicated that MVPSO showed better performance in both convergence speed and accuracy than that of VPSO, which also provided a new perspective for the optimization of fracturing parameters. Besides, the multispindle-shaped fracture-distribution pattern reached a higher net-present-value (NPV) contrast to that of homogeneous fracture distribution. The decrease of gas price leads to smaller and more nonuniform half-length distribution.

Effect of the Particle Size and Matrix Strength on Strengthening and Damage Process of the Particle Reinforced Metal Matrix Composites.

Roles of the particle, strengthening, and weakening during deformation of the particle reinforced metal matrix composite, were studied using in situ technique. Composites with three different strengths Al-Cu-Mg alloy matrices reinforced by three sizes SiC particles were manufactured and subjected to in situ tensile testing. Based on in situ observation, damage process, fraction and size distribution of the cracked particles were collected to investigate the behavior of the particle during composite deformation. The presence of the particle strengthens the composite, while the particle cracking under high load weakens the composite. This strengthening to weakening transformation is controlled by the damage process of the particle and decided by the particle strength, size distribution, and the matrix flow behavior together. With a proper match of the particle and matrix, an effective strengthening can be obtained. Finally, the effective match range of the particle and the matrix was defined as a function of the particle size and the matrix strength.

Soil Water Dynamics Under Different Land Uses in Loess Hilly Region in China by Stable Isotopic Tracing

Exploring soil water dynamics under different land use types is important for water resource management and vegetation restoration in the Loess Plateau. In this study, we investigated the hydrogen and oxygen isotopic compositions of soil water from four different land use types to explore the mechanism of soil water movement and transformation and analyse the influence of land use. The results show that the range of stable isotopes (δD and δ18O) in soil water was smaller than that in precipitation. Values for δD and δ18O in soil water showed relatively similar temporal variation, heavy isotopes were enriched in the soil water in July and depleted in October. Stable isotope values in shallow (<100 cm depth) soil water and deep (>200 cm depth) soil water were low. The δD and δ18O values in woodlands decreased gradually with increasing depth. Across the four land use types, the maximum variation in δD and δ18O was in the shallow depth of the soil profile. Groundwater was recharged mainly from precipitation and then from soil water. The ratio of groundwater recharge by soil water under different land use types followed this rank order: woodland (35.70%) > grassland (31.14%) > shrubland (29.47%) > cropland (29.18%). Matrix flow and preferential flow coexisted during infiltration, and the occurrence of preferential flow was related to the land use type. The main reason for the variation in isotopic composition in soil water is the difference in soil evaporation, which is influenced by different vegetation cover. Owing to the difference in soil evaporation and fractionation, precipitation on cropland, shrubland, and grassland can recharge more soil water than on woodland.

Study on Pressure Propagation in Tight Oil Reservoirs with Stimulated Reservoir Volume Development

The stimulated reservoir volume fracturing development in tight oil reservoirs is characterized by multiscale flow of the reservoir matrix, fracture network, and hydraulic fracture. Therefore, the flow field structure is extremely complex. Multiscale flow characteristics have been revealed through the systematical experiments including the threshold pressure gradient and the stress sensitivity. Based on the theory of elliptical flow, a comprehensive and practical mathematical model of multiregion coupling flow is established to characterize the multiscale flow, and the pressure distribution equation is derived. The calculation method of moving boundary is established to simulate the dynamic supply boundary and the dynamic pressure distribution by using the steady-state sequential replacement method. The characteristics of multiscale flow, multistage development state, and stress sensitivity are considered, especially the different stress sensitivity characteristics in different regions. Finally, the pressure propagation in tight reservoirs is clarified and the influence of matrix permeability, stress sensitivity characteristics, and drawdown pressure on the distance at the dynamic supply boundary are revealed. The research results provide theoretical basis for the development effect evaluation.

Determination of groundwater recharge processes and evaluation of the ‘two water worlds’ hypothesis at a check dam on the Loess Plateau

Areas with shallow groundwater and check dams have relatively abundant water resources compared to tablelands with deep loess layers on the Loess Plateau in China, but little attention has been paid to their potentially substantial role in groundwater recharge to date. In addition, during groundwater recharge preferential flow through macropores can replenish groundwater quickly, leaving immobile bound water relatively unmixed. Different water paths result in the separation of groundwater and plant xylem water isotopic signals, according to the ‘two water worlds’ (TWW) hypothesis. The hypothesis has received wide attention in the last decade due to observations of stronger 2H and 18O enrichment (higher δ2H and δ18O values) in plant xylem water than in groundwater. Thus, aims of this study were to analyze processes involved in groundwater recharge and evaluate the TWW hypothesis by isotope (δ2H and δ18O) tracing at a check dam on the Loess Plateau. We report three main findings. First, the groundwater table began to rise 35 days after the first large rain event during our study period, showing that recharge times at the site are orders of magnitude shorter than those in tablelands of the Loess Plateau (decades or hundreds of years). Second, preferential and matrix flows contributed 36% and 64%, respectively, of the total recharge. Third, when preferential flow prevailed, no significant isotopic differences between any measured water sources were found at the check-dammed site (e.g. precipitation, soil water, groundwater and plant xylem water) (p > 0.05), which conflicts with isotopic patterns predicted by the TWW hypothesis. In conclusion, check dams in areas with shallow groundwater can substantially promote water cycling from precipitation to groundwater on the Loess Plateau and in similar areas. This study also provides a link between hydrological and ecological processes (groundwater recharge paths and plant water uptake), with significant implications for fundamental ecohydrological understanding and practical hydrological predictions.

Characterizing Preferential Flow Paths in Texturally Similar Soils under Different Land Uses by Combining Drainage and Dye-Staining Methods

Preferential flow paths have been widely characterized by many visualization methods. However, the differences in preferential flow paths under various land uses and their relationships to hydraulic properties remain uncertain. The objectives of this study are to (1) characterize preferential flow paths under various land uses (forest and orchard) by combining drainage and dye-staining methods and to (2) build a connection between preferential flow paths and hydraulic-related parameters and extract the proportion of preferential flow paths from the compounding effects of matrix flow and preferential flow. The dye-staining experiments were conducted in five sandy soils and one sandy clay loam in situ, including four soils from forest and two soils from orchards. A total of 47 soil cores, 4 cm in height and 9 cm in diameter, were collected in each layer of the dye-stained soils for drainage experiments in the laboratory. Dye coverage and hydraulically equivalent macropore parameters (macroporosity, pore size distribution, and number of macropores) and their relationships were analyzed. The results show that the volume of preferential flow is partly affected by the total macropore volume. The effect of macropores on preferential flow varies by macropore size distribution. Dye coverage exhibited a significant (P < 0.01) correlation with macroporosity (correlation coefficient 0.83). Based on the value of macroporosity or steady effluent rates, the part of the dye coverage that was due to preferential flow on the surface dye-stained soil (resulting from both matrix and preferential flow) could be identified in this study. Compared with orchards, forestland has more preferential flow paths in both surface soil and subsoil. Further studies are needed to quantify the 3-D preferential flow paths and build a connection between preferential flow paths and hydraulic properties.

Flow-based pultrusion of continuous fibers for cement-based composite material and additive manufacturing: rheological and technological requirements

The vast majority of applications of extrusion-based concrete additive manufacturing deal with unreinforced mortar. The lack of reinforcement is a serious limitation for the industrial development of 3d printed concrete, because of the brittleness and lack of tensile strength of unreinforced mortar. In this paper, a new reinforcement method inspired by pultruded composite manufacturing, called flow-based pultrusion (FBP), is described and used. The principle is that continuous fiber rovings are impregnated and pulled by the matrix flow, avoiding motorisation, and increasing the apparent yield stress of the mortar and consequently its usability. The hardened resulting material, called anisotropic concrete, is unidirectionally and homogenously reinforced. For such composite material, the reinforcement ratio is an important material parameter that relates to specific process variables: roving type, roving count, and output diameter. This article further investigates the effect of the percentage of reinforcement on the process. It also highlights technical requirements to provide the first specifications for flow-based pultrusion.

Determining the influencing factors of preferential flow in ground fissures for coal mine dump eco-engineering.

Ground fissures (GF), appearing in front of dumps, are one of the most obvious and harmful geological hazards in coal mining areas. Studying preferential flow and its influencing factors in the ground fissures of dumps may provide basic scientific support for understanding the rapid movement of water and vegetation restoration and reconstruction in mining areas. Based on field surveys of ground fissures, three typical ground fissures were selected in the studied dump. The morphological characteristics of preferential flow for ground fissures were determined through field dye tracing, laboratory experiments, and image processing technology. The results indicated that the lengths of the three ground fissures ranged from 104.84 cm to 120.83 cm, and the widths ranged from 2.86 cm to 9.85 cm. All of the ground fissure area densities were less than 10%, and the proportion of ground fissure surface area was small in the dump. The maximum fissure depth was 47 cm, and the minimum was 16 cm. The ground fissure widths ranged from 0 cm to 14.98 cm, and the fissure width and fissure width-to-depth ratios decreased with increasing soil depth. The stained area was greater than 90% in the 0–5 cm soil layers of the three fissures, and water movement was dominated by matrix flow. The stained width decreased from 90 cm to 20 cm with increasing soil depth. The preferential flow was mainly concentrated on both sides of the fissure, which was distributed as a “T” shape. The preferential flow stained area ratios were 27.23%, 31.97%, and 30.73%, respectively, and these values decreased with increasing soil depth. The maximum stained depths of the preferential flow among the three fissures were different, and the maximum stained depth of GF II was significantly larger than that of GF I and GF III (P < 0.05). The stained path numbers of the three fissures ranged from 0 to 49. With increasing soil depth, the stained path number first increased and then decreased. The stained path widths of the three fissures ranged from 0 cm to 90 cm. With the increase in soil depth, the stained path width decreased. The stained area ratio was significantly positively correlated with ground fissure width, the ground fissure width-to-depth ratio, soil saturated hydraulic conductivity, soil organic matter, and sand content and was significantly negatively correlated with soil water content and clay content. The stained path number was significantly positively correlated with ground fissure width, the ground fissure width-to-depth ratio, soil saturated hydraulic conductivity and soil organic matter. The stained path width was significantly positively correlated with the ground fissure width-to-depth ratio, soil saturated hydraulic conductivity, soil organic matter and sand content and was significantly negatively correlated with clay content. Plant roots could significantly increase the stained area ratio, stained path number, and width and promote the formation and development of preferential flow.

Quantifying the Importance of Preferential Flow in a Riparian Buffer

HighlightsRiparian buffers and vegetative filter strips are uniquely susceptible to preferential flow.An innovative method is proposed to partition infiltration into matrix and macropore domains.Riparian buffer matrix and plot-scale infiltration experiments were simulated with HYDRUS-1D and VFSMOD.Preferential flow accounted for 32% to 47% of infiltration depending on hydrologic conditions.Preferential flow mechanisms should be incorporated into riparian buffer design tools and models.Abstract. Riparian buffers are uniquely susceptible to preferential flow due to the abundance of root channels, biological activity, and frequent wetting and drying cycles. Previous research has indicated such susceptibility and even measured the connectivity of preferential flow pathways with adjacent streams and rivers. However, limited research has attempted to partition the riparian buffer infiltration between matrix and preferential flow domains. The objectives of this research were to develop an innovative method to quantify soil matrix infiltration at the plot scale, develop a method to partition infiltration into matrix and macropore infiltration at the plot scale, and then use these methods to quantify the significance of macropore infiltration at a riparian buffer site. This research further demonstrated the importance of considering preferential flow processes in design tools and models to evaluate riparian buffer effectiveness. Sprinkler and runon field experiments were conducted at an established riparian buffer site with sandy loam soil. Trenches were installed and instrumented with soil moisture sensors along the width of the riparian buffer (i.e., along the flow path toward the stream) for detecting non-uniform flow patterns due to preferential flow. Riparian buffer parameters, including soil hydraulic parameters, were estimated using HYDRUS-1D for the sprinkler experiments and VFSMOD for the runon experiments. This research partitioned the infiltration into matrix and preferential flow domains by assuming negligible exchange of water between the soil matrix and preferential flow pathways in comparison to the magnitude of soil matrix flow. For these experimental conditions with 0.20 to 0.48 L s-1 of runon and initial soil water contents of 0.29 to 0.32 cm3 cm-3, preferential flow accounted for at least 27% to 32% of the total runon water entering the riparian buffer. This corresponded to approximately 32% to 47% of the total infiltration. While increasing the riparian buffer plot soil hydraulic conductivity in single-porosity models can adequately predict the total infiltration and therefore the surface outflow from the buffer, design tools and models should specifically consider preferential flow processes to improve predictive power regarding the actual infiltration processes and correspondingly the non-equilibrium flow and solute transport mechanisms. Keywords: Flow partitioning, HYDRUS, Matrix flow, Preferential flow, Riparian buffer, VFSMOD.

黄河三角洲刺槐群落土壤优先流及养分分布特征

黄河三角洲湿地面临严重的水资源短缺、湿地退化、土壤盐碱化等问题，湿地土壤干旱缺水，土壤收缩产生裂隙等优先流路径，在小尺度上改变湿地内部以及湿地板块之间水文连通性，小尺度水文效应往往制约大尺度水文连通性，基于当前社会对湿地修复的迫切需求，从小尺度对水文连通研究有必要加以重视。然而，目前的研究多集中在大尺度水文连通的阐述，为进一步明确黄河三角洲湿地小尺度水文连通和养分随优先流路径运移分布情况，以此区域典型刺槐群落为研究对象，基于室外染色示踪实验和室内图像处理技术，分析土壤优先流形态和分布特征，并探究优先流和土壤养分含量之间的关系。结果表明：（1）刺槐群落染色面积比随土壤深度变化主要包括2个阶段，第一阶段：优先流和基质流相互影响作用显著，第二阶段：优先流和基质流相互作用不显著，优先流强度逐渐增强，明显存在指流现象，极少部分管流现象。（2）刺槐群落染色面积比、基质流深度和百分之五十染色深度的数值较大，优先流区染色面积比较小，基质流深度发生在土壤深10-15 cm区间，该类型植被群落水流均匀入渗深度较大，优先流和基质流相互作用程度大，水文连通性较高。（3）优先流区土壤有机碳、有机质、全氮、全磷和有效磷含量值均高于基质流区；（4）土壤优先流区染色面积比与5种养分指标均呈负相关关系，有机碳、有机质和有效磷含量受优先流路径影响显著。（5）土壤中有机碳、有机质和有效磷含量在一定程度上可以衡量土壤优先流发育水平。研究结果可为黄河三角洲湿地生态系统保护和管理提供参考。;More serious environmental problems including water shortage, wetland degradation, and soil salinization are gradually recognized in the Yellow River Delta wetland. Due to water deficiency of wetland soils, preferential pathways including soil cracks occur as soil shrinkage which changes the hydrological connectivity of inside wetlands or between wetlands at small scale, and small scale hydrological processes tend to affect the large scale hydrological cycle, depending on our urgent needs to restore wetlands, it is necessary to pay attentions to the study of hydrological connectivity research from microcosmic perspectives to promote the understanding of connotation of hydrological connectivity. However, most of the current studies focused on the hydrological connectivity from macroscopical perspectives. In order to clarify the small-scale hydrological connectivity in the Yellow River Delta, the morphology and distribution characteristics of preferential flow in soil were analyzed based on outdoor dyeing tracer experiment and indoor image processing technology. The relationship between preferential flow and soil nutrient content was explored. The results show that:(1) There are two stages in the variation of the dry coverage of Robinia pseudoacacia Linn community with soil depth. The first stage:the interaction between preferential flow and matrix flow is significant; the second stage:the interaction between preferential flow and matrix flow is not significant, and the intensity of preferential flow is gradually increasing, finger flow phenomenon and a small part of pipe flow phenomenon is observed. (2) Vertical soil-profile indexes (dry coverage, dry coverage of preferential pathway, depth of diffusion area, and depth of 50% dry coverage) reveal a strong interaction between preferential flow and matrix flow, and depth of diffusion area occurs in the range of 10-15cm in the soil, which indicates that this type of vegetation community has large depth of water infiltration flow uniformly and the hydrological connectivity is high; (3) The contents of soil organic carbon, organic matter, total nitrogen, total phosphorus and available phosphorus in preferential flow area were higher than those in matrix flow area; (4) The dye coverage ratio of preferential flow area was negatively correlated with the five nutrient indexes, and the contents of organic carbon, organic matter and available phosphorus were significantly affected by preferential pathway. (5) The content of organic carbon, organic matter and available phosphorus in soil can measure the development level of preferential flow to a certain extent. These results can provide references for wetland ecosystem protection and management in the Yellow River Delta.