Bedding Surfaces Research Articles

Experimental study on flow heterogeneity of shale bedding fractures based on full-diameter cores

A self-designed full-diameter core experimental facility was used to evaluate the flow heterogeneity of bedding fractures at four radial directions under different closure pressures and injection rates, using full-diameter cores retaining original natural bedding fractures. The distribution morphology of bedding surface affects the conductivity of bedding fractures, and the flow capacity of bedding fractures in four radial directions varies greatly with the closure pressure and injection rate. The rougher the fracture surface, the greater the flow capacity varies with the closure pressure. For unsupported bedding fractures, the mean percentage error (MPE) of the conductivity in four radial directions increase gradually with the increase of the closure pressure. This phenomenon is especially prominent in deep rock samples. It is indicated that the flow heterogeneity of bedding fractures tends to increase with the closure pressure. When proppant is placed in the fracture, at a low closure pressure, due to the combined effects of self-support of rough fracture surface, proppant instability and uneven proppant placement, the flow heterogeneity is greater than that when proppant is not placed at the same closure pressure; however, with the increase of the closure pressure, the proppant becomes compact and stable, and the flow heterogeneity is mitigated gradually.

The Influence of the Main Factors on the Accuracy of Hydrological Modelling of Flooded Lands

This paper proposes a general methodological approach to hydrological modeling for determining the areas of flooded land in the plain part of the Dniester riverbed, the second largest river in Ukraine. The purpose of the study is the selection of parameters for modeling flooded zones in the plain section of the Dniester riverbed, taking into account the rising water level caused by the freshet that occurred in the western part of Ukraine on 22–24 June 2020. The proposed study examines the workflow of hydrological modeling of the flooded land zone and the main components of this scheme: the construction of the DTM, considering the roughness of the riparian territory by Manning’s coefficients, and indications of water rise. In the experiment, the influence of DTM reproduction resolution and Manning coefficients was analyzed, and their optimal values were selected, which allowed obtaining the parameters of hydrological modeling with a higher probability. The identified flooding areas were tested using a high-resolution space image during the flood in June 2020. The distance between the profiles affects not only the value of the modeling area but also their detail. The accuracy of the modeled flooded area is 5.1% for a 5 m interval between the profiles, 6.9% for 50 m, 8.2% for 100 m, and 10.8% for 200 m. These results allow determining the degree of influence of the distance between intervals on the modeling accuracy. Using different values of Manning’s coefficients for individual sections with different bedding surfaces and the selected spacing between profiles, which was 50 m, the accuracy of the modeling was investigated. After the modelling, the simulated flood areas were obtained in vector form, which allows for determining their areas and comparing them with the test flooded area. In the presented research, the RMSE of determining the flooded areas is about 5%. The test area of 600.6 hectares was determined with an accuracy of 0.8%.

Application of SVR models built with AOA and Chaos mapping for predicting tunnel crown displacement induced by blasting excavation

This study utilizes the support vector regression (SVR) model to predict the tunnel crown displacement (TCD) induced by the blasting excavation. 95 blasting operations considering the tunnel shape parameters, the blasting parameters, the explosive parameters, and the rock mass properties are used to train and test models. To enhance the model prediction performance, a novel combination of the arithmetic optimization algorithm (AOA) with the chaos mapping (CM) called CMAOA, is proposed to select the optimal hyperparameters of the SVR model with advantages of the computing speed and the optimization ability. Sixteen benchmark functions are adopted to verify the authenticity of the prediction performance improvement of the CMAOA, and six statistical indices are used to evaluate the performance of the proposed hybrid models for predicting the TCD. The evaluation results show that the SVR optimized by the AOA with Circle map (CirAOA-SVR) model has the best prediction accuracy among all models. The results of parameter analysis indicate that the degree of bonding of bedding surface is the most important input parameter for predicting the TCD, but the blasting and explosive parameters with fine importance scores cannot be ignored either.

Experimental investigation on the sudden cooling effect of oil-based drilling fluid on the dynamic compressive behavior of deep shale reservoirs

Oil-based drilling fluids are widely used in deep drilling, and their cooling effect on shale reservoirs is critical for wellbore stability. To reveal the sudden cooling effect of oil-based drilling fluid on the dynamic compressive behavior of deep shale reservoirs, oil-based fluid cooling experiments were conducted on heated Longmaxi shale samples (25–200 °C) with five bedding orientations, followed by a series of dynamic compression tests using same incident energy. Anisotropic macroscopic thermal fractures occurred in the samples at 200 °C after oil-based fluid cooling, and the imbibed oil-based fluid entirely covered the bedding surfaces during cooling. The post-peak behavior of the dynamic stress-strain relationships of samples illustrated bedding- and temperature-dependent characteristics. Dynamic strength of the samples decreased with increasing temperature for each bedding orientation, and the dynamic strength versus bedding orientation showed a V-shaped curve at all temperatures. Anisotropic fragment degree of the samples was classified into three groups using debris screening analysis. Further, the dynamic strength reduction responses and the possible nature of dynamic strength anisotropy in shale reservoirs after oil-based drilling fluid cooling were discussed. This study helps to improve the oil-based drilling fluid application in shale reservoirs and understand the behavior of deep reservoirs during drilling.

Effects of Production–Living–Ecological Space Patterns Changes on Land Surface Temperature

Rapid economic and social development has triggered competition for limited land space from different industries, accelerating the evolution of Beijing’s urban landscape types. The increase in impermeable surfaces and the decrease in ecological land have led to an increase in the impact on the urban thermal environment. Since previous studies have mainly focused on the impact of a single urban landscape on the urban thermal environment and lacked an exploration of the combined impact of multiple landscapes, this study applied standard deviation ellipses, Pearson correlation analysis, land surface temperature (LST) profile analysis, and hot spot analysis to comprehensively explore the influence of the evolving production–living–ecological space (PLES) pattern on LST. The results show that the average LST of various spaces continued to increase before 2009 and decreased slowly after 2009, with the highest average temperature being living space, followed by production space, and the lowest average temperature being ecological space for each year. The spatiotemporal shift path of the thermal environment is consistent with the shift trajectory of the living space center of gravity in Beijing; LST is positively correlated with living space (LS) and negatively correlated with production space (PS) and ecological space (ES). LST is positively correlated with LS and negatively correlated with PS and ES. Influenced by the change in bedding surface type, the longitudinal thermal profile curve of LST shows a general trend of “low at both ends and high in the middle”. With the change in land space type, LST fluctuates significantly, and the horizontal thermal profile curve shows a general trend of “first decreasing, followed by increasing and finally decreasing”. In addition, the hot spot analysis shows that the coverage area of very hot spots, hot spots, and warm spots increased by 0.72%, 1.13%, and 2.03%, respectively, in the past 30 years, and the main expansion direction is southeast, and very cold spots and cold spots are distributed in the northwest ecological space, and the area change first decreases and then increases.

The role of organic carbon in the Southern Uplands-Down-Longford Terrane accretionary prism, Scotland and Ireland

Carbonaceous shales in the Southern Uplands-Down-Longford Terrane accretionary prism had extremely high potential for hydrocarbon generation in the Lower Paleozoic. Structural thickening in the prism enhanced the rapid generation of oil. Shale horizons are separated by thick turbidites composed of low-permeability greywackes, so oil under high fluid pressure either pooled along shale bedding surfaces or migrated into fractured greywackes. Pooled oil became solidified to bitumen, which locally formed deposits on a scale of tonnes, mined as coal. The carbon-rich shale also sequestered large amounts of sulfur from seawater, which precipitated as pyrite firstly during early diagenesis, then further during fluid flow through the shale beds. The oil was also sulfur-bearing. Deformation focused on the shale beds during the evolution of the accretionary prism would have been closely related to the fluid flow which precipitated bitumen and sulfides. The palaeo-fluids were also anomalously rich in methane and hydrogen, similar to fluids venting from modern accretionary prisms. Supplementary material: details of localities searched for coal in Lower Palaeozoic, Southern Uplands-Down-Longford Terrane (locations shown in Fig. 6 ) are available at https://doi.org/10.6084/m9.figshare.c.6691597

Ammonia Emissions from Cattle Manure under Variable Moisture Exchange between the Manure and the Environment

When reducing ammonia emissions from cowsheds, it is recommended to reduce the ventilation intensity, air temperature in the barn, manure moisture by using bedding and manure-contaminated surfaces, and to prevent urine from accumulating in the airways. Using the mass flow method in the wind tunnel, after research on seven types of cattle manure with different moisture contents, it was found that ammonia evaporates up to 3.9 times more intensively from liquid manure than from solid manure. There is a strong correlation between ammonia and water evaporation from manure. Ammonia emission from liquid manure decrease by 2.0–2.3 times, emissions from solid manure decrease by 1.9–2.1 times. Different cowsheds have different opportunities to reduce air pollution and conditions for manure to dry and crusts to form on the surface. The best results will be achieved by applying complex measures to reduce air pollution.

Experimental Study on the Mechanical Properties and Acoustic Emission Characteristics of Different Bedding High-Rank Coals.

Bedding has an important influence on the macroscopic and microscopic mechanical properties of coal, and the mechanical properties of coal and rock mass and acoustic emission characteristics are very important for rock burst monitoring and warning. In order to explore the influence of different beddings on the mechanical properties and acoustic emission characteristics of high-rank coal, using the RMT-150B electrohydraulic servo rock mechanics test system and the DS5 acoustic emission analyzer, the uniaxial compression and acoustic emission characteristics of high-rank coals with different beddings (parallel bedding 0°, oblique bedding 30, 45, 60°, and vertical bedding 90°) were investigated. The results show that the uniaxial compressive strength and deformation modulus of vertical stratified coal samples are the largest, which are 28.924 MPa and 2.95 GPa, while the average values of uniaxial compressive strength and deformation modulus of oblique stratified coal samples are the smallest, which are 10.91 MPa and 1.776 GPa, respectively. With the increase of the bedding angle, the uniaxial compressive strength of high-rank coal decreases first and then increases. The stress-strain process of coal varies greatly with different high stratification grades (parallel bedding 0°, oblique bedding 30°, 45°, 60° and vertical bedding 90°). The loading times of parallel, oblique, and vertical beddings are 700, 450, 370, 550, and 600 s, and the acoustic emission mutation point values are 495, 449, 350, 300, and 410 s. The mutation point value can be used as precursor information to judge the failure of high-rank coal in different beddings. Research results to find the high-rank coal destruction instability prediction method and the index provide a basis; further damage to the acoustic emission testing high order coal provides reference significance; and the use of acoustic emission in the monitoring and early warning of percussive ground pressure, the bedding surface of coal, and the actual stress on site should be considered.

Robotic-biological systems for detection and identification of explosive ordnance: concept, general structure, and models

The subject of this study is systems for detection and identification (D&I) of explosive ordnance (EO). The aim of this study is to develop a concept, general structure, and models of a robotic-biological system for D&I of EO (RBS-D&I). The objectives are as follows: 1) to classify mobile systems for D&I of EO and suggest a concept of RBS-D&I; 2) to develop the general structure of RBS-D&I consisting of robotic (flying and ground) and biological subsystems; 3) to develop models of RBS-D&I including automaton, hierarchical, and operational ones; 4) to describe tasks and planned results of the article-related scientific project; and 5) to discuss research results. The following results were obtained. 1) The general structure of the RBS-D&I. The structure comprises the following levels: control and processing centres (mobile ground control and processing centre (MGCPC) and virtual control and processing centre); forces for detection and identification (fleet of unmanned aerial vehicles (FoU), biological detection information subsystem (BDIS), and robotic detection information subsystem (RDIS)); interference; natural covers and a bedding surface; and target objects (all munitions containing explosives, nuclear fission or fusion materials and biological and chemical agents). 2) A concept of RBS-D&I. The concept is based on RBS-D&I description, analysis, development, and operation as an integrated complex cyber-physical and cyber-biological system running in changing physical and information environments. 3) The RBS-D&I automata model. The model describes RBS-D&I operating in two modes. In mode 1, FoU and BDIS operate separately and interact through the MGCPC only. In mode 2, depending on the specifics of the tasks performed, FoU and RDIS can directly interact among themselves or through the MGCPC. 4) hierarchical model. The model has two sets of vertices: EO detection and platforms equipped with the necessary sensors. 5) An operational cycle model. The model describes land release operations via a methodology of functional modeling and graphic description of IDEF0 processes. Conclusions. The proposed concept and RBS-D&I solutions can provide high-performance and guaranteed EO detection in designated areas by the implementation of an intelligent platform and tools for planning the use of multifunctional fleets of UAVs and other RBS-D&I subsystems.

Synsedimentary slump folding: Examples and consequences of an under-recognized process in epicratonic basins

Small-scale synsedimentary folds in the Eagle Ford Formation in southwest Texas formed during deposition in a shallow epicratonic basin on the North American craton. These folds formed by slumping (i.e., gravity-driven deformation) and are characterized by (i) plastic deformation of limestone beds, (ii) discontinuous bedding and thickness changes of limestone beds, (iii) horizontal compactional fabric at high angle to folded bedding, (iv) low-angle or recumbent axial surfaces, and (v) erosion of slumped intervals and/or onlapping by resumed sedimentation. Exposures of the Eagle Ford near the northwest margin of the Maverick Basin reveal slump folds in 1–2 m thick zones with observed lateral extents of 10's to 100's of meters to >1 km – actual extents are likely much larger. Curvilinear fold hinges changing trend by >90° over short distances, indicating markedly non-cylindrical fold geometries. Bed thickness changes reflect flowage of unlithified sediment during slumping, and subsequent compaction. Previous descriptions of slump folds in the Eagle Ford Formation have focused on examples from the hydrodynamic facies of the basal portion, but slump folds are also present in pelagic facies of the upper Eagle Ford. Slump folding locally represents significant shortening (e.g., 50%) and thickening (e.g., 100%) of slumped intervals, with up-dip absence of the same interval represented by missing section, bed terminations, boudinage, and/or extensional faulting. Repeated slumping can result in significant cumulative updip thinning and downdip thickening along depositional slopes. Our experience working unconventional reservoirs in a variety of North American epicratonic basins indicates that slump folding is common and underrepresented in the literature. Criteria for recognizing slumping in the subsurface include a combination of discontinuous and contorted beds, thickening into fold hinges, compactional fabric postdating folds, and inverted or repeated section. Natural fracture networks may be less regular than in similar but flat-lying beds. Occurrence of slump folded intervals adds complexity and unpredictability to unconventional reservoirs, with implications for reservoir quality, drilling, and optimal stimulation during completion.

Stinging News: ‘Dickinsonia’ discovered in the Upper Vindhyan of India not worth the buzz

A recent report of Dickinsonia tenuis ‘hiding in plain sight’ at the Bhimbetka rock shelters in rocks of the Maihar sandstone (Upper Vindhyan) has important implications for paleogeography and the age of the Upper Vindhyan. We visited the site in December 2022 and found the evidence for Dickisonia lacking. The ‘fossil’ resembles decayed parts of modern Apis dorsata (giant honeybees) hives. In this contribution, we note the structural similarities between “Dickinsonia” and honey and pollen stores of recently decayed bee nests. A closer view of the photos provided in the original paper reveals honeycombed structures within the purported fossil. We also note that the fossil is not located on a bedding surface and is not a part of the rock, but rather is attached as a ‘tracery of waxy material’ above the surface. The remaining paleogeographic conclusions of that paper are also negated by this new discovery.

Pore network characteristics as a function of diagenesis: Implications for epigenic karstification in shallow-water carbonates

Karst processes may be critical for developing secondary porosity and permeability within carbonate reservoirs and aquifers. Karstification can significantly influence reservoir storage capacity and subsurface fluid flow. This study investigates the interplay between fluid-flow pathways and diagenesis in an epigenic karst setting in Cretaceous shallow-water carbonate rocks, Potiguar Basin, Brazil. The results indicate that dissolution occurred at all diagenetic stages, but the last stage is the most important because it is related to the most recent subaerial exposure. Dissolution produced during the last karst stage occurred in the vadose and phreatic environments, leading to the precipitation of isopachous, blocky and syntaxial cements. Dissolution affected the original pore network within the matrix and localized on preexisting discontinuities such as stylolites, fractures, and bedding surfaces. The resulting karst cavities are classified as vugs, channels and caverns, varying from a few millimeters to hundreds of meters in size. In epigenic karst systems, high porosity percentages are commonly due to moldic, intragrain/intrafossil and intercrystal porosities. The results of our study show how such porosities can be connected to fractures, veins and stylolites at the microscale. Karst dissolution can further increase porosity and the overall rock permeability due to aragonite and calcite cement dissolution during eodiagenesis, microfractures and stylolites dissolution during mesodiagenesis and all cements, fractures and stylolites during telodiagenesis.

Steinkern spiders: A microbial mat-controlled taphonomic pathway in the Oligocene Aix-en-Provence Lagerstätte, France

The Aix-en-Provence Formation is an Oligocene (22.5 Ma) Lagerstätte in southern France that contains an abundance of soft-bodied fossils preserved in exceptional detail. Many taxa have been described from this formation, including insects, spiders, fishes, and plants, suggesting a diverse ecosystem in a subtropical, brackish, lacustrine paleoenvironment. Fossil spiders from this deposit are preserved as compression fossils and internal and external molds. Recently, compression fossils of spiders from Aix-en-Provence were hypothesized to be a product of a taphonomic pathway based on diatoms and sulfurization. Here, we examine fossil spiders preserved as molds to uncover a second taphonomic pathway based on microbial mats. Evidence of microbial mats include wrinkles, pustular textures, and possible microbial mat chips on the bedding surfaces as well as a matrix fabric that contains possible microbial sheaths and bacterial spherules. The evidence presented here supports prolific microbial mat communities during deposition of the Aix-en-Provence Formation, and suggests that they are likely responsible for the moldic preservation of the spiders. Our work shows that the paleoenvironment of the Aix-en-Provence Formation promoted at least two possible taphonomic pathways that resulted in the differing modes of preservation observed.

Anisotropy and microcrack-induced failure precursor of shales under dynamic splitting

The dynamic anisotropy and failure mechanism of shales are greatly affected by bedding surfaces. To reveal the influence of beddings on anisotropic characteristics of shales under dynamic impact, the Brazilian splitting tests were conducted by the split Hopkinson pressure bar system. The fracturing process were monitored by the high-speed camera. Meanwhile, to understand crack initiation and propagation mechanism, the stress buildup, stress shadow and stress transfer were modelled based on the digital image processing and the rock failure process analysis method. The effect of dip angle and bedding spacing on crack initiation, propagation and coalescence was analyzed. Simultaneously, the spatial distribution and energy magnitude of crack-induced acoustic emissions were captured numerically. The results show that the shale discs continue to produce parallel cracks and cambered cracks induced by the high stresses at the tips of initial cracks; the tensile strength under dynamic splitting changes in the U-shaped trend with the bedding dip angle increasing; the cracking percentage of bedding surfaces decreases, and the cracking percentage of rock matrix increases with the bedding dip angle increasing. In addition, the acceleration of crack growth and the rapid growth of AE energy can be regarded as the effective precursors of shale failure.

The effects of planar structures on reservoir quality of Triassic Kangan formation in the central Persian Gulf, an integrated approach

Planar structures including stylolites, solution seams, dense streaks, bedding and cross-bedding surfaces, and fractures of carbonate Early Triassic Kangan Formation were evaluated in the central Persian Gulf. Core X-ray computed tomography (CT), Fullbore Formation Micro-Imager (FMI) log, and cores were used to detect the structures. Furthermore, conventional borehole logging data, routine core analysis, and petrographic thin sections were employed to assess the reservoir properties. Stereographic projections demonstrated that the reservoir has two sets of planar structures. The first group contains shallowly-dipping structures, consisting of stylolites, solution seams, dense streaks, bedding and cross-bedding surfaces. These structures have been created by sedimentary and diagenetic processes. The other set includes deformation-related structures that are steeply-dipping and occurred scarcely, like fractures. Spearman correlation coefficients between the frequency of each planar structure and reservoir characteristics (porosity and permeability) were calculated to reveal the numerical relationship between the parameters. The significant correlation coefficients indicate that stylolites, solution seams, and filled tension gashes diminish reservoir quality while bedding and cross-bedding surfaces have enhanced reservoir characteristics. The strongest correlation was observed as a strong negative relationship between the frequency of stylolites and permeability in K2 reservoir unit. This study implies that sedimentary and diagenetic factors are more effective in controlling the reservoir quality than deformation factors in the studied formation. The present work also indicates that each used method has unique advantages and disadvantages. Therefore, simultaneous utilization of the methods to investigate planar structures would have the best efficiency.

Invertebrate trace fossils from Paleogene fluvial strata in Western Washington, USA

Fluvial strata in the Paleogene Chuckanut Formation contain a wealth of trace fossils that include both vertebrate and invertebrate ichnofossils. Vertebrate traces have received careful scrutiny, but invertebrate traces have not previously been described. Trace fossils occur in two sediment types. Mudstones deposited in subaqueous environments include highly bioturbated layers that are dominated by horizontal meniscate burrows. Siltstone and fine-grained sandstone beds primarily contain vermiform trails on bedding surfaces, with sparse vertical traces. Some of these sandy beds also contain vertebrate tracks. Chuckanut Formation beds commonly show rapid facies changes in sediments that were deposited along a meandering river during a time of semitropical climate. Transitions from subaqueous to subaerial conditions may have been related to changing water levels caused by variations in seasonal precipitation or episodic flood events. Other possible factors include ponds created by abandonment of channels, overbank sediment s deposited by floods, and breaching of levees to form crevasse splay deposits. Trace fossil assemblages show local variations in the diverse depositional environments of the Chuckanut Formation. Many of the ichnofossil suites clearly represent Scoyenia Ichnofacies, but some occurrences may involve overprinting of Scoyenia and Mermia ichnofacies.

Petrographic and Kinematic Analysis of the Itombwe Synclinorium Formations Exposed at Tshondo and Bugoy (South-Kivu region, Democratic Republic of the Congo)

The purpose of this study is to investigate the Itombwe synclinorium formations that were affected by the late Pan-African orogenesis in the nordeastern Congo. A variety of controversies surround Tshondo and Bugoy's tectonic evolution and associated metasedimentary formations. A field-based approach combined with paleostress inversion techniques and petrographic analyses were conducted on 190 structural measurements and 6 rock samples used in this study. The results reveal the presence of four major petrographic facies: conglomeratic facies (conglomerate and diamictite), carbonate facies (travertine), greenschist facies (graphitic black shale and pelite), as well as quatzitic facies (quartzite and sandstone). The mineral assemblages consisting of high contents (>70%) of muscovite/sericite and biotite albite, plagioclase, quartz, and some opaque minerals. With the assistance of Win-Tensor software, the kinematic analysis reveals two major deformation phases, (1) a ductile deformation phase (D1-2), which is associated with isoclinal folds and strike-slip faults, and (2) a submeridian brittle deformation phase (D2), which generated extensive faults trending NNW-SSE to NE-SW directions, while reactivating bedding surfaces (NE-SW) in a series of secondary faults. The findings of this research may assist geologists in conducting core logging operations and provide a baseline for understanding the relationship between rock, minerlization, and tectonics in mineral-rich areas.

Study on the influence of mechanical characteristics of multi-rhythm inter-salt shale oil on fracture propagation in Qianjiang formation, China

There are many inter-salt rhythmic shale reservoirs in Qianjiang sag, and the mineral composition content with different rhythms is different. The thin interbedding characteristics of inter-salt shale oil reservoirs bring technical challenges to hydraulic fracturing. Taking one shale oil well in Qianjiang depression as an example, the mechanical properties and interface characteristics of rock under temperature and confining pressure are analyzed. The physical simulation test of fracture propagation under different fracturing fluid is completed, and the effects of four different factors on fracture propagation are analyzed by numerical analysis method. The results show that the mechanical characteristic and failure modes with different rhythms are obvious differences. Under uniaxial and triaxial compression, glauberite mudstone and shale have high strength, and salt rock shows obvious plastic deformation characteristics. The interbedded rock has clear interface characteristics. The cohesion of glauberite mudstone and shale bedding surface obtained from direct shear test is 0.60 MPa and 0.99 MPa. The fracture morphology of inter-salt shale is mainly affected by the development degree of rock bedding. The mechanical parameters, in situ stress difference, and the displacement have an important impact on the longitudinal propagation of fracturing fractures. The width and height of fracture propagation decrease, with the increase in the minimum horizontal principal stress in the salt layer, and the width of fracture in shale increases. The crack height decreases with the increase in tensile strength of the interlayer. With the increase in fracturing fluid injection rate from 3.0 to 7.0 ml/min, the propagation height of hydraulic fractures and the width of fractures in shale increase significantly. The research results can apply to understanding the mechanism of hydraulic fracture propagation in inter-salt shale formation.

The Effect of Bedding Plane Angle on Hydraulic Fracture Propagation in Mineral Heterogeneity Model

The bedding planes of unconventional oil and gas reservoirs are relatively well developed. Bedding planes directly interfere with hydraulic fracture expansion. Determining how bedding planes influence hydraulic fractures is key for understanding the formation and evolution of hydraulic fracturing networks. After conducting X-ray diffraction analysis of shale, we used Python programming to establish a numerical model of mineral heterogeneity with a 0-thickness cohesive element and a bedding plane that was globally embedded. The influence of the bedding-plane angle on hydraulic fracture propagation was studied. Acoustic emission (AE) data were simulated using MATLAB programming to study fracture propagation in detail. The numerical simulation and AE data showed that the propagation paths of hydraulic fractures were determined by the maximum principal stress and bedding plane. Clearer bedding effects were observed with smaller angles between the bedding surface and the maximum principal stress. However, the bedding effect led to continuous bedding slip fractures, which is not conducive to forming a complex fracture network. At moderate bedding plane angles, cross-layer and bedding fractures alternately appeared, characteristic of intermittent dislocation fracture and a complex fracture network. During hydraulic fracturing, tensile fractures represented the dominant fracture type and manifested in cross-layer fractures. We observed large fracture widths, which are conducive to proppant migration and filling. However, the shear fractures mostly manifested as bedding slip fractures with small fracture widths. Combining the fracture-network, AE, and fractal dimension data showed that a complex fracture network was most readily generated when the angle between the bedding plane and the maximum principal stress was 30°. The numerical simulation results provide important technical information for fracturing construction, which should support the efficient extraction of unconventional tight oil and gas.

Study on the failure mechanism in shale-sand formation based on hybrid finite-discrete element method

During the hydraulic fracturing in shale formation, the mechanical properties and stress field caused by sand and shale interbedding were the major factors affecting the hydraulic fracture propagation. In this paper, a hydraulic fracturing simulation model was proposed based on the hybrid finite-discrete element method, and the factors affecting the fracture propagation were analyzed. The simulation results indicated that the fracture width experienced a sudden decrease when the hydraulic fracture interacted with a bedding plane. The reservoir with high stress difference and Young’s modulus was favorable for the opening of bedding plane, and high injection rate was favorable for the longitudinal expansion of hydraulic fracture. This was mainly because the lower stress difference and Young’s modulus was, the opening degree of the bedding plane was much smaller, and the less the energy consumption of opening the bedding surface was.