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ABSTRACT Carbonate cementation in the Upper Permian sandstones informs the timing and temperature of hydrothermal activity in the south‐western Ordos Basin. This study presents a detailed examination of these hydrothermally influenced carbonate cements, constraining their age, carbonate diagenesis and relationship with regional geodynamic evolution. Sedimentological analyses demonstrate the development of deltaic plain and front sand bodies in the study area, which resulted in interbeds of volcanic matrix‐rich sandstones with matrix‐free sandstones. Petrography and electron microprobe analysis reveal four carbonate mineral growth phases of matrix‐free sandstones in the following sequence; scarce pure siderite, scarce Mg‐rich siderite, abundant blocky calcite and moderately abundant grain‐replacing calcite. The fluid inclusion data show anomalies of homogenization temperature of blocky carbonate cements during early diagenesis, over a wide range of ca 148 to 228°C. In addition, blocky carbonate cements show low δ13C (−5.9 to −13.1‰ Vienna PeeDee Belemnite) and δ18O (clustered tightly from −12.4 to −14.6‰ Vienna PeeDee Belemnite) values, interpreted to result from elevated temperatures during cementation, associated with activation of basement faults and concomitant hydrothermal fluid intrusion triggered by oceanic crust subduction in the south‐west margin of the Ordos Basin. Using in situ calcite U–Pb geochronology, the timing of hydrothermal activity was constrained to ca 247.0 ± 11 to 248.2 ± 4.7 Ma. This work provides a case study for applying intergranular calcite U–Pb dating to determine the absolute timing of fluid flow in sedimentary basins, offering tremendous potential to capture snapshots of various diagenetic evolution stages in sediments. The proposed diagenetic model can also provide new insights and understanding regarding hydrothermally influenced sediments. More importantly, hydrothermal activity may have commenced earlier than previously thought. The North Qinling Orogen uplift and associated Mianlue oceanic crust subduction may have begun at the Permian–Triassic transition with a protracted hydrothermal event in the south‐western Ordos Basin. [ABSTRACT FROM AUTHOR]

Tunnel stability is influenced by the rheological properties of the surrounding rock. This study, based on the Ganshen high-speed railway tunnel project, examines the rheological characteristics of siltstone and sandstone through laboratory tests and theoretical analysis. Rheological curves and parameters are derived, revealing the time-dependent deformation mechanisms of the surrounding rocks. A numerical simulation model is created using these parameters to analyze deformation and stress characteristics based on different rock levels and inverted arch closure distances. Results indicate that sandstone follows the Cvisc model, with the Maxwell elastic modulus increasing under higher loads while the viscous coefficient decreases. The vault displacement is mainly affected by the surrounding rock strength; lower strength leads to greater displacement, which also increases with the closure distance of the inverted arch. These findings are crucial for determining the optimal closure distance of inverted arches in sandstone conditions. [ABSTRACT FROM AUTHOR]

Undercutting anchors are structural elements used in construction and geotechnics to stabilize both structures and soils. Their main applications include stabilizing slopes and embankments, reinforcing foundations, and providing support during tunnel construction and other underground works. The authors propose the use of these anchors in rock mass detachment technology. This article presents a comprehensive analysis of the mechanism behind rock mass detachment using an undercutting anchor. Particular attention is given to the influence of parameters such as the fracture energy of the medium and the coefficient of friction between the medium and the anchor head on the detachment process of rock elements during anchor expansion in the drilled hole. Numerical FEM analysis was employed to model the effect of changes in the shape and size of failure cones under varying simulation conditions. The discussed problem is crucial for evaluating the effectiveness of this anchor design under non-standard conditions, particularly in the unconventional destruction of rock media. [ABSTRACT FROM AUTHOR]

ABSTRACT Despite a growing interest, few studies integrate quantitative supercritical‐flow deposit (SFD) distribution into paleoenvironmental reconstructions in deep‐marine settings. We present a re‐evaluation of the Oligocene Annot Sandstones (SE France) in terms of flow criticality, by examining the spatial distribution of SFDs. Proximal settings exhibit SFD proportion varying from ~90% to 40% with a common occurrence of cyclic steps, whereas distal environments show <40% SFDs with antidunes and upper‐plane beds prevalent. Comparing SFD spatial distribution to other proxies in the Annot Sandstones and published proxies from the Eocene Ainsa‐Jaca basins (Spanish Pyrenees), SFD trends appear more consistent than those in sandstone percentages, or the distribution of the Bouma sequence divisions. Our approach achieves two goals: (1) establishing the reliability of quantitatively analysing SFDs as a proxy for paleoenvironment reconstructions, characterising the proximal‐to‐basin floor transition at ~40% SFDs and (2) providing fundamental insights into the formation processes compared to other proxies. [ABSTRACT FROM AUTHOR]

Well workover operation in some blocks of the Tarim Basin resulted in mud leakage to the gas-producing formation, resulting in damage to gas well production. Therefore, refracturing of mud leakage wells in the future is necessary. To explore the means of refracturing to promote the recovery of gas well productivity, studying the evolution law of the production-induced stress field is necessary. Therefore, in this study, a prediction model of the production-induced stress field of fractured high-yield gas wells was established, and the evolution law of the stress field under the set production conditions of high-yield gas wells was clarified. The results show that with an increase in matrix permeability, main fracture conductivity, and natural fracture density, the decrease in the amplitude of stress decreases under the set production. At lower matrix permeability and natural fracture density, the low-stress areas on both sides and the front end of the main fracture are larger. At a higher main fracture conductivity, the low-stress area at both sides and the front end of the main fracture is larger. The change in the in situ stress of the gas well is smaller under set production than constant-pressure production. [ABSTRACT FROM AUTHOR]

Low salinity Waterflooding (LSWF) is a proven EOR technique which has emerged in the last few decades. In this, the salinity of the injected water is reduced. Studies have unveiled that the LSWF can improve about 5–38% of oil recovery. This work aims to study the effectiveness of LSWF by modeling Sea waterflooding (SWF) and LSWF using a commercial reservoir simulator and justifying the proposed mechanisms responsible for the incremental oil recovery due to LSWF. A critical parametric study has been undertaken, which led us to conclude that LSWF is a cost-effective, and environment-friendly method that results in additional oil recovery for sandstone reservoirs. [ABSTRACT FROM AUTHOR]

Formation damage caused by organic contaminates is one of the most important issues that undermine well injectivity and productivity. Anionic surfactants are commonly used to mobilize the organic contaminates trapped by capillary forces. The electrostatic repulsion between anionic surfactant heads reduces the surfactant density at the oil-water interface, consequently reducing the interfacial efficiency. In this paper, a novel surfactant blend of anionic-cationic surfactants (Sa/c) was designed and showed much higher efficiency than traditional ones because of the strong electrostatic attraction between anionic and cationic surfactants. The oil-water interfacial tension (IFT) was reduced to 3 × 10−4 mN/m, and the viscosity of crude oil was reduced by 92%. In addition, the freezing point of the residual oil was decreased by 20.9%. Moreover, Sa/c altered the wettability and inhibited the clay swelling. Field applications were conducted in sandstone reservoirs at temperatures of about 120 °C and permeabilities below 10 mD. The average injection pressure drops were greater than 30%. The findings of this study can help for better understanding the advantages of Sa/c with high interfacial activity and revealed that formation damage caused by the organic contaminates in high temperature and low permeability sandstone reservoirs can be effectively remediated by this novel surfactant blend. [ABSTRACT FROM AUTHOR]

The study of fracture propagation in heterogeneous shale is a crucial prerequisite for the investigation of heterogeneous cluster and perforation parameters optimization. In this paper, we conduct a physical simulation fracturing experiment on heterogeneous shale to investigate the effects of various influencing factors, such as shale bedding, near-wellbore fractures, lithological changes, and the presence of fractures surrounding the perforation hole, on fracture propagation law and morphology. Our research demonstrates that during shale fracturing, shear dislocation typically occurs between layers, resulting in the separation of different layer planes. The main fracture primarily propagates through layers in a stepped manner. The presence of sandstone in heterogeneous shale significantly impedes fracturing fractures, causing significant distortion and deviation. As the scale of natural fractures increases, it tends to cause the fracturing fracture to twist and change direction. The natural fractures network can also lead to the distortion of fracturing fractures, albeit to a lesser extent than large-scale natural fractures. The presence of micro fractures parallel to the perforation axis surrounding the perforation hole enhances the ability of the main fracturing fractures to pass through natural fractures. [ABSTRACT FROM AUTHOR]

Terrestrial sedimentary basins are influenced by rapid changes in the clay content, resulting in significant differences in the lateral sealing capacity (shale gouge ratio, SGR) of different parts of the fault. In the present study, we used a method of setting virtual wells and using seismic inversion data to accurately calculate the SGR of faults in strata composed of interbedded sandstone and mudstone calculate the clay content of the strata and to compensate for the low level of the actual well control. Optimal well spacing was determined based on the variable clay content of the formation. The planar variation of the fault throw was characterized via seismic interpretation. We also examined the lateral sealing of faults. The Putaohua oil layer in the S14 area of the Sanzhao Sag in the Songliao Basin was chosen as an case study. Based on the calculation of fault SGR values and the oil distribution, the evaluation criteria for the fault sealing capacity were determined, and the changes in the lateral sealing capacity of the faults were analyzed. This approach accurately estimates fault SGR values and predicts the effective oil-bearing area within the fault zone. It is also suitable for evaluating the lateral sealing of faults in strata composed of interbedded sandstone and mudstone. Our findings provide an in-depth understanding of the lateral sealing of faults and can aid in further research on petroleum distribution patterns. [ABSTRACT FROM AUTHOR]

This paper systematically investigates the dynamic mechanical response and fragment characteristics of sandstone with asymmetrical conjugate fissures subjected to preexisting static stress based on the split Hopkinson pressure bar apparatus. The cross-fissured sandstone exhibits significant variations in mechanical behavior when exposed to higher dynamic strain rates under identical static pre-stress. The cross-fissured sandstone with a higher dynamic strain rate is characterized by a greater coupled strength under the same static pre-stress; for a given dynamic load, the highest coupled strength occurs under the static pre-stress of 60% UCS. The failure mode of the cross-fissured sandstone is predominantly governed by the dynamic strain rate, independent of static pre-stress variations considered in this investigation. At lower dynamic strain rates, the specimens typically exhibit a mixed tensile-shear failure mode, characterized by the dominance of larger fragments in the broken specimens. In contrast, under high dynamic impacts, the sandstone tends to fail in a shear-dominated manner, resulting in smaller fragments with a more uniform size distribution. Furthermore, the study explores how varying dynamic strain rates and static pre-stress influence the fragment characteristics of the cross-fissured sandstone. Higher dynamic strain rates and increased static pre-stress generally lead to smaller mean fragment sizes. This phenomenon is quantitatively described by fitting fragment size distributions using the Generalized Extreme Value (GEV) distribution, revealing a decrease in the location parameter (μ) and an increase in fractal dimension. These metrics indicate that higher dynamic strain rates and static pre-stress result in sandstone specimens breaking into smaller fragments with a more homogeneous size distribution. These findings contribute to a deeper understanding of rock dynamics, with potential implications for engineering applications involving similar geological configuration under dynamic loading conditions. [ABSTRACT FROM AUTHOR]

Tight sandstone reservoirs have extremely low porosity and permeability. Bedding-parallel fractures (BPFs) contribute prominently to the storage and seepage capability. However, the distribution of BPFs is remarkably heterogeneous, impeding the prediction and modeling of sweet spots. BPFs are controlled fundamentally by laminations, which are widely distributed in lacustrine tight reservoirs and provide most weakness planes. Based on core and thin section data, BPFs of the upper Triassic Chang 7 tight oil reservoir are characterized microscopically. The lamination combination unit, which is defined by distinctive lamination assemblage and relatively stable lamination thickness and space, is utilized as a homogeneous unit to measure the density of lamination and related BPFs. The influence of laminations on BPFs is discussed further. Results show that most bedding-parallel fractures are unfilled, with apertures generally <40 μιτι, mainly <10 μιτι. Larger apertures correlate with low filling degrees. The distribution of BPFs is intricately controlled by lamination type, density, and thickness. (1) BPFs tend to develop along different types by a priority sequence which reflects their mechanical strength. The development degree of BPFs also depends on the mechanical contrast with adjacent laminations; (2) When controlled by a single type of lamination, the density of BPFs increases with lamination density under a turning point and then decreases; (3) BPFs prefer to develop along the thinner lamination and are usually inside it, while controlled by thick lamination, BPFs tend to extend along the edge. The change in the thickness of laminations leads to a change in the development position of BPFs, indicating that the position of the weak plane controls the development position of BPFs; (4) When multiple types of lamination coexist, the type and thickness of laminations jointly influence the development of BPFs. Plastic thin laminations are conducive to the development of BPFs, while brittle thick laminations are not conducive. When the thickness of the plastic lamination is close to or less than that of the brittle, the influence of lamination type dominates BPFs, while the thickness of the plastic laminations is much larger than the brittle, the influence of lamination thickness will dominate. [ABSTRACT FROM AUTHOR]

ABSTRACT The Erlangping back‐arc Basin separated the North Qinling Arc Terrane (NQAT) from the North China Block during Cambrian‐Ordovician. An enhanced knowledge on its evolutionary history would greatly improve our understanding on tectonic evolution of the Proto‐Tethys Ocean. In this study, we undertook sandstone petrologic and detrital zircon U–Pb geochronologic investigations on the Erlangping Group around the Tianshui Area. The Erlangping Group meta‐sedimentary rocks yielded two general types of zircon U–Pb age patterns. The first type is composed mostly of Neoproterozoic ages with a major age peak at ca. 900 Ma and has a youngest age population peaking at ca. 440 Ma. The second type is dominated by ca. 500–400 Ma components with a youngest age peak of ca. 380 Ma. The two contrasting age patterns and youngest peak ages, along with known ages of volcanic interbeds and intruded granites, allow us to subdivide the Erlangping Group to a Silurian unit and a Devonian unit, which are both stratigraphic higher than the Cambrian‐Ordovician ophiolitic unit of the Erlangping Group. In comparison with age spectra of coeval sediments in the nearby tectonic units, we found that sources of the Erlangping Group were from the Neoproterozoic NQAT basement rocks and associated Early Palaeozoic volcanic and plutonic rocks. The increasing numbers of younger zircons hint at a significant structural unroofing of the NQAT at the Silurian‐Devonian transition before having been subjected to greenschist‐facies metamorphism in the Devonian in response to the elimination of the Shangdan Ocean. [ABSTRACT FROM AUTHOR]

Fractures play a crucial role in tight sandstone gas reservoirs with low permeability and low effective porosity. If open, they not only significantly increase the permeability of the reservoir but also serve as channels connecting the storage space. Among numerous fracture identification methods, seismic data provide unique advantages for fracture identification owing to the provision of three-dimensional information between wells. How to accurately identify the development of fractures in geological bodies between wells using seismic data is a major challenge. In this study, a tight sandstone gas reservoir in the Kuqa Basin (China) was used as an example for identifying reservoir fractures using deep-learning-based method. First, a feasibility analysis is necessary. Intersection analysis between the fracture density and seismic attributes (the characteristics of frequency, amplitude, phase, and other aspects of seismic signals) indicates that there is a correlation between the two when the fracture density exceeds a certain degree. The development of fractures is closely related to the lithology and structure, indirectly affecting differences in seismic attributes. This indicates that the use of seismic attributes for fracture identification is feasible and reasonable. Subsequently, the effective fracture density data obtained from imaging logging were used as label data, and the optimized seismic attribute near the well data were used as feature data to construct a fracture identification sample dataset. Based on a feedforward neural network algorithm combined with natural fracture density and effectiveness control factor constraints, a trained identification model was obtained. The identification model was applied to seismic multi-attribute data for the entire work area. Finally, the accuracy of the results from the training, testing, and validation datasets were used to determine the effectiveness of the method. The relationship between the fracture identification results and the location of the fractures in the target reservoir was used to determine the reasonableness of the results. The results indicate that there is a certain relationship between multiple seismic attributes and fracture development, which can be established using deep learning models. Furthermore, the deep-learning-based seismic data fracture identification method can effectively identify fractures in the three-dimensional space of reservoirs. [ABSTRACT FROM AUTHOR]

This research investigated the effect of ion concentration on the performance of low salinity water under different conditions. First, the effect of injection water composition on interparticle forces in quartz-kaolinite, kaolinite-kaolinite, and quartz-oil complexes was tested and modeled. The study used two oil samples, one with a high total acid number (TAN) and the other with a low TAN. The results illustrated that reducing the concentration of divalent ions to 10 mM resulted in the electric double layer (EDL) around the clay and quartz particles and the high TAN oil droplets, expanding and intensifying the repulsive forces. Next, the study investigated the effect of injection water composition and formation oil type on wettability and oil/water interfacial tension (IFT). The results were consistent with the modeling of interparticle forces. Reducing the divalent cation concentration to 10 mM led to IFT reduction and wettability alteration in high TAN oil, but low TAN oil reacted less to this change, with the contact angle and IFT remaining almost constant. Sandpack flooding experiments demonstrated that reducing the concentration of divalent cations incremented the recovery factor (RF) in the presence of high TAN oil. However, the RF increment was minimal for the low TAN oil sample. Finally, different low salinity water scenarios were injected into sandpacks containing migrating fines. By comparing the results of high TAN oil and low TAN oil samples, the study observed that fine migration was more effective than wettability alteration and IFT reduction mechanisms for increasing the RF of sandstone reservoirs. [ABSTRACT FROM AUTHOR]

Lithological cycles with a repetitive pattern of sandstone overlain by limestone or shale comprise the middle Minnelusa Formation in the Black Hills region, USA. At least eight lithological cycles were observed at Boulder Canyon Road (BCR), eight at Vanocker Canyon Road (VCR) and four at Hot Brook Canyon (HBC). These rocks were deposited in a nearshore to offshore marine environment, and the shale and limestone intervals at the top of these cycles yielded conodonts. Five biostratigraphically distinct Middle to Upper Pennsylvanian conodont faunas were recovered from these marine units. An upper Desmoinesian assemblage was recovered from VCR cycle 1. Two Missourian assemblages were recovered, one indicative of the Idiognathodus cancellosus Zone from VCR cycle 3 and the other indicative of the Streptognathodus gracilis Zone from VCR cycle 6. Two Virgilian assemblages were recovered, one indicative of the Heckelina simulator Zone from VCR cycle 7, VCR cycle 8 and HBC cycle 2 and the other indicative of the lower part of the "Streptognathodus" group 4 virgilicus Zone from HBC cycle 3. Two conodont biofacies were recognized within the middle Minnelusa, the low energy, offshore, Idiognathodontidae-Idioprioniodus biofacies and the higher energy, nearshore Idiognathodontidae-Hindeodus biofacies. [ABSTRACT FROM AUTHOR]

The isotonic point of the Chang-6 tight oil reservoir in the Qilicun Oilfield in the Ordos Basin cannot be determined from experimental data on the waterflooding permeability. Furthermore, there are missing data near the isotonic point. Previous research on phase permeability trends has uncovered significant uncertainties in the application of data normalization, standardization, and interpolation processing. In current research, there is a consensus on the existence of a relationship between the relative permeability and saturation indices. However, since measurements contain relative permeabilities of 0, coefficient indices cannot be directly derived. Researchers have theorized that the first data number in the wet phase relative permeability must be changed from a 0 to non-0 minimum in order to directly establish the relationship between the relative permeability and the water saturation index. The missing intervals after the first data point are not interpolated. Not only does this method incorporate the knowledge of the permeability indices obtained by researchers, but it also eliminates the processes of normalization, standardization, and interpolation of missing data intervals for permeability data in order to avoid errors introduced by these processes. Without introducing new parameters, interference with raw data was reduced and the accuracy was improved. The results indicate that the relative permeability is highly correlated with the water saturation indices. Furthermore, the relationships between the water content and relative permeability and between the water saturation and water content were analyzed using the Leverett displacement function and percolation theory. The water saturation corresponding to the water content of 17 samples was calculated under the condition of displacement, and the relationship between the critical water saturation and water content was further established. These results provide support for studies on the relationship between phase permeability and oil-water saturation within this area. [ABSTRACT FROM AUTHOR]

This paper presents a comparative analysis of the mechanical characteristics of claystone and sandstone soils of natural humidity and in a completely water-saturated state. The results of field and laboratory tests showed a significant decrease in the values of strength characteristics during saturation up to 65% for claystone and 33% for sandstone. The decrease in uniaxial compressive strength during water saturation reached 70% for claystone and 90% for sandstone, and the decrease in the deformation modulus was up to 80%. [ABSTRACT FROM AUTHOR]

Tight oil reservoirs require fracturing techniques to create complex fracture networks for efficient development. It is frequently accompanied by a dynamic matrix-fracture imbibition process, promoting enhanced recovery. At present, the mechanism of three dimensions (3D) matrix-fracture dynamic imbibition at the pore scale has not been fully elucidated. In this paper, the dynamic imbibition process of oil-water two phases in matrix-fracture was simulated based on the Navier–Stokes equations, and the level set method was used to capture the real-time interfacial changes between the two phases. It was found that during matrix-fracture dynamic imbibition process, oil-phase droplets in a single pore remain in the pore mainly due to the "stuck" effect. Cluster residual oil in the pore space is mainly retained due to the "flow around" effect. Continuous residual oil in the deeper regions of the matrix is due to insufficient capillary force. Water phase in the micro-confinement space of a tight reservoir intrudes into the pore space along the pore corners, forming the "fingering" phenomenon is beneficial for enhancing the efficiency of micro-dynamic imbibition. It differs from cognition obtained in the micro-view space during conventional water flooding. The enhancement of imbibition efficiency is often accompanied by the occurrence of fluctuations in the average pressure within the matrix. Therefore, a method involving impulse type of high-frequency and short-period for supplemental energy and imbibition is suggested to enhance recovery in tight sandstone reservoirs. This study reveals the detailed mechanisms of oil-water two-phase transport at different stages in the dynamic imbibition process and holds significant guiding implications for enhancing recovery in this type of reservoirs. [ABSTRACT FROM AUTHOR]

Flood geologists linve been using tile Coconino Sandstone in the Grand Canyon to argue that a global flood rezoorked much of the plmietary geologic record. Flood geologists' arguments may seem convincing at first, but after analyzing their articles and videos tls well as the conventional geology literature, it is found t/mt the Coconino does not support a global flood. Flood geologists hai)e been largely directing their efforts toward re®ting generalized statements from critics in the popular science literature. By placing so much emphasis on finding data that could be used to refute critics, opportunities were missed to increase scient@ understanding of the Coconino. Flood geologists' filidings zoere consistently framed to argue for aqueous deposition, which they implicitly equate to global flood deposition. It is slioum flint an astronomical difference exists between the two. [ABSTRACT FROM AUTHOR]

Borehole breakout is an important geological phenomenon that provides a significant reference for the estimation of the direction and magnitude of in situ stress. To better understand the water content effect on borehole breakouts, a series of experiments on red sandstone was conducted. The standard cylindrical samples and cubic samples containing a circular hole were loaded under uniaxial compression in four water states including dry, unsaturated, short-term saturated, and long-term saturated. Then, the water effect was discussed in terms of rock strength and deformation, initiation stress of borehole breakout, failure evolution process, geometries of breakout, fragments, and acoustic emission (AE) signals. In the water-bearing state, rock properties significantly deteriorate with the decline of strength, the enhancement of plastic deformation, and the restrain of the brittle failure mode. For cubic samples, the axial stress required for borehole breakout increases under the short-term saturated condition but decreases after the long-term immersion. In addition, obvious extrusion deformation of borehole walls toward the borehole space is observed in saturated samples, resulting in vague V-shaped void notches. Before the overall failure, more obvious AE signal fluctuation occurs in the water-bearing state and AE locations reveal a larger fracture concentration region. Two mechanisms are proposed to explain the water content effects on the borehole breakout initiation stress, involving strength weakening effect and stress redistribution mechanism. The shrinkage of borehole breakout geometries is discussed further on the influence on the in situ stress estimation. Highlights: The influence of water content on sandstone properties and borehole breakout is comprehensively investigated. Short-term immersion results in a slight rise of borehole breakout initiation stress, while it declines again after long-term immersion. Double mechanisms are proposed to explain the water content effect on borehole breakout initiation. Water induces the shrink of both breakout depth and angular span, which may further affect the in situ stress estimation. [ABSTRACT FROM AUTHOR]

To investigate the influence of joints on the rockburst process in the surrounding rock of deep tunnels, a free-face true-triaxial test was conducted on the rockburst of the jointed surrounding rock of a tunnel. The rockburst ejection failure process (i.e., failure mode, rock strength, and deformation characteristics), rockburst debris characteristics, and acoustic emission evolution characteristics of red sandstone specimens under different joint conditions were investigated. The experimental results indicate that the failure modes of red sandstone specimens with a joint condition perpendicular to the tunnel direction generally exhibit 'internal shear and external splitting', which can suppress the occurrence of rockbursts, whereas the failure mode of red sandstone specimens with a joint condition parallel to the tunnel direction is mainly shear fracture, which can also promote the occurrence of rockbursts. The variation trend of the peak strength and the joint angle of the specimen exhibits a 'U' type shape, and the debris pertaining to the joint specimen's rockburst exhibits fractal characteristics. With the increase of the joint inclination angle, the fragment's ejection kinetic energy exhibits a trend of decreasing first and subsequently increasing. There are significant differences in the total number pertaining to impacts of acoustic emissions during rockburst under different joint conditions. The distribution range pertaining to the dominant frequency signal of the acoustic emission signal at the time of rockburst is mainly a low-frequency signal, and the intermediate-frequency, high-frequency, and ultra-high-frequency signals also exist, thus indicating that the rockburst damage exhibits strong complexity. Finally, the strain rockburst mechanism of jointed rock is significantly different from that of jointless intact rock. The rockburst mechanism is mainly divided into two typologies: shear fracture and slab buckling-shear fracture. The research results contribute to a deeper understanding of the underlying mechanisms of rockburst disasters in jointed rock formations. Highlights: Successfully performed a free-face true-triaxial rockburst test in the jointed surrounding rock of a tunnel. The presence of different joints under true-triaxial conditions significantly alters the mechanism of strain-induced rockburst. Joints perpendicular to the tunnel direction reduce the occurrence of rockbursts, whereas joints parallel to the tunnel direction promote the occurrence of rockbursts. With the increase of the joint inclination angle, the fragments' ejection kinetic energy exhibits a trend of decreasing first and subsequently increasing, where the joint inclination angle of 30° is the transition's inflection point. [ABSTRACT FROM AUTHOR]

In deep underground geotechnical engineering, water–rock interaction emerges as a critical factor influencing the stability of engineering rock masses. This study delves into the macroscopic nonlinear mechanical behavior and fracture characteristics of natural water-bearing and saturated sandstone samples under various true triaxial stress states. The objective is to systematically elucidate the impact of actual three-dimensional stress conditions and water content on the mechanical behavior and fracture mechanisms of sandstone formations. The findings reveal significant anisotropic deformation in sandstone under 3D stress, while saturated sandstone exhibits robust plastic deformation capacity. Moreover, changes in the strain ratio coefficient are identified as precursors to rock fracture. The dilatation behavior of sandstone weakens progressively with increasing σ2, indicating reduced expansion in saturated sandstone. To account for the influence of σ2, a novel dilation index is proposed. The true triaxial strength of sandstone demonstrates a close relationship with both σ2 and water content, with strength initially increasing before declining as σ2 rises. In addition, water significantly undermines sandstone strength, although this weakening trend moderates with increasing σ2. Morphological analysis and SEM observations unveil three-dimensional volume fracturing characteristics in rock fractures, primarily characterized by shear accompanied by tensile fractures. Saturated sandstone samples, in contrast, exhibit numerous tensile and intergranular fractures. Overall, this study holds paramount practical significance for the evaluation of rock stability and the design of water-bearing engineering rock masses, encompassing domains, such as geology, water conservancy, and mining. Highlights: The influence of the σ2 and water on the nonlinear mechanical behavior of sedimentary sandstone was studied. A new dilatation index and strength criterion considering the effect of the σ2 was proposed. The 3D fracture morphology and failure network characteristics of sandstone was analyzed. Mesoscopic fracture mechanism of the σ2 and water on sandstone was revealed. [ABSTRACT FROM AUTHOR]

Fluid injection into subsurface reservoirs may cause existing faults/fractures to slip seismically. To study the effect of temperature on injection-induced fault slip, at a constant confining pressure of 10 MPa, we performed a series of injection-induced shear slip experiments on critically stressed sandstone samples containing saw-cut fractures (laboratory-simulated faults) under varying fluid pressurization rates (0.1 and 0.5 MPa/min, respectively) and temperatures (25, 80, and 140 °C, respectively). At 25 °C, slow fault slip events with a peak slip velocity of about 0.13 μm/s were observed on a tested sample in response to a low fluid pressurization rate of 0.1 MPa/min. In contrast, fluid injection with a high pressurization rate of 0.5 MPa/min caused fault slip events with a peak slip rate up to about 0.38 μm/s. In response to a given fluid pressurization rate, several episodes of slip events with a higher slip velocity were induced at an elevated temperature of 140 °C, indicating an appreciable weakening effect at elevated temperatures. We also experimentally constrained the rate-and-state frictional (RSF) parameters at varying effective normal stresses and temperatures by performing velocity-stepping tests. The obtained RSF parameters demonstrate that for a relatively high normal stress, increasing temperature tends to destabilize fault slip. Post-mortem microstructural observations reveal that elevated temperatures promote the generation of abundant fine-grained gouge particles associated with injection-induced shear slip. Our experiments highlight that injection-induced fault slip is affected by temperature-related wear production over the fault surface. [ABSTRACT FROM AUTHOR]

The mechanical properties of fissured sandstone will deteriorate under water–rock interaction. It is crucial to extract the precursor information of fissured sandstone instability under water–rock interaction. The potential of each acoustic emission (AE) parameter as a precursor for instability in the failure process of fissured sandstone was investigated in this study. An experimental dataset comprising 586 acoustic emission experiments was established, and subsequent classification training and testing were conducted using three machine learning (ML) models: AdaBoost, MLP, and Random Forest (RF). The primary parameters for identifying the instability risk state of fissured sandstone include acoustic emission ringing count, energy (mV·ms), centroid frequency, peak frequency, Rise Angle (RA), Average Frequency (AF), b value, and the natural/saturated state of fissured sandstone: state. To enhance data utilization, a 10-fold cross-validation method was employed during the model training process. The machine learning models were developed and designed to identify the instability risk of fissured sandstone under the natural and saturated states. The results demonstrated that the established RF model was capable of identifying fissured sandstone instability risks with an accuracy of 97.87%. Feature importance analysis revealed that state and b value exerted the most significant influence on identification results. The Spearman correlation coefficient was utilized to assess the correlation between input features. This study can provide technical support to identify the risk of instability of fissured sandstones under both natural and saturated water conditions. Based on the models developed in this study, it is possible to implement an early warning method for instability in fissured sandstone that meets realistic working conditions. Compared with the traditional empirical and formulaic methods, the machine learning method can more quickly process huge amounts of AE data and accurately identify the damage state of fissured sandstone. [ABSTRACT FROM AUTHOR]

The Dina 2 gas field in the Kuqa Depression of the Tarim Basin is one of China's most critical oil and gas exploration areas. Natural fractures have played an important role in the low-permeability reservoirs in the Tarim area. Tectonic fractures are dominant in such reservoirs. In fact, the factors influencing tectonic fracture development have always been the source of important issues in tight reservoirs. Cores, thin sections, and borehole image logs were used to analyze the types, basic characteristics, and factors influencing tectonic fractures in the tight sandstone reservoirs of the Dina 2 gas field in the Tarim Basin. The results showed that the tectonic fractures are dominated by high-angle and upright shearing fractures, and they mainly show ENE–WSW strikes. The thin sections suggest that 60% of the fractures are fully filled with minerals, 20% are unfilled, and 20% are partially filled. The analysis also shows that lithology, faults, and in situ stress are the main factors controlling the development of the tectonic structures. Furthermore, the correlation between the unimpeded flow from a single well and the apertures of the tectonic fractures indicates that tectonic fractures play an important role in the production of hydrocarbons. [ABSTRACT FROM AUTHOR]

The H8 and S1 reservoirs in the lower Shihezi Formation and Shanxi Formation of the central block in the Sulige Gas Field are typical fluvial tight sandstone reservoirs. Due to frequent river channel migrations during deposition, the reservoirs exhibit complex spatial structures with developed intra-sand mudstone interlayers. As the field has entered the middle and late stages of development, the distribution of remaining gas is intricately controlled by these interlayers, necessitating research on their distribution to understand the remaining gas patterns and types for effective extraction enhancement. However, the thinness of interlayers presents a challenge for precise prediction. Addressing this, this study delineates different interlayer types and their origins, applies reservoir architecture theory, and utilizes bounding surfaces characterization, planar and sectional distribution studies, unit scale analysis, horizontal well data, and quantitative characterization methods to investigate the internal reservoir architecture bounding surfaces. The study finely portrays the interlayer distribution, analyzes the control of reservoir architecture bounding surfaces on remaining gas, and establishes a multi-tiered reservoir architecture model in the study area. Numerical simulation of the gas reservoir clarifies the types of remaining gas enrichment. This study also identifies and quantitatively characterizes the 5–3 level architecture bounding surfaces within the sandbody, categorizing the remaining gas into bounding surfaces-controlled, well-network uncontrolled, and single-layer unperforated types, proposing targeted enhancement measures for each type. Based on the findings, four vertical wells and three horizontal wells were deployed, improving the well network density to three wells per square kilometer. The first completed horizontal well encountered an effective drilling rate of 61.7%, marking significant implications for the exploitation and recovery enhancement of similar tight sandstone gas reservoirs. [ABSTRACT FROM AUTHOR]

Amber appears in the archaeological material of the Eastern Baltic region in the Middle Neolithic and continues in intensive use until the end of the Neolithic. Four major amber-working centres are known in the Eastern Baltic, the least studied of which is the settlement of Siliņupe (4th mill. BC). Not yet widely applied in research on Stone Age amber technologies, but very promising, is an analytical approach combining experimental archaeology and traceology to study processing choices made in specific production stages (i. e., individual technological elements), revealing that fine-grained sandstone was favoured for surface grinding, while flint tools were most widely used for shaping and surface treatment, distinguishing three techniques, namely shaving, scraping and pressure flaking, each identifiable by characteristic features. The study suggests that amber and flint processing were largely intertwined, as pressure flaking along with indirect percussion (used for splitting amber) derive from flint processing, and in the case of pressure flaking, required specific know-how. [ABSTRACT FROM AUTHOR]

Sandstone-type deposits have emerged as the primary uranium deposits being mined in China, utilizing in-situ leaching technology. However, the leaching of uranium resources within sandstone formations faces significant constraints due to their extremely low permeability, accounting for approximately seventy percent of the total. This paper aims to present recent advancements in the field in enhancing seepage-leaching mass-transfer properties in in-situ leaching of uranium from low-permeability sandstone. We will provide an overview of the main factors influencing the in-situ leaching process in low-permeability ore layers, discussing the technical characteristics of applicable methods designed to enhance permeability in low-permeability uranium-bearing sandstone based on the existing research. Additionally, we will also explore the suitability of various approaches from other fields in the in-situ leaching of uranium process. Finally, we will outline the prospects regarding process methodologies, leaching agents, and enhancing permeability methods. [ABSTRACT FROM AUTHOR]

Tetrapod tracks from the Lower Jurassic strata of the Hartford Basin (Newark Supergroup) abound in areas of Massachusetts and Connecticut, USA. However, tetrapod skeletal fossils are uncommon, and the co-occurrence of tracks with skeletal material in the same bedding plane is exceedingly rare. Due to this paucity of skeletal material, spatiotemporal gaps and taxonomic incompleteness continue to hamper research on bones preserved as natural casts. Here we report on the discovery of new bone cast specimens and three grallatoroid (Anchisauripus) tracks on a single sandstone slab originating from Portland or Middletown, Connecticut. The Nash Specimen – BRCM 2021.10 – holds important implications for Early Jurassic dinosaur taphonomic history and the paleoenvironment in which dinosaur footprints and bones as natural casts were preserved. Results from this study describe three Anchisauripus tracks and five natural bone casts from an early theropod dinosaur. Judicious comparisons to the only comparable specimen of dinosaur bone casts (MOS.2001. 248) are made in describing the morphological structures and the taphonomic pathways leading to cast formation. The unique taphonomic circumstances of bone casts preserved alongside dinosaur tracks on the same substrate layer provide an intriguing case of the non-preservation of body fossils that merits further research in the Hartford Basin. [ABSTRACT FROM AUTHOR]

Chandarpur–Raipur sequence in Chattisgarh basin is represented as siliciclastic-dominated Chandarpur Group and carbonate-dominated Raipur Group. Here, we introduce 'Devsagar Sandstone Member', the only sandstone-dominated member in the carbonate-dominated Charmuria Formation of Raipur Group, that marks a period of rapid siliciclastic deposition identifying a phase of forced regression between two carbonate platforms of Charmuria–Chandi formations, thereby indicating a drastic change in palaeogeography of Raipur Group. In addition, this study revised the litho-stratigraphy of Mesoproterozoic Chattisgarh basin to clarify the confusion raised due to the existence of different stratigraphy in different basinal parts and different nomenclature for the same lithologic units. Detailed geological mapping with facies analysis in the eastern part of the basin manifests the entire basin-fill succession as part of the Chattisgarh basin itself, rather than sub-dividing some parts as Baradwar sub-basin and Singhora proto-basin. Singhora Group deposited in Singhora proto-basin has already been presented as equivalent of Chandarpur Group. Here we propose, Bamandihi–Saradih–Raigarh formations of Raipur Group in Baradwar sub-basin, as lateral equivalent of Gunderdehi–Chandi–Tarenga formations of Raipur Group and Sarnadih–Nandeli formations of Kharsiya Group in Chattisgarh basin. Inferred depositional environment and tectonic setting of Chattisgarh basin support the lithostratigraphic revision, which will help in basin analysis as well as intrabasinal–interbasinal correlation in regional and global contexts. Research highlights: Devsagar Sandstone Member introduced as the only sandstone-dominated member in carbonate-dominated Charmuria Formation of Raipur Group. Devsagar Sandstone Member represents a tidal shelf in between two carbonate ramp platforms (Charmuria and Chandi), marking a period of rapid siliciclastic deposition and the only phase of forced regression in overall sea-level rising scenario of the carbonate-dominated Raipur Group. Stratigraphy of Chattisgarh basin revised. The entire Chattisgarh succession is represented as deposits of Chattisgarh basin only, without further subdivision into sub-basin and/or proto-basin, thus resolving the stratigraphic and basinal correlation problem. [ABSTRACT FROM AUTHOR]

Since the dawn of civilization, sandstone has been a fantastic building material. Numerous causes have been observed in the past for sandstone damage or deterioration, one of which is sulphur-oxidizing bacteria (SOB) and cyanobacteria. In general, SOB is present in the soil, air, water, humidity, and human activity. The oxidation of sulphur compounds, such as hydrogen sulphide, thiosulphate, or elements of sulphur, provides energy for SOB. These microorganisms contribute to the decay of buildings materials, especially those made of stone, metal, or concrete. The sulphur oxidizing process affects the mechanical properties of sandstone. Mechanical properties are related to strength. Losses of mechanical properties may be the reason for deflection, cracking, collapse, and catastrophic failure of sandstone. Monitoring and evaluation gives an idea about the behavior of structure and the prevention of catastrophic failure. This research paper contains the application of electromechanical impedance with surface bonded Piezoelectric Lead Zirconate Titanate. The sandstone samples and soil samples have been collected from the historical site. Two sets of cylindrical types of sandstone specimens have been used in experimental work. The conductance signature and susceptance signature have been measured every six months. The variation and shifting of the signature curve have been used to identify the structural behavioral change. Statistical methods like the root mean square deviation have been used for the quantification of damage. An equitation has been generated on the basis of the percentage root mean square deviation to quantify the prediction of damages. [ABSTRACT FROM AUTHOR]

Engineering rock mass often involves bedding planes and U-shaped defects. Under the combined influence of bedding planes and defects, the fracture mode of engineering rock mass is more complicated. To better understand the failure law of U-shaped defects in rock mass with bedding planes and better control or predict the crack propagation morphology, three-point bending tests were conducted on semicircular disk specimens with U-shaped notches under three different notch radii. The load variation law, different stages of the load–displacement curve, and fracture characteristics of specimens under five bedding angles were analyzed. Then, numerical models of semicircular three-point bending tests were established in particle flow code 2D (PFC2D). The influence of notch radius and bedding angle on the failure path of semicircular specimens was simulated and analyzed, and the three failure modes were verified again. In addition, the stress at the notch tip of the model was monitored by the measurement circle in the discrete-element method, and the fracture toughness of the model specimen was calculated, which provides a new calculation idea for fracture toughness. The research results help us more comprehensively understand the mechanical behavior and failure characteristics of layered rock mass with notch under external load. [ABSTRACT FROM AUTHOR]

The study is concerned with the rate of evaporation from porous rock, including the second stage of evaporation characterised by the existence of a dry surface layer separated from the wet capillary zone by a sharp evaporation front. The main objective is to investigate the relationship between the depth of evaporation front and the rate of evaporation as the drying process progresses, and to compare measured evaporation rate with the corresponding calculated values. Sandstone core samples saturated with water were allowed to dry naturally under room conditions, while the changes in the evaporation rate and the depth of evaporation front, among other quantities, were measured. We demonstrate that the evaporation rate can be very accurately determined from the depth of the evaporation front and the ambient air temperature and relative humidity using Fick's law for water-vapor diffusion. During the second stage of evaporation, the diffusion flux through the dry surface layer is computed using the water-vapor diffusion coefficient of the rock, determined from a separate wet cup experiment. In order to cover the first stage of evaporation, an additional parameter characterising the diffusion layer of air above the surface is required, either determined by the best fit to the measured evaporation rates, or adopted from previous studies. The calculated evaporation rate was in good agreement with measurements, with Pearson correlation coefficient 0.98 and relative error of the calculations averaging 15% over the evaporation front depths ranging from 0 to 29 mm. A workflow for determining the evaporation rate from sandstone outcrops is suggested, along with possible applications in sandstone weathering research. [ABSTRACT FROM AUTHOR]

Tight sandstone reservoirs are significant sources of natural gas reserves. As traditional reserves become increasingly scarce and costly, optimizing the development of these reservoirs becomes crucial. This study introduces a novel two-phase gas–water flow model for single wells, incorporating both Darcy and non-Darcy flow equations. These equations are derived from mass conservation and momentum principles for both gas and water phases. Using data from a real tight gas well, our model, which includes stress-sensitive phases for gas and water, outperforms traditional Darcy flow models. Specifically, the average relative deviations in daily production rates were 0.1815% for gas and − 0.2677% for water, which are significantly smaller compared to traditional Darcy flow models. Further application of the non-Darcy flow model reveals strategies to enhance well performance. For example, mitigating liquid lock damage within a 2 m radius near the well could restore the permeability from 0.045 to 0.143 mD, thereby tripling the daily gas production. This non-Darcy flow model is easy to implement and shows significant potential in forecasting production yields in tight sandstone reservoirs, highlighting its importance in the petroleum and natural gas industry. [ABSTRACT FROM AUTHOR]

In low-pressure gas reservoirs, water-based fracture fluid is difficult to flowback, which is unfavorable for several tight sandstone gas reservoirs in the Sichuan Basin with low pressure and high permeability geological characteristics. Supercritical CO2 possesses a number of remarkable physical and chemical features, including a density near to water, a viscosity close to gas, and high diffusion. Supercritical CO2 fracturing is a new type of non-aqueous fracturing method that is favorable to fracturing flowback in low-pressure tight sandstone and has a wide range of applications. To discuss on whether supercritical CO2 fracturing with low pressure tight sandstone is feasible. Tight sandstone cores from the Jinqiu gas field in the Sichuan Basin were used to study the influence of supercritical CO2 on the physical properties of sandstone reservoirs. Supercritical CO2 was used to interact with tight sandstone samples, and then the changes in porosity, permeability, and rock microstructure of tight sandstone were observed under various time, pressure, and temperature conditions. After the interaction between tight sandstone and supercritical CO2, new dissolution pores will appear, or the original pores will be increased, and the width of some natural fractures will also be increased, and the porosity will increase by 1.09%e8.85%, and the permeability will increase by 2.34%e21.26%, quantifying the influence of supercritical CO2 on physical properties of tight sandstone, and further improving the interaction mechanism between supercritical CO2 and tight sandstone. This study improves in the understanding of the tight sandstone-supercritical CO2 interaction mechanism, as well as providing an experimental foundation and technological guarantee for field testing and use of supercritical CO2 in low-pressure tight sandstone gas reservoirs. [ABSTRACT FROM AUTHOR]

Rock abrasiveness is an important factor affecting the tool's lifetime and efficiency in breaking a rock. Characterizing rock abrasiveness helps in the design, optimization, and mean-life prediction of tools. X-ray diffraction, cast thin section analysis, and CERCHAR abrasiveness tests were performed on 18 different sandstones to characterize rock abrasiveness and explore new methods for characterization. The relationship between the mineral composition and microstructure of sandstone and abrasiveness was investigated. The results show that different structural maturities have varying effects on abrasiveness. In addition, the higher the structural maturity, the more the abrasiveness. Furthermore, in sandstones of the same structural maturity, the abrasiveness increases with equivalent quartz content (EQC). The texture coefficient (TC) and CERCHAR abrasiveness index (CAI) of sandstones with the same structural maturity showed a good linear relationship. Moreover, the correlation coefficients considering the combined parameters are above 0.85. Therefore, obtaining the microstructure and mineral composition of sandstone can effectively characterize rock abrasiveness. It also provides a new method for predicting the abrasiveness of the rock in the well. [ABSTRACT FROM AUTHOR]

To solve the development problems caused by the geological characteristics of heterogeneous sandstone reservoirs, such as uneven interlayer exploitation, a method for improving uneven interlayer exploitation differences by applying interlayer injection–production coupling technology is proposed. A physical model of interlayer injection–production coupling is elaborated in detail, and its mechanism of enhancing oil recovery is analyzed. The reservoir physical property parameters are measured, and a productivity numerical model for the two-phase flow of oil–water was established based on measurement results. Then, the effectiveness of interlayer injection–production coupling was evaluated. The results showed that the mechanism of interlayer injection–production coupling can be summarized as reservoir elastic energy adjustment and reservoir flow field reconstruction, based on the established physical model. The application of interlayer injection–production coupling technology can significantly improve the interlayer exploitation differences in strongly heterogeneous sandstone reservoirs. The injection rate, liquid production rate, half-period ratio, and coupling period all have a significant influence on the interlayer injection–production coupling effect. Specifically, for the J1 well group, the injection rate and liquid production rate can be appropriately increased by a factor of 2 and 1.5, and corresponding oil recovery will increase by 6.4% and 5%. Meanwhile, when the half-period ratio increases to 3:1, the oil recovery will increase by 7.08%. Therefore, during the design of the interlayer injection–production coupling scheme, the injection rate and liquid production rate can be appropriately increased, the injection time should be increased for the under-exploitation layer, and the optimal coupling period should be selected based on the characteristics of the oilfield. [ABSTRACT FROM AUTHOR]

Poorly producing wells in sandstone gas reservoirs are often refractured to enhance production. Considering the mutual interference of initial/refractured fractures, conductivity dynamic evolution, non-uniform inflow, and variable mass flow in the fracture comprehensively, a semi-analytical reservoir-fracture coupled production model fusing spatial and time separation methods is introduced to model refractured well performance. The proposed model is verified by CMG. The field applications indicate that the refracture job should be carried out when production is lower than the desired value. Restoring the Cf-ini and constructing the Cf-ref can increase productivity, which increases over 8 D•cm. The production growth rate just obtained a slight improvement. The production increased significantly with Lf-ini increasing from 120~270 m and Lf-ref increasing from 100~150 m. Hence, it is essential to extend the Lf-ini under engineering conditions. The ks/km = 10 can obviously increase production, but further enlarging ks does not contribute to well performance. Conversely, further producing larger bs is vital to enhancing production. Subsequently, the optimal parameter combinations (ds > Lf-ini > Lf-ref > Cf-ini > ks > Cf-ref) for well(X1) are carried out by orthogonal experiments. This work proposes a novel method to simulate refractured vertical well performance in tight gas reservoirs for refracture optimization. [ABSTRACT FROM AUTHOR]

The nuclear magnetic resonance T2 spectrum was used to identify the T2 cut-off value, which is the key to determining the irreducible water saturation of a reservoir. In this paper, the saturation and centrifugal T2 spectra of sandstone and coal samples were used to explore the correlation between each parameter and the T2 cut-off value, using a single fractal dimension, a multifractal dimension and a spectral morphology discrimination method. The conclusions are as follows: (1) The T2 spectra of nine sandstone samples in this paper can be divided into four types. Type A is represented by sample 2, wherein the T2 spectrum shows a bimodal state and the area of the right T2 spectrum (2.5~100 ms) is larger than that of the left T2 spectrum (T2 < 2.5 ms), indicating that the sample has good pore connectivity and belongs to the macroporous development sample. The B-type T2 spectrum is unimodal, and the pore connectivity is poor, indicating that it is a large-pore development sample. The T2 spectrum of the C-type sample is unimodal, and the pore connectivity is very poor, indicating that it is a mesoporous development sample. The T2 spectrum of the D-type sample shows a single peak state, and the main T2 is distributed within 0.1~2.5 ms. The pore connectivity is very poor, which indicates that it belongs to the small pore development type sample. (2) The single fractal model shows that, compared with other single fractal parameters, D2 increases with the increase in the T2 cut-off value, but the correlation is weak. Therefore, it is not feasible to predict the T2 cut-off value using the single fractal dimension parameter. (3) The multifractal model shows that D−10–D10 increases linearly with the increase in D−10–D0, but there is no obvious linear correlation between D0–D10 and D−10–D10, indicating that the low pore volume area in this kind of sample controls the overall heterogeneity of pore distribution. (4) The related parameters affecting the T2 cut-off value include D−10–D10, D−10/D10, D−10–D0, TM and D2. Therefore, based on the above five parameters, a T2 cut-off value prediction model is constructed. The T2 cut-off value calculated by the model is highly consistent with the experimental value, which proves the reliability of the model. [ABSTRACT FROM AUTHOR]

The instability of jointed rock masses has been a persistent concern in China's underground geotechnical engineering, particularly regarding rock mass instability triggered by structural activation, such as faulting. This form of instability constitutes a significant type of dynamic geological hazard in the field of geotechnical engineering. Research on the mechanism of jointed rock mass instability typically concentrates on various characteristics associated with structural activation but frequently neglects the interplay between coalescence instability within the jointed zones and the intact zones, as well as the development and evolution of abrupt water channels. To delve into the coalescence instability characteristics between jointed and intact zones, this study conducted uniaxial compression tests on macro-scale pre-fabricated jointed sandstone. The research results show that the failure process of the specimen consists of a strong deviation linear stage, a sub-critical stage, and an unstable stage. The main failure process occurs during the sub-critical stage. [ABSTRACT FROM AUTHOR]

Given the critical role of true triaxial strength assessment in underground rock and soil engineering design and construction, this study explores sandstone true triaxial strength using data-driven machine learning approaches. Fourteen distinct sandstone true triaxial test datasets were collected from the existing literature and randomly divided into training (70%) and testing (30%) sets. A Multilayer Perceptron (MLP) model was developed with uniaxial compressive strength (UCS, σ c ), intermediate principal stress ( σ 2 ), and minimum principal stress ( σ 3 ) as inputs and maximum principal stress ( σ 1 ) at failure as the output. The model was optimized using the Harris hawks optimization (HHO) algorithm to fine-tune hyperparameters. By adjusting the model structure and activation function characteristics, the final model was made continuously differentiable, enhancing its potential for numerical analysis applications. Four HHO-MLP models with different activation functions were trained and validated on the training set. Based on the comparison of prediction accuracy and meridian plane analysis, an HHO-MLP model with high predictive accuracy and meridional behavior consistent with theoretical trends was selected. Compared to five traditional strength criteria (Drucker–Prager, Hoek–Brown, Mogi–Coulomb, modified Lade, and modified Weibols–Cook), the optimized HHO-MLP model demonstrated superior predictive performance on both training and testing datasets. It successfully captured the complete strength variation in principal stress space, showing smooth and continuous failure envelopes on the meridian and deviatoric planes. These results underscore the model's ability to generalize across different stress conditions, highlighting its potential as a powerful tool for predicting the true triaxial strength of sandstone in geotechnical engineering applications. [ABSTRACT FROM AUTHOR]

Featured Application: This concept could be used to guide the study of sandstone creep models under other conditions. The Nishihara creep model is an extremely effective method in the field of sandstone creep model research. However, the Nishihara creep model curve for sandstone under the marine environment (the coupled effect of chemical corrosion and temperature) does not fit the actual creep test data well. Based on the Nishihara creep model, we discovered that, on the one hand, the viscoelastic elements in the Nishihara model are replaced by the viscoelastic elements containing temperature and pH factors, which can accurately describe the influence of temperature and pH on rock creep characteristics; on the other hand, the viscoplastic elements in the Nishihara model are replaced by nonlinear viscoplastic elements, which can accurately describe the accelerated creep stage of the rock mass. After modifying Nishihara's creep model twice, a new temperature–pH damage nonlinear creep model is established. The creep curve of the temperature–pH damage nonlinear creep model is compared with the creep test data of sandstone. The comparison results indicate that the creep curve of the nonlinear creep model has a high degree of fit with the creep test data of sandstone (accuracy > 92%). This validates the correctness of the newly established temperature–pH damage nonlinear creep model presented in this paper, demonstrating that the new model can effectively reflect the influence of temperature and pH on the creep characteristics of sandstone. [ABSTRACT FROM AUTHOR]

This article studies the migration and settlement laws of backflow proppants after fracturing tight sandstone. This paper proposes a fitting method based on a multi-task learning network to address the issue of interference from multiple physical parameters during the transport and settlement processes of proppants. This method can effectively handle multi-dimensional interference factors and fit the mapping logic of multiple engineering parameters to transport patterns through the continuous correction of multi-layer networks. We first introduce the characteristics of tight sandstone reservoirs and their important value in mining, as well as the status of current research on the migration and settlement laws of proppants at home and abroad. Based on this, we then deeply analyze the sedimentation rate model of proppants in tight sandstone backflow and the equilibrium height of proppants under multiple factors of interference while considering the distribution characteristics of proppants. In order to more accurately simulate the transport and settlement laws of proppants, this paper introduces a multi-task learning network. This network can comprehensively consider multi-dimensional parameters, learn the inherent laws of data through training, and achieve accurate fitting of the transport and settlement laws of proppants. This study trained and tested the model using actual production data, and the results showed that the proposed model can fit the input–output relationship well, thus effectively supporting the study of proppant transport and settlement laws. [ABSTRACT FROM AUTHOR]

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Copyright of World Nuclear Geoscience is the property of World Nuclear Geoscience Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Notostracan branchiopods reported from the Lower Triassic Voltzia Sandstone Lagerstätte in northeastern France have previously been assigned to Triops cancriformis, making this species the longest persisting animal in earth history. Careful restudy of the splendidly preserved material indicates that the species can be well characterized but represents a notostracan genus and species that cannot be assigned to the Triopsidae. This undermines the concept of its being a 'living fossil'. The name Apudites antiquus, under which the first known specimens were described, is available and appropriate for the species. Some of the specimens of Apudites antiquus offer morphological details of a quality that ranks them among the best‐preserved fossil non‐marine arthropods. The Voltzia Sandstone specimens previously assigned to the extant species T. cancriformis include two additional, peculiar diplostracan branchiopods, Olesenocaris galli gen. et sp. nov. and Grauvogelocaris alsatica gen. et sp. nov., both with unique morphological features that characterize them as representatives of as yet unknown families. [ABSTRACT FROM AUTHOR]

Purpose: To effectively address the disposal issues of gold tailings and alleviate the shortage of sandstone resources,this study investigated the potential of utilizing gold tailings as a partial replacement for natural sand in the production of gold tailings concrete. Method: Through a series of cube compressive strength tests and orthogonal experimental methods,this research conducted a comprehensive analysis of the impacts of four factors: water-binder ratio, go Id tailings substitution rate, sand ratio, and fly ash content, on the 28-day compressive strength of gold tailings concrete. The systematic analysis was performed using range and variance analysis methods, which determined the relative importance of each factor in influencing the 28-day compressive strength of the concrete. Furthermore,a multiple regression equation was established for predicting the compressive strength of gold tailings concrete based on the orthogonal experimental data. Conclusion: The results indicated that the water-binder ratio had the most significant effect on the compressive strength, followed by the gold tailings substitution rate, fly ash content,and sand ratio. The regression equation,with an R2 value of 0. 872,can predict the compressive strength to a considerable degree. Significance: The research results can provide some reference value for the comprehensive utilization of gold tailings resources. [ABSTRACT FROM AUTHOR]

Layered composite rocks (abbreviated as LCRs) are frequently encountered in rock engineering, and it is difficult to predict whether the stress-induced crack propagates through the layers or is contained within a layer when subjected to external loads. There was limited advanced knowledge of the crack propagation mechanism of flawed LCRs because the previous research mainly focused on the single lithologic layer. This study conducts uniaxial compression tests on weaker and stronger interbedded sandstone with pre-existing flaws. The crack propagation process of two-layer weaker and stronger interbedded sandstone specimens is studied using acoustic emission (abbreviated as AE) and digital image correlation (abbreviated as DIC) techniques, with the focus being on the influences of the flaw position. The results indicate that the crack initiation mode and final failure mode of weaker and stronger interbedded sandstone are dominated by the tensile mechanism. Three crack initiation modes are identified, which are closely related to the flaw position. When the flaw exists inside the weaker layer or the flaw is far away from the interface, the crack initiation mode is the tensile anti-wing crack initiation from the flaw tip. Whatever the flaw positions, the upward-propagating cracks arrest within the stronger layer, but the downward-propagating cracks eventually cross the weaker layer, which is the main mechanism causing failure. This study sheds light on the failure mechanism of LCRs considering the flaw position under uniaxial compression. [ABSTRACT FROM AUTHOR]

The randomness and complexity of rock mass fissure development seriously affects the strength and stability of the rock mass. In order to analyse the influence of different numbers of rock bridges on the strength of rock mass, uniaxial compression and numerical simulation tests of fissured sandstone is carried out to study the influence law of different numbers of rock bridges on the mechanical properties and failure characteristics of rock. The results show that the number of rock bridges affects the damage mode of the rock mass, and when the fissure inclination and the inclination of rock bridges is constant, the damage microcrack development status of rocks with different numbers of rock bridges is different; the increase of the number of rock bridges has an optimising effect on the mechanical properties of the rock mass, and the peak stress and elastic modulus increase with the increase of the number of rock bridges. Under uniaxial compression, shear coalescence occurred in the intact rock specimens, while the fracture-containing specimens changed from shear damage to mixed shear-tension damage with the increase of the number of rock bridges. The damage of the rock specimens containing only 0 and 1 rock bridge are mainly caused by the extended shear cracks at the tip of the prefabricated cracks and the penetrating rock bridges; the rock specimens containing 2 rock bridges produced tensile cracks at the same time as shear cracks are formed, and the damage of the specimens is caused by the joint extension of shear and tensile cracks. When the total length of the co-linear cracks is the same, the higher the number of rock bridges, the smaller the stress intensity factor of the rock mass, and the less likely the specimen will be damaged. [ABSTRACT FROM AUTHOR]

State Highway-05A in Northern India, connects the states of Chhattisgarh, Madhya Pradesh, Uttar Pradesh, and Jharkhand. In Uttar Pradesh, it passes through steep and rugged Markundi Hill, composed of highly jointed sandstone. The current study examines road-cut slopes at six locations to quantify the instability mechanism and slope health. Detailed field and laboratory investigations were combined to ascertain the structural, petrographic, and strength attributes of the rock. Afterwards, data was collated to characterise the rockmass behaviour through widely accepted classification schemes, viz., geological strength index (GSI), Q-slope, rock mass rating (RMR), slope mass rating (SMR), and modified global slope performance index (modified GSPI). The value ranges provided by various empirical classifications are 40–62 (RMR), 39.61–58.46 (SMR) and modified 44.57–52.57 (GSPI). For structural stability, kinematic analysis was conducted. According to RMR, five locations fall in fair and one in poor rockmass classes. SMR suggest all locations are partially stable. Eventually, a novel approach for finding the ratings of GSPI is also introduced in the present work, allowing more comprehensive discontinuity characteristics incorporation. The new approach brings GSPI and SMR to the same scale, making it easy to compare the two. GSPI yields that all the locations have high chances of local bench failures. Compared to other approaches, GSPI predicts a wide range of instabilities and should be used alone or in conjunction with other systems for slope stability assessment. [ABSTRACT FROM AUTHOR]