Shape Of Dunes Research Articles

Numerical Simulation of Gas-suspended Proppant Migration Based on Equivalent Principle

Abstract The gas-suspended proppant is composed of conventional proppant particles and hydrophobic aerophilic coating. The surface coating changes the surface wettability of the proppant and increases the adhesion of the proppant surface to the gas, changing the traditional liquid-carried proppant into a gas-carried proppant. The sand carrying capacity of low concentration polymer fracturing fluid can be greatly improved by using air-suspended proppant carrying technology. At present, there is no numerical simulation method aiming at the migration law of gas-suspended proppant. Based on the principle of equivalent substitution, the density of gas and particles is set as the calculated equivalent density during simulation. Similarly, the diameter of gas-suspended proppant is set as the calculated equivalent diameter during simulation. Then computational fluid dynamics software was used to simulate the Euler-Euler method. It is found that Euler-Euler method can simulate the placement of gas-suspended proppant in fractures. Compared with conventional proppant, gas-suspended proppant is not easy to settle during migration, and the settling sand bed still has fluidity and can be re-suspended. When gas suspension proppant density increases, the proppant sedimentation trend increased. Increase the injection rate of proppant, and the shape of long and high sand dunes will become shorter and lower. As the sand rate increases, the height of the sand bank rapidly increases.

The Effect of Fracture Wall Surface Roughness on Proppant Transport

Summary Proppant transport into created fractures is crucial in maximizing hydrocarbon recovery in unconventional reservoirs. Injected proppants keep the created fractures open, enhancing final fracture conductivity and propped and effective fracture length. The roughness and width of the created fracture are also important in proppant transport and distribution within induced fractures, affecting fracture conductivity. This study investigates the effect of fracture width on proppant transport into a complex slot system with 3D-printed rough wall surfaces. This study builds upon previous experimental work (Tatman et al. 2022), exploring different fracture roughness profiles and varying fracture widths. The effects of proppant densities, sizes, and concentrations on proppant transport within rough fracture surfaces were also investigated. A laboratory-scaled slot apparatus was used to examine the effects of various parameters. The slot consisted of a 4-ft-long primary fracture intersecting with a secondary fracture at a 90o angle. The wall surface roughness was printed using 3D printing technology and average fracture widths were set at 0.1 in. and 0.2 in. Fresh water (1 cp) with proppant concentrations of 1 ppg and 2 ppg and proppant sizes of 100-mesh sand [2.65 specific gravity (SG)], 40/70-mesh sand (2.65 SG), and 35/45-mesh ultralightweight (ULW) proppant (1.07 SG) were tested. The results show that fracture wall roughness impacts proppant transport behavior. The rough wall surface formed irregular proppant dune shapes and trapped some of the injected proppant at different locations within the slot, which is distinct from smooth wall surfaces. Decreasing the fracture width of a rough wall surface had a significant impact on proppant transport. The majority of the injected proppant was transported away from the injection point due to the increased slurry velocity. This led to improved proppant transport due to redirected flow direction and associated decreased proppant settling velocities, and a decrease in the proppant-dune-building rate near the inlet point, which carried more proppant deep into the secondary slot. Increasing the proppant concentration had a positive impact on proppant transport within both tested fracture widths by increasing the proppant-dune-buildup rate along the fracture slot and increasing the proppant-covered area inside fractures. The slurry injection rate had a great impact on low-density proppant transport. Decreasing the injection rate for lighter proppant (1.07 SG) helped to build a proppant dune near the injection point, increasing the proppant-covered area. Conversely, the higher injection rate carried the majority of the lighter proppant farther and out of the slots. Larger proppant sizes, (i.e., 40/70-mesh sand) resulted in a large proppant-covered area for both tested fracture widths, due to more particle-to-wall interaction, particle-to-particle interaction, and high density, which increased the settling velocity. However, injecting lighter and larger proppant sizes, such as 35/45-mesh ULW proppant, resulted in less settling and more particles suspended during injection due to lower density.

Storms on the Western Black Sea shore: an opportunity for long-distance dispersal of coastal dunes psammophytes?

Coastal areas are among the planet's most dynamic yet vulnerable environments, the shape of sand dunes being typically modeled by two sets of forces: the relatively constant action of wind and waves and the disturbances caused by infrequent but powerful storms. These meteorological events affect the plant communities, but can also play an important role in the long-distance dispersal of some psammophytes. Plants can be exposed, uprooted, broke apart and spread by wind and waves. Then, depending on the resilience of the plant to seawater temperature, the duration of immersion, the type of fragment transported and the suitability of the new habitat, new population can be established. This study aimed to assess the resilience to seawater immersion and the ability of long-distance dispersal of three psammophytes: Convolvulus persicus, Alyssum borzaeanum, and Silene thymifolia. The experiments tested the viability of vegetative fragments and the reproductive capacity of seeds under different exposure periods (5, 10, and 15 days) and temperature conditions: 4 °C (average surface seawater temperature during winter and early spring when storms on the Black Sea Coast occur), 13 °C (average surface seawater temperature), and 23 °C (average summer surface seawater temperature). The increase of both seawater temperature and the exposure time had a significant negative influence on the viability of the vegetative fragments and on germination capacity. After exposure to seawater, Convolvulus persicus, and Silene thymifolia are able to establish new populations through vegetative fragments or seeds, whilst in case of Alyssum borzaeanum, no vegetative regeneration was observed.

Regional differences in the grain size characteristics of surface sediments from typical barchan dunes in arid zones

The grain size characteristics of aeolian sediments are the combined result of sand sources, regional airflow regimes, dune morphology, etc., and are essential for understanding the formation and evolution of barchan dunes. Based on field investigations and laboratory experimental data, in this paper, we explored differences in the grain size characteristics of the surface sediments of barchan dunes at the southeastern edge of the Taklimakan Desert (TKLM-SE), the western (QB-W) and southern parts of the Qaidam Basin Desert (QB-S), the sand belt connecting the Badain Jaran Desert and the Tengger Desert (BT-B), and their responses to sand sources, dune morphology, and wind regimes. The main results were as follows: (i) The mean grian size distribution patterns of the windward slope toe to the leeward slope toe through the dune crest/ridge varied with different transects of the barchan dunes and different deserts, showing four types including gradually fining (GF), gradually coarsening (GC), coarsening followed by fining (CF), and fining followed by coarsening (FC). The patterns were GF and CF in the TKLM-SE; GF, GC, and FC in the BT-B; GF, GC, and CF in the QB-W; and GF in the QB-S. (ii) The interdune sediments provided the source material for the formation and development of barchan dunes and their grain size varied in different deserts. The interdune sediments were composed of gravel and very fine sand in the TKLM-SE, while they were composed of medium and fine sand in the QB-W, QB-S, and BT-B. (iii) The windward side of the barchans varied with different wind directions, and dune height affected dune surface airflow velocity and direction, changing the pattern of grain size distribution on the dune surface. The wind regime over a ten-day or half-month scale could explain the variance in the grain size distribution patterns better than that on an annual scale. (iv) Grain size characteristics of dune surface sands changed with dune shape due to changes in the surface airflow velocity and direction and the sediment-carrying capacity of the airflow. With an increasing ratio of dune height to dune width, the grain size of the dune crest sands became coarser. These results help advance our understanding of the grain size characteristics of barchan dunes and regional variabilities in their patterns.

Evolutionary stages of active to vegetated coastal transgressive dunefields in the San Matías Gulf coast, Argentina

The understanding of how active aeolian coastal dunes transform into fully vegetated forms, and the specific dune configurations that emerge, particularly within coastal dune systems, is limited. This study examines the changes in dune morphology associated with vegetation growth within transgressive dunefields in NE Patagonia, Argentina. The dunefields are located along the northern coast of the San Matías Gulf, ranging from relatively small to large, and dominantly active in the past ∼80 years. Satellite and aerial images were analyzed using GIS procedures to calculate vegetation cover changes and dune migration rates and identify different dune types and their evolutionary stages of fixation. Five stages were defined in relation to vegetation cover and dune shapes and patterns, from bare sand to fully vegetated dunes. Transverse or barchanoid ridges (sometimes with aklé [fish scale], or chaotic patterns) convert to partially vegetated sub-parabolic dunes and finish as parabolic dunes. In other cases, the dunes may convert to reticulate, aklé, or chaotic patterns, completely vegetated. Various stages span a continuous range from unvegetated to fully vegetated dunes across a wide array of timeframes. In some cases, it was possible to recognize changes from stage one to five in ∼20 years. Where the initial transverse dune is largely maintained during the stabilization process a distinctive pattern named “gusanos” (worms) was identified. The vegetated surface measured in some areas changed from 11 % to 51 % vegetation cover between 1961 and 2022, with migration rates decreasing from 9.53 m/year to zero between 1991 and 2020. The increase in vegetation growth also coincides with higher temperatures, weaker winds and decreasing grazing livestock within this Patagonia area. The recorded processes are similar to coastal dune and vegetation changes in some semiarid regions from the Southern Hemisphere between 30 and 60° S, especially in South Australia.

Simulations of dune morphology under tri-directional wind regimes and application to dunes on Mars

Dune morphology was simulated using coupled models of wind flow and sand transport for 4728 tri-directional wind regimes and bed conditions. The dominant control of dune morphology is sand coverage on the bed. Dunes on a fully sand-covered bed tend to form a periodic pattern of long crests with a relatively uniform spacing. In contrast, dunes on a starved bed have greater diversity of crest orientations and shapes, including complex shapes that have not been simulated or observed in bidirectional wind regimes. These specific dune shapes resulting from the tri-directional wind regime persist regardless of whether the transport capacity of the weakest wind is comparable to or only 1/10th that of the dominant wind.On sand-covered beds, dunes generally have only a single modal orientation (approximately that with maximum gross bedform-normal transport). The exceptions are where two strong winds diverge by 90° (two dune orientations arise), where three winds have triradial symmetry (three dune orientations), or winds have modest deviations from triradial symmetry (two dune orientations).On a starved bed, increasing the divergence angle between two strong winds produces a highly generalized sequence of: barchan dunes (divergence angle ∼30° between the two dominant winds), squat barchans or domes (divergence angle of ∼60°), dunes with two or three crest orientations (divergence angles ∼90° or 120°), to slug-shaped or boomerang-shaped dunes (divergence angle 180°, i.e., reversing winds). The simulated morphologies include a wide variety of Martian dune shapes, which allows their formative wind regimes to be inferred.

The Effect of Sediment Transport Models on Simulating River Dune Dynamics

AbstractRiver dunes, dynamic bedforms in the river bed, limit navigable depths during low flows and increase bed roughness. To predict the navigable depth or where maintenance dredging is needed, a numerical dune development model can be a powerful tool. To study the effect of sediment transport on dune shape and propagation, four different sediment transport models were applied in an existing dune development model. Each sediment transport model was able to simulate dune propagation, while only sediment transport models based on the shear stress reshaped the river dunes. The tested sediment transport models can simulate dune celerity similar to observations and realistic, though different, dune shapes for low and median discharges. Implementation of a gravitational bed slope effect combined with a critical shear stress results in low angle dunes, which are representative for river dunes during low river flows. Sediment transport models with spatial relaxation, also result in low angle dunes. However, the relaxation parameters need to be redefined for low flow situation to prevent transition to upper stage plane bed at too low flow velocities. Further analysis of the resulting dune shapes shows that the sediment transport model determines the dune shape in terms of slope angles, while the dune height is related to the total transport capacity.

Study on the pump schedule impact in hydraulic fracturing of unconventional reservoirs on proppant transport law

In unconventional reservoir hydraulic fracturing, using a pump schedule with proppant concentration constant (PCC) instead of the proppant concentration stepwise increasing (PCSI) is considered an innovative approach to reduce operation costs and risks. However, the large-scale application of PCC lacks sufficient theoretical support. This study investigates the impact of two pump schedules on proppant transport behavior through visualized proppant transport experiments and the model that couples fracture propagation with proppant transport. The experiments demonstrate that PCC facilitates proppant transport and reduces flow fluctuations in complex fractures, while PCSI enhances the filling of near-well fractures. The proppant concentration (the ratio of proppant volume to fracturing fluid volume) reaches 20 % or when a combination of 40/70-mesh and 20/40-mesh quartz sands is used for injection, the pump schedule influence becomes negligible. An increase in fracture complexity, proppant size, and proppant concentration all lead to a reduced influence of the pump schedule on the dune shape. Numerical simulation reveal that the fracture area and average fracture conductivity is similar for different pump schedules, but PCC yields a slightly larger propped fracture area (ranging from 1 % to 4 %) and results in a more uniform conductivity. PCC could achieve similar fracture filling when the proppant concentration is more than 15 % or there is no significant demand for near-well conductivity. Overall, both two pump schedules have a relatively insignificant impact on the transport process of proppants and the final sand dune shape. By comparing pressure curves during hydraulic fracturing, it was evident that PCC resulted in fewer fluctuations. Analyzing production data from 93 wells with PCSI and 59 wells with PCC, it was observed that PCC enhances production and reduces proppant flowback post-fracturing. This research provides a theoretical foundation for the application of PCC pump schedule in hydraulic fracturing.

Using Texture Statistics to Identify and Map Different Dune Types within the Rub’ al Khali

Understanding the movement, direction, and shape of sand dunes can contribute to reducing their impact on infrastructure and the environment. The Rub’ al Khali desert has a distribution of dune types. This study aims to identify and map the different types of dunes within the Rub’ al Khali using a texture analysis method based on a digital elevation model (DEM). Statistical texture analysis methods (variance, skewness, and kurtosis) show three different textures of sand dune shapes, according to the geography of the dunes, using data contained in global DEMs. The analysis presented in this study focused on the use of DEMs to investigate the varied dune morphology within the Rub’ al Khali. The GMTED2010 and EarthEnv_DEM90 digital elevation models were compared. Spatial variability in dune height, spatial variability in dune texture, and profile graphs were created to examine dune surfaces in cross-section. The results provided six different dune types within the sand sea: giant compound linear dunes, simple linear dunes, simple transverse dunes, compound crescentic dunes (megabarchans), huge star dunes, and many transitional forms that defy classification. The results showed that the compound linear dune and simple linear dune were the dominant dune types, covering 41.61% and 31.7% of the total study area, respectively. The maps of variance, using either 10 × 10 and 30 × 30 focal blocks, produced a fairly sharp distinction in dune texture. It is hoped that future research in aeolian geomorphology will greatly benefit from these results, which could easily be expanded with the use of more sophisticated pattern recognition software, which clearly shows the value of using such an approach.

Bathymetric data integration approach to study bedforms in the estuary of the Saint‐Lawrence River

The identification of bedforms has an important role in the study of seafloor morphology. The presence of these dynamic structures on the seafloor represents a hazard for navigation. They also influence the hydrodynamic simulation models used in the context, for example, of coastal flooding. Generally, MultiBeam EchoSounders (MBES) are used to survey these bedforms. Unfortunately, the coverage of the MBES is limited to small areas per survey. Therefore, the analysis of large areas of interest (like navigation channels) requires the integration of different datasets acquired over overlapping areas at different times. The presence of spatial and temporal inconsistencies between these datasets may significantly affect the study of bedforms, which are subject to many natural processes (e.g., Tides; flow). This paper proposes a novel approach to integrate multisource bathymetric datasets to study bedforms. The proposed approach is based on consolidating multisource datasets and applying the Empirical Bayesian Kriging interpolation for the creation of a multisource Digital Bathymetric Model (DBM). It has been designed to be adapted for estuarine areas with a high dynamism of the seafloor, characteristic of the fluvio-marine regime of the Estuary of the Saint-Lawrence River. This area is distinguished by a high tidal cycle and the presence of fields of dunes. The study involves MBES data that was acquired daily over a field of dunes in this area over the span of 4 days for the purpose of monitoring the morphology and migration of dunes. The proposed approach performs well with a resulting surface with a reduced error relative to the original data compared to existing approaches and the conservation of the dune shape through the integration of the data sets despite the highly dynamic fluvio-marine environments.

Cellular automaton modelling of the effects of buildings on aeolian bedform dynamics

• Sediment dynamics around buildings are included in a cellular automaton model. • Simulated deposition and erosion patterns show good agreement with field experiments. • Building effects interact with local bedform dynamics, and can affect dune shape, growth and migration. Buildings affect aeolian sediment transport and bedform development in sandy environments. Cellular automaton (CA) models have, however, only been used to simulate natural bedform dynamics. This study extends a well-known aeolian CA model to include sediment dynamics around buildings, and uses this model to explore the interaction of building-induced deposition and erosion with natural bedform dynamics. New CA rules are introduced to represent acceleration, deceleration and sideward transport of sediment around obstacles. The simulated deposition and erosion patterns show good agreement with field experiments. The model reproduces the shape and location of the morphological pattern around a single building, and effects of building spacing on this pattern for building groups. Model results further demonstrate that building-induced effects interact with local bedform dynamics and can alter the shape, growth and migration of sand dunes.

Mapping 2‐D Bedload Rates Throughout a Sand‐Bed River Reach From High‐Resolution Acoustical Surveys of Migrating Bedforms

AbstractThis paper introduces a method for determining spatially‐distributed, 2‐D bedload rates using repeat, high‐resolution surveys of the bed topography. As opposed to existing methods, bedform parameters and bedload rates are computed from bed elevation profiles interpolated along the local bedform velocities. The bedform velocity fields are computed applying Large‐Scale Particle Image Velocimetry, initially developed for surface velocity measurements, to pairs of successive Digital Elevation Models (DEMs). The bathymetry data are interpolated along the direction of each bedform velocity and the mean height of the closest bedform is computed. The dune shape factor is also evaluated along each bedform direction of travel. The local bedload fluxes can be computed by multiplying the bedform velocity by its mean height averaged over the successive two DEMs, and they can be time‐averaged over a series of DEM pairs. This method is applied to a high‐resolution acoustical survey of an approximately 300 m long by 40 m wide reach of the Colorado River in Grand Canyon upstream from Diamond Creek, USA. The repeat period was about 6–10 min and bed elevation was interpolated every 0.25 m. The obtained results provide insight to the spatial and temporal variability of bedload rates, bedform parameters and bedload fluxes through cross‐sections. The method can be applied to other repeated acoustical surveys of river reaches provided that the space and time resolutions are high enough to capture the local movement of bedforms.

Desert dunes transformed by end-of-century changes in wind climate

Sand dunes in arid regions are conspicuous mobile landforms that require adaptation and mitigation strategies to protect human infrastructure and economic assets from encroachment, and play a substantial role in desertification and atmospheric dust emissions. Here we show how the shape, migration speed and direction of mobile desert dunes globally are projected to change by 2100, in response to sand-moving wind regime shifts associated with climate change under the shared socio-economic pathway SSP5-8.5 (SSP, shared socio-economic pathway) scenario. We find transformations in dune dynamics for many sand seas and dune fields across the Sahara, The Horn of Africa, the Southern Arabian Peninsula, South Asia, China and Australia—as well as an increased potential for sand sea expansion and reactivation of dormant dune fields—linked to climate change alterations in the Hadley circulation, extra-tropical cyclone activity and monsoon systems. These projected changes will affect planning for and management of dune encroachment on transportation infrastructure, industry and urban development in desert regions.

Investigation of fluid flow and heat transfer in a narrow channel with micro barchan-dune-shaped humps

• A bionic heat sink with micro barchan-dune-shaped humps is proposed. • The micro dune can reinitialize the boundary layer by inducing a secondary flow. • The micro dune generally outperforms traditional fins in overall heat transfer performance. This paper proposes an innovative bionic heat sink with micro barchan-dune-shaped humps for improving the flow and heat transfer performance of the smooth narrow channel. The effects of the Reynolds number of the main flow ( Re ) and pitch distance of the dune array ( P ) on the thermal-hydraulic performance are investigated. In this work, the air is the working flow. The Reynolds number range is considered to cover the regimes of laminar, transitional, and turbulent, Re = 860–6,050, and the pitch distance ( P ) equals 8, 12, and 16 mm. The results show that the micro barchan-dune-shaped hump can induce a secondary flow downstream of the slip face and enhance the local heat transfer performance. For the multi-dune models, the case with P = 16 mm provides the highest overall thermal performance parameter ( η ). The η value decreases monotonically with decreasing P in the considered range. In addition, compared with other micro pin fin shapes (including the hexagonal cylinder, square cylinder, and triangular cylinder), the η of the barchan dune shape is almost identical to that of the hexagonal cylinder and exceeds the other two shapes by more than 20%.

Numerical Simulation Analysis of the Formation and Morphological Evolution of Asymmetric Crescentic Dunes

Generally, typical crescentic dunes in the ideal state are symmetrical, but it is difficult to form crescentic dunes with two perfectly symmetrical horns under actual conditions. Among many environmental factors, bidirectional winds, the size of sand particles, topography, epiphyte vegetation, and dune collision are important reasons for the asymmetric evolution of sand dunes. Few existing studies have revealed the mechanism of the morphological evolution of asymmetric crescentic dunes, especially in regard to the role of wind in a complex dune’s morphology. In this study, we used the Reynolds Averaged Navier-Stokes (RANS) and mass balance models to simulate the asymmetric forms and flow fields of crescentic dunes and analyzed the potential causes of the asymmetry among the above aspects. The results showed that: (1) the angle of the bidirectional winds significantly changed the structure of vortices around the sand dune; (2) for crescentic dunes with coarser sand, the deposit continuity was better, the extension of the single horn was maintained for a long time, and the extended horn took longer to die out; (3) the crescentic dune deformed according to the direction of the inclination of the terrain, and the shear stress of a dune on a slope was related to the slope, width, or height; (4) whether there was epiphytic vegetation on a dune’s surface had a great impact on the dune’s migration; (5) the collision position of two dunes determined the shape of the two dunes after fusion. The simulation results indicated that the spatial–temporal differences in sand flux, caused by changes in flow fields that were induced by various factors, determined the evolutionary shape of crescentic dunes. These results can provide a reference for the study of the erosion of surface flow fields on various dunes and for the prevention and control of wind and sand disasters in the Gobi Desert area.

Analysis of movement and relationships between morphometric components of sand dunes (barchans) in the south-eastern of Iran

Morphology of barchan dunes plays a key role in the rate of movement of barchan dunes and accordingly, the areas that are in the path of barchan dunes can be identified. In this study, morphological parameters of eight barchan dunes in west Lut and movement were investigated. For this purpose, 8 parameters of windward slope length, back-to-wind slope length, right arm length, left arm length, length, width, right width and left width were measured in each barchan dune and Pearson correlation was calculated by SPSS software. In order to better understand the shape of barchan dunes, satellite images were extracted separately from google earth. The results of morphometry showed that barchan dune 6 was in the first place in terms of all morphometric factors. The right arm length to the width had the highest correlation (0.993). The back-to-wind slope length to the right arm length had the lowest correlation (0.815). The right arm length to the width had the highest coefficient of determination (0.9845). The movement during 2005–2019 was extracted from satellite images. It was found that until 2015, the highest movement belonged to barchan dune 3 (225.55 m) and during 2017–2019, the lowest movement belonged to barchan dune 6 (137.49 and 184.66 m). The highest movement during 2017–2019 was 288.24 and 307.67 m for barchan dune 5, respectively.

Experimental Study on Proppant Transport within Complex Fractures

Summary The process of proppant filling within complex fractures is a key factor that determines the effects of volumetric fracturing operation in shale. To address the need of simulating proppant transport within the complex fractures, a large size visual simulation equipment of particle transport within complex fractures was built. The equipment considers roughness and fluid leakoff on the fracture wall and includes branched fractures of different angles. We ensured that the fluid flow and proppant flow were similar to those in the actual fracture and carried out experiments to obtain the influence of operation parameters and fracture parameters on proppant transport, also including injection of special proppants and channel fracturing. Then, we quantified proppant placement through standardized processing of dune shape pictures. The results show that under the influence of perforation, the near-wellbore dune is distributed in a slope shape, and an increase in the perforation density causes dune migration toward the wellbore and proppant accumulation to a certain height within the wellbore. The carrier fluid flow velocity change from 1.0 to 1.5 m/s within the fracture has the greatest effect on the dune equilibrium height (DEH). The dune height only varies slightly after the proppant concentration reaches 12%. Small size proppants have better transport capacity in the slickwater, and the large size proppants are likely to settle down. Combination injection of increasing proppant size improves the filling effect of complex fractures significantly. Pulse injection and stopping pump during fracturing operation have little effect on the transport of conventional proppants. Nevertheless, stopping pump leads to a significant increase in the dune height of the lower density proppants. An increase in the flow rate within the branched fracture leads to the transition in the dune shape from triangle to trapezoid and rectangle. The dune height within the major fracture determines those within the branched fracture, and the proppant volume flowing into the branched fracture determines the dune length. Proppant accumulation within the branched fracture enhances the difficulty in the diversion of carrier fluid to the branched fracture. The proppant tumble area increases within the inclined fracture, which is conductive to proppant transport. The proppants form the dune through bridging within the horizontal fracture, and the dune is less stable than that within the vertical fracture. A reduction in the proppant density and size leads to a significant increase in the proppant transport distance, and the convection effect by concentration difference of the ultralow-density (ULD) proppant is of great significance for propping of the microfractures. In channel fracturing, the proppant and fiber agglomerates are formed at the fracture point with the relatively large fluid leakoff and wall roughness, and the agglomerates grow laterally against the fracture fluid flow direction. A reasonable increase in the injection rate enhances the connectivity between the channels within the fracture. This study provides guidance for the optimization of volumetric fracturing parameters.

The encountering dune fields in a bidirectional flow system in the northwestern South China Sea: Pattern, morphology, and recent dynamics

Dune fields are composed of differently scaled of dunes and can reflect the external environmental conditions in both eolian and subaqueous environments. However, little is known about the interplay between dune fields when they meet together. Based on recent field observations, we identified new dune fields that encountered with other dunes migrating oppositely under a bidirectional flow system on the tidal shelf in the northwestern South China Sea. The dune fields, whose front boundary was outlined by the reversal of dune asymmetry, encountered each other end-by-end or side-by-side. Six encounter patterns were identified through the relative relation between dunes over the field boundary, which was associated with the net sand transport strength and sand supply. Under the control of the residual sand transport regime, dunes became steeper and more symmetrical towards the dune-field boundary and migrated gradually slower to the minimum. The regional net bedload transport determined the convergent migration of dunes in the encountering fields and shaped the linear, symmetrical, and steep shapes of dunes over the field boundary. The field boundary was shifting from 2014 to 2016, and most dunes close to the field frontier moved to their stoss side, thereby weakening the asymmetry extent or reversing the asymmetry. The field boundary shifts reflected location inconsistency between the long-term accumulative transport balance (symmetrical shape) and the recent transport balance (non-migration) on dunes near field boundary which was ascribed to the modulation of residual sand transports associated with the systematic changes in the regional flow conditions. These results suggest that the behavior of dunes near the field boundary can sensitively reflect the field-field interplay and reveal the relevant minor environmental changes, thus providing new insights into the surface processes in a bidirectional flow system of a planet with eolian or subaqueous environments.

Sediment Transport on a Sand Bed With Dunes: Deformation and Translation Fluxes

AbstractAs dunes move along the riverbed, they change in size, shape, and arrangement. This involves sediment fluxes on top of the net downstream motion of the dune field, but how much dune dynamics affect total sediment flux remains unclear. In this study, we obtain high‐resolution and high‐frequency digital elevation models of migrating submerged dunes in the laboratory. We use the measurements to identify three‐dimensional dune sediment fluxes and characterize dune interaction processes. We show how variations in dune migration lead to flux variations. Varying bed and water surface slopes and time‐varying sizes and shapes of dunes are imprinted in the observed flux variations, with a corresponding time lag. Feedback between bed morphology and water surface slope causes (some) dunes with high crests to lengthen into long dunes (of about twice the mean dune length), until a breakup of this dune into shorter dunes occurs, typically facilitated by merging of superposed bedforms until the primary dune splits. A large number of short dunes have high overall transport rates. Dune interaction causes patterns in the dune deformation flux. Dune splitting, defect creation and defect repulsion cause a peak in the deformation flux, whilst merging triggers a temporal increase of the total flux. Estimating the fluxes with spatially varying dune migration, and contributing fluxes to either deformation or translation processes continues to be a challenge, even for the detailed data set we have presented. Our analysis suggests that sediment transport variability over dune‐covered riverbeds is best described by considering the topography of multiple dunes.

Novel Learnings of Proppant Transport Behavior in Unconventional Hydraulic Fractures

Summary Effective proppant placement has been one of the key objectives of hydraulic fracturing. Different proppant and fracture fluid characteristics and placement methodologies have been historically applied based on learnings from standard proppant transport studies with parallel plate slots. The standard test setup represents a simplified planar fracture with constant width and confined height, incorporating only basic flow characteristics, and thus, is inadequate to capture unique phenomena of proppant transport in unconventional reservoirs. In this study, proppant transport laboratory tests were conducted on a large-scale (10×20 ft) tortuous slot flow system. This novel setup incorporates many significant unconventional fracture features, including lateral and vertical tortuosity, variable width, leakoff, fluid dynamics replicating upward fracture growth, and so on. Proppant transport behavior was investigated with multiple parameters such as proppant size, density, and concentration; fracture fluid type and viscosity; pumping sequence; pump rate; and fracture properties (width, leakoff location and rate, fracture tortuosity profile, and flow directions). The detailed parametric and integrated study of test results includes analysis of proppant dune evolution, dune shape, particle-size distribution across dune, propped area, fluid, and proppant collected from leakoff and exit ports. Multiple unique phenomena occurring at tortuous interfaces were observed, including the generation of isolated pockets of proppant pack, restriction of upward movement owing to proppant bridging, and creation of discontinuous and sparsely distributed proppant pillars above the dune. The test results demonstrated a larger proppant dune angle in front of the dune peak during injection and a subsequent falloff of proppant pack with a higher percentage of smaller mesh proppant backfilling the area at and near the inlet (analogous to the wellbore). Self-segregation of proppant in slickwater as per mesh size resulted in higher percentage of smaller mesh (larger size) proppant settled near the injection point, and a higher percentage of larger mesh (smaller size) proppant placed farther in the system. These observations and novel learnings highlight that it is critical to account for tortuous fracture pathway, leakoff effects, and flow directions (both lateral and upward) to better understand proppant transport behaviors in unconventional fractures. A partially proppant-filled fracture area is recognized in unconventional fracture in addition to general classification of propped and unpropped fracture area. Using proppant with large mesh size distribution range or pumping smaller mesh proppant first in slickwater helps achieve dual benefits of higher near-wellbore conductivity and improved far-field transport. This study demonstrates and physically verifies unique proppant transport behaviors in unconventional hydraulic fractures. It also provides novel learnings that will help the industry to optimize hydraulic fracture design through the selection of optimum proppant and fluid properties with enhanced pumping strategies for overall well productivity improvement in an unconventional reservoir.