Single Sand Research Articles

Fractal Analysis of Particle Size and Morphology in Single-Particle Breakage Based on 3D Images

The accurate modeling of single-particle breakage based on three-dimensional (3D) images is crucial for understanding the particle-level mechanics of granular materials. This study aims to propose a systematic framework incorporating single-particle breakage experiments and numerical simulations based on a novel 3D particle reconstruction technique for fractal analysis of particle size and morphology in single-particle breakage. First, the vision foundation model is used to generate accurate particles from 3D images. The numerical approach is validated by simulating the single-particle breakage test with multiple Fujian sand particles. Then, the breakage processes of reconstructed sand particles under axial compression are numerically modeled. The relationship between 3D fractal dimensions and particle size, particle crushing strength, and morphology is meticulously investigated. Furthermore, the implications of these relationships on the particle breakage processes are thoroughly discussed, shedding light on the underlying mechanisms that govern particle breakage. The framework offers an effective way to investigate the breakage behavior of single sand particles, which will enhance understanding of the mechanism of the whole particle breakage process.

Application effect of temporary plugging and diverting fracturing technology in well X1

Abstract The deployment of horizontal well development in the sweet spot area of the main sand body can improve the reservoir control degree and the production of a single well. The X1 well in the X block will be put into production after fracturing in 2022. The target layer of this well is a tight reservoir with low porosity and low permeability, and natural fractures are not developed. Combined with the geological characteristics of the target well, horizontal well soluble bridge plug and temporary plugging and diversion fracturing technology are applied to well X1, and branch fractures are formed through temporary plugging and diversion in the fracture, which increases the complexity of fractures and increases the drainage area. The well is currently in production for 485 days, with a peak oil day of 17.4t and a current oil day of 7t, with a cumulative oil production of 4184.6t. In this paper, the fracturing effect of well X1 is preliminatively analyzed, and it is concluded that under the same reservoir conditions, the single particle size sand combined with temporary plugging in the fracture transformation has a relatively good development effect, which can obviously improve the development effect of tight reservoir. The application effect of soluble bridge plug and temporary plugging diversion fracturing technology in this well has certain reference significance for improving the development effect and benefit of horizontal well in ultra-low permeability reservoir.

Study on reservoir heterogeneity of Chang 8 reservoir in Hongde Oilfield, western margin of Ordos Basin and knowledge of fine stratification development practice

Abstract In view of the problems of reservoir heterogeneity and unclear understanding of main control factors in Chang8 reservoir of Hongde Oilfield in the middle part of Tianhuan, Ordos Basin, efficient development and stable production are restricted by a large number of drilling, core data and log comparative analysis, using core observation, microscopic rock slice identification, scanning electron microscopy, mercury injection and phase infiltration experiments, etc. Based on the understanding of sedimentary facies and diagenesis, the quantitative analysis of Chang 8 in the study area is carried out from the macro intra-layer, interlayer, plane heterogeneity and microscopic heterogeneity. Finally, it is concluded that Chang 8 reservoir in Hongde Oilfield presents strong heterogeneity, and sedimentary microfacies, different clastic components of sandstone, diagenesis and other main factors affecting its strong heterogeneity. At the same time, in the development practice of De 5 area of Hongde Oilfield, the multi-stage sedimentary cycle division and step production and development were taken as a breakthrough, and the differential reservoir seepage theory was innovatively used to reduce the section/plane contradiction caused by reservoir heterogeneity, and a technical system such as regional fine stratified production, differentiated reservoir fracturing and reconstruction, single sand body fine injection and production control and stratified water injection was formed. The high efficiency development and utilization of strong heterogeneous reservoir is realized.

Development Characteristics of Single Sand Body of Yan’An Formation in Western Ordos Basin

Abstract The precise characterization of the types and spatial distribution of strongly heterogeneous single sand bodies are a necessary prerequisite for the study of oil and gas reservoir formation. This article takes the Yan’an Formation in the western Ordos Basin as an example to systematically study the internal structure and profile distribution of strongly heterogeneous single sand bodies. The research results indicate that using mud interlayers, physical interlayers, and calcium interlayers as layered interfaces can effectively classify the single sand body types of each small layer in the Yan’an Formation. The drilling rate of a single sand body is between 50% and 85%, and the average number of sand layers is between 3 and 8. When two rivers intersect and are horizontally tangent, due to the mutual erosion of water flow, relatively thick sand bodies can be retained, ultimately resulting in a pattern of relatively thick on both sides and thin in the middle. The single sand body at the bottom is the most developed and has good continuity. In the transverse direction of the river, the continuity of a single sand body is relatively poor, and the extension range of a single sand body is 300m to 1200m. Along the river channel, single sand bodies are developed with good continuity, extending about 2km. The connectivity between the sand body and the well varies with the migration of the river in the transverse river profile of the research area. The lateral contact relationship of a single sand body determines the boundary of the river channel, and the profile characteristics of a single sand body also reflect its planar distribution. The intergranular pores of the Yan’an Formation sand body are relatively developed, and a large number of quartz particles offset the compressive stress of the rock. The corrosion of feldspar particles is relatively strong, retaining a large number of primary intergranular pores. There is a good quantitative relationship between the thickness of sand in a single level channel and the width of the channel. The control of the well network on the sand body is relatively small, and there is room for further densification of drilling. The vertical contact relationship of a single sand body in the Yan’an Formation of the research area can be divided into three modes: separation, stacking, and overlapping. Cut pile sand bodies with good connectivity have good production capacity. The different planar contact relationships reflect the degree of connectivity between channel sand bodies and wells, with lateral cutting type>docking type>isolation type. The side cut sand body has uniform longitudinal water absorption, small water absorption difference, and high-water drive degree. The isolation type has the greatest difference in water absorption rate and the lowest degree of water flooding.

Research and Field Application on Extreme Subdivision Water Injection Technology for Low Permeability Reservoirs

Abstract Low permeability Block A is about to enter the development phase of extremely high water cut. Due to the low overall recovery degree and residual oil scattered sporadically, the efficient development meets more difficulty and conventional residual oil analysis and adjustment methods cannot fulfil the current needs of precise development. To enhance the reservoir utilization degree, we conduct extremely concise zonal water injection experiment from three aspects: sand body description, policies establishment, and proper technique application. In terms of sand body description, four types of sand-body distribution patterns were summarized through fine characterization of vertical, planar, and spatial single sand bodies, clarifying the origin and potential distribution of residual oil. In terms of development policy, numerical simulation and reservoir engineering methods have been comprehensively applied to determine the subdivision combination of injection layers and reasonable water allocation boundaries, and subdivision principles have been established, considering single septation to thin and thick layer with combining enhancing -controlling. In terms of supporting technology, we have studied the small space subdivision process of small compartments, optimized the testing and adjustment methods, and finally achieved accurate separation, strict clamping, and accurate matching of water injection layers. Through the research and practice, it realized the finest septation of the water injection to extremely enhance the subdivision degree and the efficiency of the injection. It also have strengthened the utilization of thin and poor layers and effectively controlled the decline.

Application of 3D Geological Modeling Technology for Single Sand Body in Block S

Abstract The P oil layer is generally in a medium to high water cut period, with scattered macroscopic residual oil distribution. The reserves are mainly distributed in the microfacies of river sand bodies, and the main layer is severely flooded with water, resulting in ineffective circulation in some areas. Therefore, it is necessary to tackle the modeling technology within the layer and describe the remaining oil within the layer to meet the needs of tapping the potential of remaining oil during the high water cut period. This article takes the establishment of a three-dimensional geological model of a single sand body within the S block layer as an example. Based on the establishment of a fine structural model, the “plane and vertical dual control” modeling method is used. The implementation method is: controlling the range of sedimentary facies maps → using the top surface of the sand body as the top depth of the river channel → controlling the vertical shape of the river channel through the facies controlled sandstone contour map, thereby achieving the vertical shape of the river channel through the facies controlled sandstone contour map, and establishing a three-dimensional sedimentary facies model of a single sand body. Optimize the method for characterizing interlayers, achieve modeling of single sand body interlayers, and perform random simulation interpolation in phases to obtain attribute models. The use of a single sand body model within a layer can visually display the three-dimensional channel morphology and classify the identified connected bodies within the study area. The upscaling method of sedimentary facies models with irregular boundaries greatly reduces the number of model grids and provides a relatively refined geological model for numerical simulation.

Sedimentary Reservoir Characteristics of Fuyu Oil Formation in the Middle of Fuxin Uplift, Southern Songliao Basin

Currently, the Fuyu reservoir in the middle of the Fuxin uplift, located in the southern Songliao Basin, is in a high water cut development stage. The understanding of this reservoir is limited, and the distribution of favorable reservoirs remains unclear, necessitating detailed reservoir characterization. This study employs a combination of well and seismic technologies to accurately depict the sedimentary and reservoir characteristics of the Fuyu oil reservoir. By conducting detailed correlations of oil zones, a regional high-resolution isochronous stratigraphic framework is established. The division of sedimentary microfacies enables fine characterization of single sand bodies, identifying channel boundaries, clarifying lateral connectivity, and detailing the vertical contact relationships of sand bodies across different deposition periods. The results demonstrate that root-mean-square amplitude and total energy metrics effectively correspond to sand bodies in the study area and can be used to analyze sand body distribution. Provenance is primarily from the southwest to the northeast, with shallow deltas predominantly developed. Delta plain subfacies are found in zones 4-12, while delta front subfacies are in zones 1-3. Microfacies types mainly include the main distributary channel, distributary channel sides, crevasse splay, overbank sand, and underwater distributary channels. The main distributary channel reservoirs exhibit the best physical properties, with porosity ranging from 20-25%, while sand bodies at the sides of distributary channels reach 15-20%. Predicted favorable areas are mainly located at the intersections of distributary channels and structural highs, where drilling is likely to yield high oil flow.

DEM modelling of surface indentations caused by granular materials: application to wheel–rail sanding

The presented surface indentation model is one step towards building a DEM model for wheel–rail sanding. In railways, so-called low-adhesion conditions can cause problems in traction and braking, and sanding is used to overcome this problem. Sand grains are blasted towards wheel–rail contact, fracture repeatedly as they enter the nip and are drawn into the contact and then increase adhesion. Research on this topic has mostly been experimental, but focussed on adhesion enhancement measurement. Thus, physical mechanisms increasing the adhesion are not well understood. Previous works involved experiments and DEM modelling of single sand grain crushing tests under realistic wheel–rail contact pressures of 900 MPa, focusing on sand fragment spread and formation of clusters of solidified fragments. In the experiments, indents in the compressing steel plates were also observed, which are also observed on wheel and rail surfaces in railway operation. These are now modelled by adapting an existing surface indentation model from literature to the case of surface indentations caused by granular materials. Two test cases are studied, and experimental spherical indentation tests for model parametrisation are presented. In a proof of concept, the mentioned single sand grain crushing tests under 900 MPa pressure are simulated including the surface indentation model. This work contributes to DEM modelling of wheel–rail sanding, which is believed to be a good approach to deepen the understanding of adhesion increasing mechanisms under sanded conditions.

Research on interface precision control strategy for additive manufacturing of coplanar sand laying with multi-material composite sand mold

Compared with the additive manufacturing of single sand material, it is more difficult to form multi-material composite sand mold. The paper focuses on the influence of sand laying process for the multi-material composite sand mold, explores the change rules in the quality of different curing agent content of composite sand mold, realizes the high quality and high precision sand laying of multi-material sand, and expands the application of composite sand mold in complex structure and high performance products.

DEM modelling of particle crushing of single carbonate sand using the improved bonded particle model

Carbonate sands have complex particle morphology and abundant intra-particle pores, influencing their mechanical beahviours. This paper presents a detailed modelling procedure for single-particle crushing of carbonate sands with different shapes. The particles were scanned, and their realistic profiles were simplified using a curvature-based simplification technique before being imported into the DEM program. An improved bonded particle model using coplanar Voronoi polyhedrons as cells was adopted, and the internal porosity of carbonate sands, which follows a Weibull distribution, was modelled by randomly removing internal cells. The effects of local damping and loading rate on crushing responses are discussed and their values are carefully determined. Four loading stages were observed during platen compression. Particles with different shapes experienced varying degrees of tension and shear failure in internal bonds and exhibited different probabilities of splitting, explosive, and chipping failures. The work presented lays the groundwork for subsequent simulations of carbonate assemblies.

Nanoscale Abrasive Wear of Polyethylene: A Novel Approach To Probe Nanoplastic Release at the Single Asperity Level.

There is a growing concern that nanoplastic pollution may pose planetary threats to human and ecosystem health. However, a quantitative and mechanistic understanding of nanoplastic release via nanoscale mechanical degradation of bulk plastics and its interplay with photoweathering remains elusive. We developed a lateral force microscope (LFM)-based nanoscratch method to investigate mechanisms of nanoscale abrasive wear of low-density polyethylene (LDPE) surfaces by a single sand particle (simulated by a 300 nm tip) under environmentally relevant load, sliding motion, and sand size. For virgin LDPE, we found plowing as the dominant wear mechanism (i.e., deformed material pushed around the perimeter of scratch). After UVA-weathering, the wear mechanism of LDPE distinctively shifted to cutting wear (i.e., deformed material detached and pushed to the end of scratch). The shift in the mechanism was quantitatively described by a new parameter, which can be incorporated into calculating the NP release rate. We determined a 10-fold higher wear rate due to UV weathering. We also observed an unexpected resistance to initiate wear for UV-aged LDPE, likely due to nanohardness increase induced by UV. For the first time, we report 0.4-4 × 10-3 μm3/μm sliding distance/μN applied load as an initial approximate nanoplastic release rate for LDPE. Our novel findings reveal nanoplastic release mechanisms in the environment, enabling physics-based prediction of the global environmental inventory of nanoplastics.

Strategies for hydrogen storage in a depleted sandstone reservoir from the San Joaquin Basin, California (USA) based on high-fidelity numerical simulations

Depleted hydrocarbon reservoirs are promising candidates for underground hydrogen (H2) storage (UHS) due to their known (and often high) storage capacity, well-identified geological conditions, reduced cushion gas requirements, and adaptable existing infrastructure. While these reservoirs have shown success in natural gas storage, their performance for pure H2 storage remains untested, necessitating further research. To address this knowledge gap, we conduct high-fidelity multiphase compositional reservoir simulations to evaluate the technical feasibility of UHS in a depleted hydrocarbon reservoir within the Temblor Formation at the North Belridge Field from the San Joaquin Basin, California, USA. This reservoir zone comprises five distinct sand layers separated by four shale intervals and is bounded above and below by thick shales (overburden and underburden, respectively). This research has two primary objectives: (a) to assess the feasibility of two distinct UHS strategies—one that involves storing H2 across all sand layers and another that stores an equal amount of H2 in the single thickest sand layer, and (b) to evaluate the effect of interbedded shale permeability on UHS performance. Our findings indicate that UHS in the Temblor Formation is technically viable, showing enhanced H2 withdrawal efficiency and produced H2 purity over successive cycles with minimal leakage risk. The two storage strategies significantly impact UHS performance in the interbedded sandstone sequence. Storing H2 across all sand layers results in higher H2 withdrawal efficiency, but lower produced H2 purity. In contrast, storing H2 in a single sand layer leads to lower withdrawal efficiency but consistently high H2 purity. In addition, H2 storage in a single sand layer results in higher reservoir pressure buildup, implying greater geomechanical risks. The impact of interbedded shale permeability on UHS performance is negligible across the range of uncertainty for shale permeability. This research not only lays the groundwork for potential UHS implementation in the abundant sandstones of the San Joaquin Basin of California, but also offers valuable insights for other depleted hydrocarbon reservoirs under consideration for UHS that share similar properties.

Dynamics of oil separation from sand particle moving in the water at the Reynolds number of 500

This work represents studies of bitumen separation from a single bitumen-coated sand particle moving in a stream of water at the particle Reynolds number of 500. The transient dynamics of separation processes are calculated numerically using a three-dimensional computational fluid dynamics based model coupled with the combination of level set and volume of fluid models, known as the coupled level set volume of fluid model. The sand particle diameter is 10−4 m. The bitumen layer thickness comprises 10% of the particle diameter. The simulation results show that 20% of bitumen is removed during the onset of the water flow around the particle. Further, a bitumen layer is transformed into a bitumen tail with 77% of the original bitumen volume.

METHODS FOR HIGHLY EFFICIENT DEVELOPMENT OF OFFSHORE RIVERINE OILFIELDS

The geological reservoir conditions of offshore fluvial oilfields in the Bohai region are characterized by their complexity, including thin reservoir layers, significant lateral variations, and poor connectivity. These conditions pose challenges in deploying production wells, finely describing reservoirs, and optimizing resource allocation. This study has developed several techniques to address these challenges effectively. These include fine description and quantitative characterization of reservoirs in areas with few wells, regional sedimentation analysis and identification of individual river reservoirs, optimization of well networks for single sand bodies, and a rolling technique for tapping potential in fluvial oilfields. The effectiveness of these techniques has been confirmed through their application in the development of 11 oilfields in Bohai, resulting in the addition of newly discovered reserves amounting to 5 billion tons during rolling development evaluations. The fine description technology has achieved a drilling accuracy rate of 98%, while the recognition technique for individual river reservoirs in fluvial environments has increased the recovery rate by an average of 3%. Moreover, the rolling development technique has enabled rapid resource transformation around the oilfields, facilitating the efficient development of oil and gas resources. The Bohai oilfield, situated in the Bohai Sea, is part of the offshore Bohai Bay Basin. After four decades of exploration and development, this field has produced significant oil reserves from the Neogene Minghuazhen and Guantao formations. In 2012, these reserves constituted 56.3% of the Bohai oilfield's total proven oil reserves, underscoring the critical role of efficient development strategies for these reservoirs in maintaining and enhancing oil production. These formations have undergone extensive fracturing due to recent tectonic activities, resulting in a highly broken structure and complex fault blocks. Post-Neogene, the Bohai Sea became a major drainage center for the Bohai Bay Basin, fostering not only traditional fluvial sedimentary systems but also shallow water delta systems, primarily around distributary channels or underwater channels. The fluvial oilfields discussed in this paper include reservoirs from both meandering and braided river systems, as well as parts of shallow water delta distributary channels. The reservoirs typically exhibit curved strip, ribbon, and dendritic shapes, and feature thin reservoir layers with substantial lateral variation. This complexity is further compounded by interactions between water and oil beds, leading to a distinct "one sand body, one oil reservoir" scenario. To effectively develop these fluvial oilfields, the research employs advanced techniques. These include detailed reservoir description and quantitative characterization in areas with sparse well distribution, study of regional sedimentary evolution and identification of individual river reservoirs, optimization of well networks for single sand bodies, and a rolling technique to tap potential in fluvial oilfields. These methods are essential for overcoming the geological challenges and optimizing oil extraction in such complex environments. The development of offshore fluvial oilfields encounters specific challenges, largely due to differences in the approach and conditions compared to onshore oil field development. For onshore fields in China, the development strategy typically evolves progressively, beginning with the establishment of a basic well pattern. This initial phase is followed by integrated geological and reservoir studies to determine optimal development and infill drilling patterns. In contrast, offshore oilfields are constrained by several factors that complicate their development. High operational costs limit the number of exploratory wells that can be drilled, resulting in sparse well patterns and consequently less data for making informed decisions. Additionally, the physical limitations of offshore platforms — such as available space, the lifespan of the platform, and finite well slot resources — prevent frequent adjustments to the drilling plan. To navigate these challenges and achieve efficient development, offshore oilfields require robust preliminary research. This involves a comprehensive and detailed geological description of the reservoirs, which forms the basis for strategic deployment of development wells. By adapting the well deployment strategy to the specific geological characteristics and constraints of offshore environments, developers can enhance the effectiveness and efficiency of oil extraction from these complex reservoirs. Keywords: geological reservoir condition, fluvial oilfields, geological challenges, seismic data, conventional method

Pullout behaviour of strip plate anchors in clay overlying sand

Plate anchors are being increasingly employed to establish mooring systems for offshore floating facilities owing to their notable efficiency and cost-effectiveness. The majority of prior studies have concentrated on plate anchors, examining the vertical to horizontal orientations, particularly when embedded in single layer clay or sand. However, it is common to encounter multiple layers of soil with varying characteristics in practical engineering, leading to inaccurate estimation of pullout capacity when based solely on the properties of a single soil layer. This paper reports pullout behaviours of strip plate anchors in clay overlying sand by adopting two dimensional finite element analysis. To simulate the hardening and subsequent softening behaviour of medium to dense sand, a modified Mohr-Coulomb model is considered. The current Finite Element (FE) results are in good agreement when compared with both previous FE simulations and physical model test data. Then, the soil flow mechanisms (e.g. the accumulated plastic shear strain, the friction angle) surrounding the anchor during pullout out are revealed. Based on a series of FE results, a calculation framework considering the effect of thickness of clay layer is designed to predict the pullout behaviour of plate anchors in clay overlying sand.

ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ПРОЦЕССА СЕДИМЕНТАЦИИ ЧАСТИЦ ПЕСКА В ТУРБУЛЕНТНОМ РЕЖИМЕ

Experimental studies of the process of sedimentation of single sand particles in turbulent mode have been carried out. A criterion equation was obtained for calculating the sedimentation rate of single sand particles

Effective reservoir identification and sweet spot prediction in Chang 8 Member tight oil reservoirs in Huanjiang area, Ordos Basin

Abstract The Chang 8 reservoir of the Huanjiang Oilfield in the Ordos Basin is a tight sandstone reservoir with poor reservoir physical properties and uneven oil distribution. In this study, the effective reservoirs developed on a large scale under the condition of horizontal well volume fracturing technology of the Chang 8 Member was identified based on the data of core observation, experimental analysis, logging, oil test, and production dynamics, and the identification standard of effective reservoirs in terms of reservoir physical properties, oil content, and comprehensive logging characteristics of gas logging was established. This scheme allows for a thorough identification of the effective reservoirs for horizontal well development in tight sandstone reservoirs. The findings indicate that the study area’s tight oil resources can be successfully produced. The well logging results show that the oil-bearing property of sand bodies with oil stains is better than that of oil spots. The bottom limit of the acoustic wave time difference is 210 μs/m, the permeability is 0.03 mD, the porosity is 6.0%, the rock resistivity is 30 Ω m, and the total hydrocarbon gas measurement value exceeds five times the baseline. At the same time, the total oil thickness is greater than 6 m, and the thickness of a single sand body is above 4 m. We have defined the lower limit standard of high-efficiency reservoir of Chang 8 tight oil in L289 block of Huanjiang oilfield. According to the analysis of oil-bearing property, comprehensive logging display, and reservoir thickness, the geological “sweet spots” is optimized to provide reference for subsequent mining. Through comparative analysis, the rules and trends are found to provide a basis for selecting mining strategies. With the help of technical means such as numerical simulation and geological modeling, the prediction accuracy and decision-making effect are improved. By clarifying the lower limit standard of reservoir, optimizing geological “sweet spots” and avoiding risk areas, the mining efficiency is improved and the cost is reduced, which provides reference for similar oilfield development.

Sedimentary characteristics, formation conditions, and distribution mechanisms of beach bar sand bodies of Chang 82 submember, southwest Ordos Basin

AbstractBeach bar sand bodies are a relatively new type of reservoir in oil exploration where they can be found in the proximity of the source rock. In this study, the genetic type of sand bodies in the Upper Triassic Yanchang Formation in the southwest Ordos Basin is systematically studied through the observation of outcrops, core description, microscopic inspection of thin sections, along with petrophysical properties, mineralogy, and geochemical analysis of trace elements. The results showed that a new sedimentary type of beach bar sand bodies are developed in the Chang 82 submember of the Yanchang Formation, that are generally characterized with a flat bottom and convex top, where the contact interface between the bottom of the single sand body and mudstone is flat, and the bottom lacks erosion and filling structure. Moreover, a variety of wave‐induced sedimentary structures are developed, showing a reverse sedimentary sequence from fine to coarse grains. The structural and compositional maturity of the beach bar sand bodies are higher than that of the river channel sand body. Moreover, reservoir pore type is mainly comprised of intergranular pores, and the porosity and permeability of the samples retrieved from the sand body are much higher than that of the river channel sand body. Finally, the main controlling factors of the formation and preservation of beach bar sands are found to be the lake basin bottom topography, the base level cycle, and the stable paleo‐lake shoreline. During the Chang 82 submember depositional period, a large area where the beach bar sand bodies were deposited parallel to the lake shoreline in the southwest of the basin was created. Such depositional environments had suitable conditions to host large‐scale oil‐bearing reservoirs, as major exploration and development targets in the Ordos Basin in the future.

Efficient chemical and microbial removal of iron and manganese in a rapid sand filter and impact of regular backwash

Aeration followed by rapid sand filtration is a common method in drinking water treatment to remove iron (Fe) and manganese (Mn) from anoxic groundwater. To ensure the successful removal of Fe and Mn within a single filter, several factors such as raw water characteristics, backwash procedures and chemical and microbial interactions with the filter medium need to be considered. Here, we assess the characteristics of a single medium rapid sand filter with highly efficient removal of Fe and Mn. Using synchrotron X-ray spectroscopy, we show that formation of ferrihydrite-type Fe oxides in the top of the filter (0–50 cm) accounts for >95 % of the removal of dissolved Fe2+ in the filter. Birnessite-type Mn- oxides, which are thought to be biogenic, form over a wider depth interval (0–110 cm). Results of 16S rRNA gene amplicon sequencing indicate a corresponding distinct vertical stratification of the microbial community, with potential iron-oxidizing Gallionella, Leptothrix and Sideroxydans dominating in the upper part of the filter, and nitrifiers being more prevalent deeper in the filter. Besides Fe and Mn-oxide, Fe-flocs and bacteriological hollow sheets form in the upper part of the filter. Both the Fe-flocs, hollow Fe-sheets and part of the Fe and Mn coatings are removed through backwashing, thereby reducing the pressure difference measured over the filter medium linked to clogging of pores (from 14 kPa to 1.5 kPa) and ensuring continued water flow. Backwashing removes part of the Gallionella, but this does not negatively impact the filter performance. Strikingly, SEM imaging with EDS mapping revealed alternating layers of Fe and Mn-oxides on the coated grains throughout the filter. This indicates slow mixing of the filter medium between the upper 30 cm and the rest of the filter during backwashing. Slow mixing likely contributes to continued success of the filter by ensuring homogeneous filter bed growth, while still allowing for stratification of the microbial community.

Fractures in faulted sandstone reservoir

The Chang 8 Member of the Yanchang Formation in southwest Ordos Basin is a typical faulted sandstone oil reservoir. The development characteristics of fractures have obvious controlling effect on the formation of sweet spots of tight reservoir. In this paper, taking the Chang 8 Member of the southwestern Ordos Basin as an example, the development characteristics and controlling factors of fractures in faulted sandstone oil reservoir are systematically studied. The results show that the faulted sandstone oil reservoirs in the study area are distributed along the main strike-slip faults. The fracture system includes vertical fractures and horizontal bedding fractures. The fracture surface of vertical fractures generally has good oil display, and mostly presented as oil spot and oil immersion level; however, the oil level of horizontal bedding fractures is usually presented as oil spot level. The development frequency of horizontal bedding fractures is 62.5 %, while that of vertical fractures is 37.5 %. The fractures are mainly developed in fine sandstone and a small amount of medium-grained sandstone and siltstone. The factors that affect the fracture development degree in faulted oil reservoir include the distance from main fault, sand thickness, lithology and structural location. For the Jinghe and Honghe Oilfields, the degree of fracture development decreases sharply when the distance from the main fault is greater than 1.25 km and 1.5 km, respectively. Single sand body thickness also controls the degree of fracture development. Single sand bodies with thickness within 6 m have more developed fractures, and the fracture development decreases sharply when the thickness exceeds 6 m. The sand body in the wing part of river channel is relatively thin with fine grain size and small compacted space, which is easy to break under tectonic activity. The fractures of the Chang 8 Member in the study area are mainly developed near the faults, the top of anticline and its wing part.