Sandstone Research Articles

Enhancing understanding of fracture mechanisms in brittle rocks under static and cyclic loads: Experimental insights and theoretical model

The present study investigates the fatigue behavior of granite and red sandstone under coupled static and cyclic loading. Experimental results reveals a three-stage evolution trend for the deformation modulus, Young's modulus, peak strain, residual strain, dissipated energy as a function of the absolute cycle. Acoustic emission (AE) rate also exhibits a three-phase stage, including the initial phase, uniform velocity phase, and accelerated phase. To explain the mechanical responses and fracture mechanisms related to rock fatigue, this study built a micromechanics-based damage model that adopts a modified Paris law and considers both the effect of static stress and stress amplitude. The theoretical model successfully reflects the effect of static loading history and reproduces the three-stage evolution of residual strain during rock fatigue. The SIF deduced by the theoretical model exhibits a distinct behavior: it initially decreases during the first few cycles, stabilizes at a lower value, and subsequently undergoes a significant increase as it approaches the final cycles. The internal cause behind the three-stage evolution of crack growth rate, AE rate, and mechanical properties is attributed to subcritical crack growth during cyclic loading. Finally, the theoretical model successfully characterizes the effect of static stress and stress amplitude of cyclic loads on fatigue damage evolution.

Acidification characteristics and its influencing factors of red paddy soil derived from four parent materials in Southeast of China

There are many types of parent materials of red paddy soil, however the acidification characteristics of paddy soil derived from different parent materials remain unclear in the past 40 years at Jiangxi Province, and the degree of soil properties on acidification is also still unknown. Therefore, four red paddy soil derived from Quaternary red earth (Qre), plate shale (P), red sand stone (Rs), granite (G) in Jiangxi Province was selected. We investigated the acidification characteristics and quantified the effect of soil factors on the acidification of red paddy soils derived from different parent materials. Results showed that related to the second soil survey (1980s), the pH of red paddy soil derived from four parent materials decreased by an average 0.47 unit, base saturation (BS%) decreased 13.93%, exchangeable acid (EA) and exchangeable aluminum (EAl) increased by 0.43 and 0.20 cmol·kg−1, respectively. The ∆pH of red paddy soil derived from Rs (0.56) and G (0.54) were higher than that of Qre (0.34) and P (0.32). The ∆BS% of of red paddy soil derived from G (22.53%) > Rs (17.61%) > Qre (8.27%) > P (2.66%). The ∆EA (0.76) and ∆EAl (0.62) of Rs is highest. The results of redundancy analysis (RDA) shown that soil properties of red paddy soil derived from four parent material could explain 43.1% variation of soil acidification. CEC7, silt, sand, and SOM could explain 36.3%, 4.6%, 1.4% and 1.2% variation of acidification, respectively. The difference of soil mineralogy including oxide and clay minerals is also the reason for the different acidizing characteristics of paddy soil derived from four parent material. Therefore, compared with the 1980s, there is significant acidification of red paddy soil in 2018 in Jiangxi Province. The acidification of red paddy soil derived from red sand stone and granite is higher than that of plate shale and Quaternary red earth. And, parent material affect acidification by soil physicochemical and mineralogical properties.

Late Paleozoic and Early Mesozoic radiolarians from chert conglomerates embedded in the Kaeng Raboet Formation (Upper Mesozoic continental red-bed), Kanchanaburi Province, western Thailand

Continental clastic red-beds including red and fine- to medium-grained sandstone, pebbly sandstone, and conglomerates are distributed in the Sai Yok area of western Thailand. This rock sequence is known as the Jurassic Kaeng Raboet Formation by Thai geologists. A similar clastic rock succession, approximately 150 m thick, outcropping at Wat Phu Toey, near Ban Tha Sao, was investigated for paleontological analysis. The lithostratigraphy of the sequence in this section consists of Units 1-3 in ascending order. Unit 1 is characterized by pebble-sized chert conglomerates, Unit 2 by medium-grained massive sandstone, and Unit 3 by fine-grained red sandstone and shale with ripple marks and cross-laminations. Several thin layers of chert conglomerate are intercalated in the lower parts of Units 2 and 3. We collected six chert conglomerate samples from this section and distinguished Tournaisian (Lower Carboniferous) to middle Norian (Upper Triassic) radiolarian-bearing chert clasts that have not been not separated as single clasts but treated as one sample. Although the preservation of radiolarians is generally poor, we identified 40 species belonging to 26 genera with one radiolarian gen. et sp. indet. The exact age of the Kaeng Raboet Formation is still equivocal; however, we assigned the age of the red-bed to the Late Jurassic to Early Cretaceous based on the presence of Middle Jurassic foraminifer-bearing limestone clasts in the limestone conglomerate embedded in equivalent formations. The present chert conglomerate, including radiolarian-bearing chert clasts is interpreted to have been derived from the grained siliceous rock bodies of the Mae Sariang Zone and deposited in an alluvial depositional basin and braided river system during the Late Jurassic to Early Cretaceous.

Experimental study on the spatial fracture behaviour of red sandstone containing weak inclusions with non-unitary geometric features under uniaxial compression

The failure of weakly filled rock masses is an important factor in the safety and stability of rock engineering. To investigate the spatial fracture behaviour of red sandstone with non-unitary geometric weakly filled joints, uniaxial compression experiments were carried out. The 3D DIC and acoustic emission (AE) technology were used to obtain the three-dimensional spatial deformation information and AE parameters of specimens containing both planar and circular weak inclusions, respectively. The main results are as follows: (1) The results show that the ductility of planar weak inclusions has a certain deterioration effect on the bearing capacity of the specimens, and the extent of this deterioration effect is not constant. (2) When the ductile length of the planar weak inclusions increases, the three-dimensional spatial displacement and strain fields of the specimens are divided by the planar weak inclusions, showing different degrees of deformation behaviour and different distribution patterns in the upper and lower parts. (3) The three-dimensional spatial fracture characteristics of specimens with different ductile lengths of planar weak inclusions will be dominated by the ductility, and the main failure mode of the specimen will gradually evolve from shear failure to splitting failure. The “guiding effect” of circular hole weak inclusions on the main crack is no longer evident when the ductility of planar weak inclusions is at its maximum. (4) The growth trend of the cumulative AE counts can be divided into four stages, and the stage characteristics are significantly influenced by the ductile length of the planar weak inclusions; different failure modes show different AE signal distribution characteristics. In addition, the key points of stability control of rock masses containing weak inclusions in practical engineering are discussed.

Ichnology of a Middle Devonian regression: Environment, terrestrialization and true substrate controls on trace fossils of the Hangman Sandstone Formation, SW England

The Eifelian-aged Hangman Sandstone Formation of southwest England is a unit of the ‘Old Red Sandstone’ that has traditionally been considered relatively barren of trace fossils. Following recent investigations, we here show it to contain the most diverse Middle Devonian non-marine ichnofauna known globally. The fluvial-lacustrine facies of its constituent Trentishoe Member contain representatives of 21 ichnogenera including Archaeonassa, Arenicolites, Beaconites, Bifungites, Circulichnis, Cruziana, Diplichnites,?Gluckstadella, Gordia,?Halimededes,?Lockeia, Merostomichnites, Palmichnium, Petalichnus, Planolites, Polarichnus, Rusophycus, Siskemia, Spirophyton, Steinsfjordichnus, and Taenidium, as well as several microbially induced sedimentary structures. The transitional marine-influenced facies of the underlying Hollowbrook Member are less extensively exposed but contain four additional unique ichnogenera in the form of Cochlichnus, Halopoa, Phycodes, and Teichichnus, as well as further examples of Cruziana, Planolites and Taenidium. A revised ichnological assessment of the unit reveals several notable features, including the youngest known example of the arthropod refugia trace, Polarichnus, as well as a transitional trace fossil form of Diplichnites-Beaconites, demonstrating that the latter was produced by arthropods. As the Hangman Sandstone Formation records the culmination of a marine regression, its ichnofauna can be compared with that of the conformably underlying Lynton Formation, which was deposited in a sandy marine shelf setting. The ichnological differences between these units are acute, with 88% of the Hangman Sandstone Formation trace fossils being exclusive to non-marine or transitional facies. This observation sheds light on the terrestrialization process and indicates that the segregation of marine and non-marine trace fossil communities had accelerated by the Middle Devonian. Internal variability in ichnological signatures within the Hangman Sandstone Formation is also pronounced, with greater ichnodiversity and ichnodisparity in distal fluvio-lacustrine facies than in proximal fluvial facies. By viewing the depositional environment of the unit as a regionally extensive distributive fluvial system, this variability can be partly explained by a bias arising from the distribution of true substrates (bedding planes that demonstrably preserve an ancient sediment-air interface) in the unit, with such phenomena being more readily preserved in the lower energy outer reaches of the depositional system.

Damage evolution and failure characteristics of red sandstone with prefabricated crack under coupled dry–wet cycle-fatigue loading

Dry-wet cycle and fatigue loading both severely affect the stability of hydraulic engineering. In this paper, the deterioration process of red sandstone under coupled dry–wet cycle-fatigue loading was investigated from three aspects of deformation, energy, and damage. And the variation law of crack propagation mode of the samples was revealed. The results showed that the elastic modulus and the deformation modulus had a negative correlation with the number of dry–wet cycles, while the maximum strain and the residual strain were the opposite. Coupled damage variable was defined based on porosity and dissipated energy density, and a damage evolution model was established. With the increase of dry–wet cycles, the failure characteristics of the samples evolved from predominantly wing cracks to anti-wing cracks.

Influence of water on rockburst proneness of sandstone: Insights from relative and absolute energy storage

To fully understand the influence of water on rockburst proneness from energy storage viewpoint of rock and to check the performance of energy-related rockburst proneness indexes, several groups of uniaxial compression tests at different stress levels are performed on three types of red sandstone specimens (i.e., the saturated, natural, and oven-dried specimens). The method of combining strain energy calculations and experimental observations is used to assess the rockburst proneness. The progressive failure of the three types of specimens is recorded by a high-speed (H-S) camera, and the rockburst proneness is comprehensively assessed by considering the failure features of the rock specimens (including the fracture sound, fragment ejection, fragment distribution, and far-field ejection mass ratio). The laboratory results reveal that the rockburst proneness of the natural and oven-dried sandstone specimens is similar. However, compared with the natural and oven-dried specimens, the water-saturated specimens exhibit a visible weaker rockburst proneness. It is also evident that the linear energy storage and dissipation laws are applicable to the saturated, natural, and oven-dried specimens. Most interestingly, the energy storage and energy dissipation coefficients are nearly immune to the presence of water in the red sandstone. Based on this, the definitions of absolute energy storage capacity and relative energy storage capacity are adopted to describe the influence of water on the energy storage performance of rocks. It is found that water dramatically weakens the absolute energy storage capacity, but has little and negligible effect on the relative energy storage capacity of sandstone. Considering the direct and indirect indexes, the accuracy of several representative energy-related rockburst proneness indexes is comprehensively compared in terms of the far-field ejection mass ratio, and their performance is discussed from the relative and absolute energy storage aspects. According to their performance, it is strongly recommended to use the direct index for determination of rockburst proneness when energy-related indexes are used.

Subcritical crack growth and fracture behavior of three rocks containing quartz under wet and dry conditions

Understanding the subcritical crack growth behavior of rocks under water-rock interaction is essential to evaluate the stability of deep resource extraction and the integrity of nuclear waste disposal or CO2 sequestration. Using the double-torsion technique, we performed relaxation tests on three rocks with different quartz contents, namely, quartz amphibolite, white sandstone and red sandstone, in air environment and in introduced water conditions to systematically investigate the effect of water on subcritical crack growth behavior of three rocks. The subcritical fracture behavior of the rock responded very rapidly to water: when water entered the crack tip, 1) the crack velocity increased with decreasing of stress intensity factor; 2) the slope of the load decay curves increased abruptly; and 3) the strain values at the bottom monitoring points extracted from three-dimensional digital image correlation increased immediately. Compared with the air environment, the subcritical crack growth indexes of quartz amphibolite, white sandstone and red sandstone in fluid conditions were reduced by 27.08 %, 80.81 % and 86.14 %, respectively, and the crack velocities were increased by one order of magnitude. The Ka values of quartz amphibolite, white sandstone and red sandstone were less sensitive to water than the subcritical crack growth indexes, decreasing by 9.43 %, 17.35 % and 37.33 %, respectively. Data from these tests reveal that the subcritical fracture properties of the three rocks were degraded despite the small amount of water flowing through the sample, proving that the long-term effects of water on actual engineering cannot be ignored.

Hydraulic Fracturing Unlocks Potential of Europe’s Largest Reservoir

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 205250, “Hydraulic Fracturing at Clair: Unlocking the Potential of Europe’s Largest Reservoir,” by Alistair M. Roy, SPE, Graeme H. Allan, SPE, and Corrado Giuliani, SPE, BP, et al. The paper has not been peer reviewed. _ Greater Clair, Europe’s largest oil field, has two existing platforms, Clair Phase 1 and Clair Ridge, on production with the potential for a third platform targeting the undeveloped Lower Clair Group (LCG) to the South of Phase 1. In some areas, however, low matrix quality and lack of natural fractures were the dominant characteristics, resulting in lower production rates. The authors write that fracturing technology brings opportunities to unlock poorer Phase 1 and Ridge reservoir areas. Introduction Reservoir Description. The Clair field is 75 km west of the Shetland Islands on the UK Continental Shelf within the extensional Faroe-Shetland Basin. The Old Red Sandstone reservoir is divided into two lithostratigraphic units, the Upper and LCG. The LCG contains the bulk of the oil in place and underpins the Clair development. The reservoir is characterized by large variations in facies and permeabilities. The LCG is subdivided into six units, I through VI, based on variations in sedimentary facies and heavy mineral assemblages. Development drilling preferentially targets the highest quality reservoirs in Units V and III. The LCG is defined as a dual-permeability system with a variable fracture distribution. Three main scales of fracture features are observed in the core: fault-related, fracture corridors unrelated to large offsets, and lower-density joint-type networks. Open fracture density varies considerably by stratigraphy, with the highest densities in Units V and VI. Field Development Phases. Exploration Well 206/08-1A discovered Clair in 1977. Clair underwent a protracted appraisal program before sanction of the Phase 1 development in 2001. The commercial viability of the reservoir was not confirmed until an extended well test in 1996 proved that oil rates from 15,000 to 20,000 BOPD could be sustained from long horizontal wells targeting high-productivity fracture zones. The scale and complexity of the reservoir drove a phased multiplatform development. Phase 1 began production with 20 producer and injector wells coming online between 2005 and 2011. Five additional wells drilled during 2016–2017 increased platform production by 70%. The second platform, Clair Ridge, came online in 2018, and the development wells drilled to date have encountered varying degrees of natural fractures. Design and Characteristics of Well A23 for Hydraulic Fracturing Candidate Well for Propped Fracturing. Horizontal Well A23 was completed in March 2017. The lateral penetrated the low-permeability unfractured Unit VI of the LCG in the Graben area of Phase 1. It crosses the core/graben fault, which is assumed to act as a baffle to communication between Zones 1 and 2 and Zones 3 and 4. Well A23 was put on gas lift production in 2017, producing approximately 1,300 BOPD. The well trajectory was designed to remain 40 m above the shale layer and to facilitate roughly transverse fractures within Unit VI Upper by drilling perpendicularly to the maximum horizontal stress to overcome vertical drainage issues.

Jurassic system stratigraphy at the Kamianka village. Part 1. The Kozhulinska formation

Introduction. Despite the active research of Jurassic system deposits of Donbas during the last two centuries the issue of their stratigraphy principles is still debatable. The authors` analysis of the available scientific material revealed a conflict between the normatively established requirement to rely on lithostratigraphy during the sediment research, and the fact that the existing common system relies on biostratigraphy. This contradiction could be solved by the study of lithostratigraphic units of the Donetsk Jurassic deposits. Therefore, the purpose of the article was to research the Kozhulinska formation, establish its composition and describe its main features. The Lower Jurassic deposits from the outcrops near the village of Kamyanka (Izyum district) were studied, since the studyable stratotypes of most of the Jurassic strata, which are well exposed in the studied area, are available in this area. Material and methods. The scientific search was based on the analysis of literature, results of previous drilling and mapping works, as well as own field, laboratory and analytical studies. Clay samples were taken from different stratigraphic levels of the formation and from different lithological varieties. Laboratory research was carried out in two directions: using an electron microscope and a spectrometer to detect the material composition of clays. A palynological analysis of these deposits was also carried out, namely, stratified deposits with the help of microphytofossils, including, for the first time, according to the orthostratigraphic group – dinocysts. Results and discussion. The results showed that the Kozhulinska formation was mainly represented by shaly clays (gray, dark gray, sometimes blue-gray) that contained relatively thin layers of sandstones. The main mass of clays consisted of hydromicas, with an insignificant content of montmorillonite, and an even smaller amount of kaolin. The layers of quartz sandstones cemented with clay material were separately traced along the strike. Paleontological remains in the clays of Kozhulinska formation were rare. Palynological groups are mainly represented by pollen and spores, as well as remains of dinophytes, green algae and acritarchs. According to paleoalgological analysis, the sediments of the Kozhulinska formation are dated for the first time. Stratigraphy. Kozhulinska formation is represented by a fairly uniform layer of montmorillonite clays of hydromica composition. The clays are mainly siltstone, but there are, especially in the lower part, washed clean varieties. It is impossible to divide the formation into sub formation based on lithological features. According to biostratigraphic data, deposits are divided into Plinsbachian, Toarian and Aalenian. Paleogeography. At the end of the Early Jurassic and the beginning of the Middle Jurassic era, the territory of the Kamianska area was a low-lying part of the seabed that was drained during low tides. The land that surrounded it was also low-lying and almost did not supply detrital material, however, there is a regular removal of plant detritus. The source of the terrigenous material was the sedimentary complexes of the Donetsk Ridge. Features of the chemical composition of the world are a significant amount of iron (siderite, ocher, chamosite) and the absence of calcium and magnesium carbonates. At the end of the early Aalene, this unstable basin finally regresses, which leads to the accumulation of a powerful subaerial pack of sandstones, which weigh and border the cores of anticlines - elevated areas of the bottom or small islands.

English

Mining nonmetallic sand, gravel, and river stones in a river channel has previously caused damage to the river channel itself and the surrounding environment. Technical studies are needed to minimize the damage in mining in the river channel. Ongkag Dumoga River channel is planned to be the location of mining non-metal material for sand, gravel and river stone. The research method was carried out by geodesic mapping to determine the profile of the river cross-section, hydrological analysis with a return period of 25, 50, and 100 years and hydraulic analysis on the river flow along the 600 m. The design of the post-mining river technical profile is carried out to improve the capacity of the existing river cross-section. Changes in river profile are the potential volume of nonmetallic material in the form of sand, gravel and river stones that become river sediment. The results of the analysis of flood discharge HSS-Snyder method get a discharge of 1249.1692 m³ / s at the 100-year return, 1178.8064 m³ / s at the 50-year return period, 1104.2372 m³ / s at the 25-year return period. The simulated flood discharge shows a flood overflow in the existing river during the 25, 50 and 100-year floods. The new technical river cross-section will use the economic cross-section of the trapezium cross-section. Changes in river profiles are calculated to get a potential excavation volume of 88,347.42 m³.

Thermal Infrared Precursor Information of Rock Surface during Failure Considering Different Intermediate Principal Stresses

Rock failure generally leads to serious consequences, and it is significant to obtain the precursor information prior to failure using associated techniques. Thus, it is essential to acquire and probe the relevant precursor information. In this study, true triaxial tests are performed on red sandstone specimens under varying intermediate principal stress conditions. The thermal infrared image evolution and the temperature-induced change characteristics of rock failure are also analyzed using infrared thermal imaging technology. In addition, with the assistance of a high-speed photography technique, these characteristics during the true triaxial compression and unloading processes are systematically investigated to determine how the intermediate principal impacts on thermal image, temperature, and fracture propagation. Finally, the evolution mechanism of the specimens is summarized, and a non-contact thermal infrared rock failure precursor indicator is proposed, which can give significant advance notice of rock collapse before the abnormal temperature change. The results show that there exist thermal infrared temperature precursors, thermal image precursors, and rapid development of rock macroscopic cracks before rock failure. Abnormal thermal images are prior to the abnormal temperature changes. As the intermediate principal stress increases, thermal abnormalities will change accordingly. Both temperature changes and thermal image anomalous patches can be utilized as precursor information of rock collapse, and the mechanism and specific information of thermal infrared failure precursors can be preliminarily determined in time and space. Our results can function as a significant frame of reference for the analysis and prevention of rock failure due to sudden instability.

Engineering Characteristics and Application Analysis of Red Sandstone in the Lanzhou Metro

The water-rich red sandstone strata at the Lanzhou Metro site area have special engineering properties and vary greatly in their speed of disintegration when exposed to water. There is an urgent need for a comprehensive and systematic study of the engineering properties of red sandstone and their classification. From the disintegration speed of red sandstone encountered during the excavation of Lanzhou metro lines 1 and 2, the relationship between physical parameters such as particle size, composition, dry density, and permeability coefficient as well as mechanical parameters such as shear wave speed, dynamic penetration test (DPT), natural uniaxial compressive strength, and disintegration speed of red sandstone was analyzed through indoor and outdoor tests and geological exploration data statistics, and classification guidelines for red sandstone are given. The results show a significant correlation between dry density, permeability coefficient, natural uniaxial compressive strength, and disintegration speed. The red sandstone can be classified as I, II, and III according to the disintegration speed, dry density, permeability coefficient, and natural uniaxial compressive strength. The design of the foundations is differentiated according to the classification, and different support systems are used for the deep foundation pits of the metro stations. The category I red sandstone pit is supported by diaphragm walls with internal bracing, the category II pit by bite piles with internal bracing, and the category III pit by row piles with internal bracing. The study results can provide technical support and experience reference for the investigation, design, and construction of metro projects in similar red sandstone distribution areas.

Provenance and Paleoclimate Analysis of Sandstone from Meluhu Formation in Kendari, Southeast Sulawesi.

The Meluhu Formation consists of quartz sandstone, red shale, siltstone, and mudstone at the bottom part, and alternating black shale, sandstone, and thin limestone in the upper part. Petrographic components composition can reveal the litho-tectonic setting of a region. The composition of sandstone can be done by determining and counting the proportion of a quartz minerals, feldspar, and rock fragment from thin sections using petrographic analysis. This result can guide to depositional setting and paleoclimates of a region. The Provenance and paleoclimates studies of Meluhu Sandstone are still relatively new. The purpose of this study was to find out the provenance and paleoclimate of sandstone of Meluhu Formation which is exposed in 14 sites along Kendari and adjacent area. The method used to make petrographic observations followed by data processing using the point counting method. The sandstone in this study area was classified into sublitharenite, lithic arenite, arkose, subarkose, and greywacke. The origin of rock was interpreted as located in the recycled orogenic province, interior craton, and undissected arc. The paleoclimate that occurred in the formation of these siliciclastic rocks was arid to humid. Keywords: Meluhu Formation, provenance, paleoclimate, sandstone.

Experimental investigation of temperature effects on the physical and mechanical properties of red sandstone

With the rapid development of transportation in China, the number of tunnels is also increasing. However, tunnels may be affected by fire accidents during operation, and the surrounding rock will be irreparably damaged by high temperature. To investigate the influence of temperature on the physical and mechanical properties of red sandstone, physical properties were determined and the Brazilian splitting test was conducted on red sandstone after thermal treatment from 25 to 600°C. The results show that with increasing temperature, the apparent colour of the red sandstone gradually deepened. The P-wave velocity and Leeb hardness decreased with increasing temperature, and a typical temperature threshold (300°C) was identified. The tensile strength of red sandstone decreased quickly from 25 to 300°C and then decreased slowly from 300 to 600°C. The acoustic emission (AE) signal shows a high degree of consistency with the stress–time curve. As the temperature increased, the AE became more active, and when red sandstone was near failure, the AE signal increased sharply. The failure mode of sandstone mainly consists of a throughgoing main crack and a secondary crack, and the crack width also grows gradually. Moreover, we observe from polarized light microscopy images that the number of fractures in the sandstone also increases as the temperature rises. The variation of physical and mechanical properties of red sandstone is closely related to microstructure. These findings demonstrate that temperature has an obvious weakening effect on the physical and mechanical properties of sandstone, and provide theoretical guidance and significant engineering information for tunnel fire restoration. Thematic collection: This article is part of the Engineering Geology and Hydrogeology of the Anthropocene collection available at: https://www.lyellcollection.org/topic/collections/engineering-geology-and-hydrogeology-of-the-anthropocene

Tight Sandstone Reservoir Characteristics and Controlling Factors: Outcrops of the Shanxi Formation, Liujiang River Basin, North China

Tight sandstone reservoirs are of interest due to their potentially favorable prospects for hydrocarbon exploration. A better understanding of tight sandstone outcrop reservoir characteristics and their influencing factors is thus needed. By laboratory observation, thin section analysis, and experimental analysis, the current work carried out a detailed investigation of densely sampled tight sandstone outcrops of the Shanxi Formation in the Liujiang River Basin, paving the way for further research on rock types, reservoir spatial distribution, physical properties, and their key controlling factors. The application of the Pressure Pulse Attenuation Method made it possible to determine the porosity and permeability, as well as the analysis of debris composition and filling content. The findings indicate that the main rock type of the tight sandstone outcrop reservoirs in the Shanxi Formation in the Liujiang River Basin is lithic quartz sandstone, some of which contains fine sand-bearing argillaceous siltstone, giving them very low porosity (average porosity of 4.34%) and low permeability (average permeability of 0.023 mD) reservoirs. Secondary pores—mostly dissolved pores among and in grains—are widely developed in the target region. In addition, diagenesis primarily includes mechanical compaction, cementation, and dissolution. The main controlling factors of tight sandstone reservoirs in the target region are sedimentation, diagenesis, and tectonics, whereby sedimentation affects reservoir physical properties that become better as the clast size increases, reservoir properties are negatively impacted by compaction and cementation, and reservoir properties are somewhat improved due to dissolution and the impact of tectonism. In addition, the tilt of the crust will produce faults during the tectonic action, generating reservoir cracks that improve the reservoir’s physical properties. This study tends to be helpful in the prediction of high-quality reservoirs in the Permian Shanxi Formation in North China and can also be used for analogy of high-quality reservoirs in similar areas with complete outcrops.

Identification of the Potential Quartz Sandstone in the Sambong Area By the Dipole-Dipole Configuration of Resistivity Method

The presence of quartz sandstone outcrops in the southeast of the study site suggests the presence of quartz sandstones in the northwest, according to the strike direction. This geoelectrical measurement aims to determine the distribution of these sandstones below the surface. The method used is geoelectric with a dipole dipole configuration.
 Measurements were made on four track lines. Tracks A and B indicate the existence of a layer of sandstone below the surface. Tracks C and D do not show any layers of quartz sandstone. This is caused by the presence of faults or folds that cause the layer to shift from the previously predicted path.

Experimental Study on Mechanical Properties and Deterioration Mechanism of Red Sandstone from the Panjiatuo Landslide under Action of Acidic Drying−Wetting Cycles

Due to frequent water level fluctuations and complex hydrochemical environments, rock slopes in reservoir areas progressively deteriorate and become unstable. This study investigated the coupling effect of drying−wetting cycles and acidic solutions on the physical and mechanical properties, strain field evolution, failure mode, and micro-mechanism of red sandstone using a series of laboratory experiments (wave velocity tests, uniaxial compression tests, the digital image correlation method, scanning electron microscopy, and X-ray diffraction). The results showed that with increasing drying−wetting cycles, the mass, P-wave velocity, elastic modulus, and uniaxial compressive strength decreased monotonically, while the water absorption and apparent strain in the strain localization band increased. Moreover, the failure mode transitioned gradually from tensile failure to shear failure or tensile-shear composite failure. The decrease in the solution pH values aggravated the changes in the physical and mechanical parameters and contributed to an increase in the secondary cracks and the occurrence of shear behavior. In addition, the coupling effect of drying−wetting cycles and acidic solutions accelerated the worsening of the microstructure and the dissolution of minerals, resulting in a loose structure with well-developed pores and fissures. These changes provide a favorable explanation for the mechanical property deterioration of red sandstone subjected to acidic drying−wetting cycles.

Acoustic emission precursor information of rock failure under true triaxial loading and unloading conditions

Rock failure generally leads to serious consequences, and it is significant to obtain the precursor information prior to failure using associated techniques. Acoustic emission (AE) is one of the indispensable methods for disaster warning of hard and brittle rock. Acoustic emission detection technology can effectively monitor real-time information about changes in the rock interior and predict the process of rock damage failure. To probe the relationship among the AE precursor information of red sandstone under different intermediate principal stresses, an experimental study was conducted by us to examine the alterations in AE parameters during the failure of red sandstone under both loading and unloading circumstances. The study shows that the ringing count rate and absolute energy versus time curves are divided into four stages, namely, quiet, frequent, sudden increase and decline periods. The cumulative count curve is also divided into four phases: pre-unloading period, post-unloading period, sharp increase period, and decrease period. With the rise of the intermediate principal stress, the ringing count rate and energy exhibited during the frequent period of AE demonstrate a consistent increase, with a larger increase in the maximum value and a smaller increase in the average value. In addition, the peak value of AE signals during failure also increases accordingly. The occurrence moment and clarity of the frequent period determine the reliability and priority of the information related to the rock’s failure precursor; moreover, the reliability and priority of the AE precursor information will increase with the increase of the intermediate principal stress. After comparison, it is found that the AE precursor information occurs prior to the thermal infrared precursor information.

Rock fragmentation and drilling experiment of electric impulse drilling and structural optimization of electrode bit

Due to the low rate of penetration (ROP) and high cost of natural resource development projects such as oil and gas, and mining, it's urgent to explore some new rock-breaking methods with subversive characteristics for rising ROP. Electric impulse drilling (EID) has attracted widespread attention due to its unique advantages, but its rock-breaking effect is affected by various factors such as the structures of electrode bits, pulse voltage characteristics, liquid medium type, rock type, circulation conditions, and the contact between electrode bit and rocks, etc, leading to the lack of theoretical basis for the design of the electrode bit. Therefore, to more truly reflect the interaction among electrode bits, rocks, and liquid media of EID, this study carried out rock fragmentation and drilling experiment of EID on red sandstone. Firstly, an electrical impulse generator (EIG) with a stable frequency range of 1–5 Hz and a pulse voltage of about 0–80 kV at the load terminal was developed. And an indoor EID and rock fragmentation experimental system was built, then three kinds of electrode bits with typical structures: petal-type (H type), cross-type (S type) and, coaxial type (T type) were designed. Subsequently, the rock fragmentation and drilling experiment of EID was carried out on red sandstone in three liquid media (tap water, deionized water, and hydraulic oil) under the capacitors' charging voltage of 13–17 kV (the load terminal voltage was about 45–60 kV). The general influence law of electrode bit structure, liquid medium type, circulation condition, and pulse voltage on EID was studied. Finally, the electrode bits used in the experiment were evaluated, and suggestions for structural optimization of the electrode bit were put forward based on the experiment. This study is expected to further promote the design of electrode bits and the development of EID technology.