Parallel Bedding Research Articles

Influences of hydraulic fracturing on microfractures of high-rank coal under different in-situ stress conditions

The influence of hydraulic fracturing on coal microfractures (10–1000 μm) is an important part of the stimulation mechanism of coalbed methane (CBM) hydraulic fracturing, which is of great significance to improve the productivity of CBM. In this work, high-rank coal samples from Chengzhuang and Sihe Mine were selected for hydraulic fracturing simulation experiments, using stereoscopic microfracture scanning and binarization image processing methods, the mean aperture, surface density, connectivity, mean length and fracture porosity of microfractures were compared and analyzed under different in-situ stress conditions before and after hydraulic fracturing, and the permeability changes were calculated. It is found that hydraulic fracturing could increase the permeability of microfractures in coal seam significantly, and the permeability of microfractures after hydraulic fracturing was 0.43–14.82 times that before fracturing. Hydraulic fracturing did not induce new microfractures, but only the expansion of the original microfractures, which caused the mean aperture, mean length and fracture porosity of microfractures to increase by 39.85%, 47.70% and 115.59%, respectively. Stronger heterogeneity and larger horizontal in-situ stress difference will inhibit the expansion of microfractures. Under the same hydraulic fracturing conditions, there are larger interlayer differences for coal samples with stronger heterogeneity, and hydraulic fracturing will prioritize microfractures in the direction of parallel bedding planes. In addition, the smaller the horizontal in-situ stress difference, the microfractures expand uniformly during hydraulic fracturing, which is more conducive to the connection of microfractures.

The Yunmengshan iron formation at the end of the Paleoproterozoic era

Precambrian iron formations (IF) carry rich information on the interplay between the geosphere and the early biospheres. Here, we report mineralogical and petrologic characterizations of the Yunmengshan IF deposited at ~1.7 Ga, the transition between Paleoproterozoic and Mesoproterozoic eras. The IF contains granular hematite with evident oncoidal structures and lacks laminated structures, which are distinct to the typical banded iron formation (BIF) or granular iron formation (GIF) deposited early in the marine environments with high silica content. Based on observations of clastic sedimentary structures around the Yunmengshan IF, including parallel bedding, cross-bedding, asymmetric ripple mark, and mud crack, we deduce that the Yunmengshan IF was mainly deposited in a supratidal to intertidal zone. Electron microscopic observations show two types of hematite euhedral crystals: laminar hematite of 200–500 nm thick and 3-5 μm in diameter and granular hematite of 200–300 nm in diameter randomly distributed in the matrix. Stilpnomelane and minnesotaite, as Fe2+-silicates in the typical Archean- and the early Paleoproterozoic BIF, are identified but with extremely low abundance. The morphology and paragenetic association of these minerals imply that the Yunmengshan IF was probably deposited in an aquatic environment with low Si concentration. These results indicate that the Yunmengshan IF represents a transitional type of iron deposition between BIF/GIF and ironstone in the geological history of the iron cycle on the surface of Earth.

Shale Brazilian split test and numerical simulation of the discrete element method

This study performs a shale Brazilian split test by setting the loading and bedding directions at different angles to study the effect of anisotropy caused by the bedding structure and heterogeneity caused by different particles on the mechanical properties of shale. The results indicate that the cracks of the specimens splitting and breaking at 0° and 90° all extend vertically through the disk. A combination of Neper and 3DEC software is applied to the numerical simulation study of the shale splitting process through the discrete element method. The shale splitting failure modes are divided into three categories. At 0° and 90°, the cracks extend linearly through the loading ends, and the models show bedding and particle tensile failures, respectively. At 15°, the cracks run diagonally through the rock sample from the end of the upper loading section, parallel to the bedding direction. At 30° to 75°, the cracks propagate from the loading end along the parallel bedding direction. The experiments and numerical simulations verify that bedding influences the shale splitting failure mode and the mechanical behavior. Therefore, the investigation of the mechanical problems caused by the destruction of shale gas mining has a far-reaching significance.

Validity of series and parallel bed layer permeability equations from experimental and numerical modeling

It is rare to encounter a homogeneous reservoir in nature. More formations have a space variation of permeability than porosity and difficult to measure. Yet adequate knowledge of permeability distribution is critical in predicting reservoir depletion or by-pass hydrocarbon recovery. Average method is used to determine the average permeability of series (or vertical) and parallel (or horizontal) beds with different permeability. We used the experimental results to verify the accuracy of existing model. The Computed Permeability (CP) results vis a vis numerical method were compared with Experimental Permeability (EP) and it was found that there is no significant change between them for Series Beds Layered (SBLP). However, there is a significant change for Parallel Beds Layered (PBLP). The CP is greater than EP for the PBLP by a multiple of 1.534. This was taken as Correction Factor (CF) to obtain a modified average PBLP equation. This modified equation is more accurate for computation of scaling up in reservoir flow modeling.

Diagenetic evolution and associated dolomitization events in the middle Jurassic Samana Suk Formation, Lesser Himalayan Hill Ranges, NW Pakistan

The Jurassic carbonates of the Samana Suk Formation are extensively exposed in the foreland areas of Himalayas and form major reservoir of the upper Indus basin. These carbonates are composed of oolitic, pelitic fossiliferous and micritic limestone units which have been extensively modified by diagenetic alterations, particularly dolomitization. Field observations show two distinct types of dolostone geobodies (i) bedding parallel stratiform, and (ii) patchy dolostone units respectively. Bedding parallel stratiform dolostones are present in the basal part of the formation, while patchy dolostones are present at the middle and upper parts of the Samana Suk Formation. The dolomitization intensity of both geobodies increases from NW to SE in the study area. Petrographic studies reveal six phases of dolomites and three phases of calcites based on texture, crystal size and morphology. These phases are: matrix replacive dolomites (MD-I to MD-III); cementing dolomites include, replacive cementing dolomite (RD), saddle cementing dolomite (SD) and late stage cementing dolomites (CD); and calcite phases include CC-I and CC-II. XRD analyses reveal that stratiform matrix dolomites (MD-I, MD-II) are stoichiometric (51.08–51.86 mol percent of CaCO3) and contain up to 95% of the mineral dolomite. The patchy dolomite cement is non-stoichiometric (33.39–55.08 mol% of CaCO3) and contains around 65% of the mineral dolomite, whereas saddle dolomites is also non-stoichiometric (51.57 to 53.50 mol% of CaCO3) in origin. Stable isotope studies reveal non-depleted δ18O and δ13C values of matrix dolomites (MD-I, MD-II) represents coeval sea-water signatures of Jurassic carbonates, hence may have been formed by evaporative process. Dolomite cements (RD, SD) shows depleted δ18O values which represent elevated temperature, related to hydrothermal fluid source for their formation. The fracture filling calcite (CC-II) exhibits less depleted values indicative of meteoric fluids affected by shallow to moderate burial. The dedolomites shows depleted δ13C values suggests their formation from the meteoric water. Field, petrographic and geochemical studies suggest that diagenetic evolution of the Samana Suk Formation is the multistage process. In the first phase, marine diagenetic processes including marine cementation, stratiform dolomitization may have formed due to surface processes of marine water in peritidal to intertidal settings, while the second phase of diagenesis is due to burial associated processes which includes hydrothermal dolomitization occurred due to movement of magnesium rich fluids along weak planes such as fractures, faults, bedding planes and stylolites. Last stage includes formation of hydrothermal fracture filling calcites, replacive pyrites and dedolomites due to the uplift related processes.

Signs of Polychronous Deformations in Carbonate Rock Microstructures of the Katav–Yuryuzan Transpression Zone (Southern Urals)

The Katav-Ivanovsk transpression zone experienced at least two stages of tectonic deformations and the sequence of deformations was approximately the same throughout the entire zone from the Bakal–Satka fault in the south to the Suleya fault in the north. Three stages of the development of structural parageneses were identified. The parageneses of the first and the second stages were formed in a pure shear environment, while the paragenesis of the third stage formed in a simple shear environment. Stylolites (S1) occur, arranged parallel to the bedding, and mineral veins (V1) in the paragenesis of the first stage. The paragenesis of the second stage includes stylolites (S2), mineral veins (V2), and intergranular cleavage (S2). The paragenesis of the third stage includes schistosity (S3), mylonites (S3), cataclasites, mica sheets (SC-structures), and the rotation structures of porphyroblasts.

Impact of Aqueous Phase Trapping on Mass Transfer in Shales with Multiscale Channels

Large amounts of fracturing fluid retained in a shale reservoir could not only restrict gas production through aqueous phase trapping (APT) but also cause potential contamination of groundwater. In this work, experiments modeling in situ aqueous imbibition and flowback were conducted to quantitatively investigate the APT behavior in shales, including matrix, natural fracture, and artificial fracture. Results show that the forward imbibition process is slower than the reverse imbibition process. The aqueous phase tends to be imbibed through parallel bedding. Permeability reduction rates of matrix, natural fracture, and artificial fracture cores after imbibition could reach 100, 85, and 70%, respectively. Meanwhile, it is demonstrated that the aqueous flowback efficiency depends upon flowback timing, pressure difference, initial permeability, and bedding direction. The flowback efficiencies of matrix, natural fracture, and artificial fracture cores could be less than 7%, less than 30%, and less than 15%, respectively. Additionally, the permeability recovery rates of those could be less than 19%, less than 39%, and less than 67%, respectively. Finally, shale APT mechanisms are analyzed from both geological and engineering aspects. The quantitative investigation of shale APT is conducive to economically and environmentally developing shale gas reservoirs.

Static and dynamic mechanical properties and deterioration of bedding sandstone subjected to freeze\u2013thaw cycles: considering bedding structure effect

The bedding rock widely exists in nature and its mechanical properties are complex. In this study, the Φ100 mm split Hopkinson pressure bar (SHPB), freeze–thaw(F–T) cycle test system joint with scanning electron microscope and other facilities are applied to investigate the static characteristics, impact characteristics, and damage microstructure of the bedding rock under freezing and thawing conditions. Our experimental results show that under the F–T cycle conditions, the peak point deteriorating path of the static stress–strain curve and the post-peak strain softening curve of the vertical and parallel bedding sandstone specimens have obvious anisotropic characteristics. Parallel bedding specimens have a “pressure bar” effect when loaded. Under the dynamic mechanical test, the peak stress of the vertical bedding specimen is always larger than that of the parallel bedding specimen, and the difference between the two becomes larger while the impact velocity increases. Finally, our microscopic analysis indicates that the main reason for the formation of fissures in the bedding sandstone under the F–T cycle is the cracking of the cement and the shedding of the mineral particles, while the fracture of the mineral particles rarely occurs. The results can provide theoretical guidance for geotechnical engineering in alpine regions.

Revisiting the “Gercus Basalt within the Eocene Gercus Formation”: Implications for sedimentary origin of a basaltic body, Kurdistan Region, Northern Iraq

• A previously described intrusive basaltic body is re-studied. • The re-study proved that it consists of sedimentary rocks (greywacke sandstone). • The greywacke transported from remote volcanic source to its present area. • More than ten evidences are found to prove its sedimentary origin. • These evidence are such as content of bitumen, conglomerate imbricated pebbles. • Other evidences are presence of graded beds, parallel and planer bedding and ooids. • More evidences are absence of basaltic flows and chilled borders and others. Recently, a basaltic body is described geochemically and mineralogically by previous authors within the Gercus Formation, in the Bekhair Anticline (Duhok Governorate, Northern Iraq). They indicated feldspar, anorthoclase, diopside, forsterite and olivine as main minerals of the body with many accessary ones. They added that the body is anorogenic (non-tectonic), extruded on continental crust of Arabian Plate and affected by pervasive alteration with a thickness of 16 m and a width of 4 km. The present study discussed in detail the sedimentary origin of the claimed basaltic body, contesting its intrusive or extrusive igneous origin. We proved that the body consists of a volcaniclastic succession (greywacke), which was derived from remote volcanic source areas and deposited by running water in the basin of the Gercus Formation. These sediments had been transported from a northeastern source area toward southwestern by streams to the deltaic basin of the Eocene basin. For proving its sedimentary origin, we presented many field and petrographic evidences such as content of bitumen, ooid bed, thick or thin planar layers (with parallel and sharp contacts), graded bedding, conglomerate, imbricated pebbles and hosting limestone beds in addition to absence of contact metamorphism, lack of structures such as pillow lava, basaltic flow, crystals zoning, xenoliths, peperites, digitation into host rocks and dilatations features. The previous authors depended on the geochemical and thin section studies for proving its igneous origin but these methods cannot prove if the constituents (whole or broken minerals) of the body are transported or indigenous. While accurate field survey and boundary conditions studies can indicate its origin. The proof of the sedimentary origin is achieved via conjugating evidences of the body boundary, those from its internal architecture and composition. Therefore, we are sure 100% that the body is sedimentary succession not basaltic one.

Formation and re-orientation of the Suerkuli Basin within the Altyn Tagh in northeastern Tibetan Plateau since late Miocene

The Altyn Tagh Fault (ATF) had formed in Eocene time, however, the temporal evolution of slip along the fault remains elusive, despite its important role in governing growth of the Tibetan Plateau (TP) and evaluating models of continental lithosphere deformation. Here we perform a high-resolution magnetostratigraphic study on Neogene sediments from the Suerkuli Basin, a small basin located between two active strands of the ATF. Results show that both depositional environment and the sedimentation rate changed at about 5.4 and 6 Ma, respectively. Lithofacies shift from conglomerates and sandstones with parallel and cross beddings to mudstones with horizontal bedding, suggesting a sedimentary fluvial to lacustrine depositional environment shift. Even aridification increased over northern China, from the Tarim Basin to the Qaidam Basin, during late Miocene to early Pliocene times, lacustrine deposition had started in the Suerkuli Basin still. We argue that an increase in slip rate along the ATF triggered concurrently forming of the intermontane basin. The inferred timing of increased slip-rate on the ATF coincides with a variation in movement of the Indian Plate relative to Eurasian Plate since Late Miocene-Early Pliocene time. Our work suggests that a re-orientation of crustal shortening and deformation in the northern TP began since late Miocene and supports the idea that the modern boundary of the TP orogen had rejuvenated to adjust deformation in plate boundary conditions.

Study on mechanical properties and wellbore stability of deep brittle shale

As the drilling depth of shale's oil and gas wells increases worldwide, the problem of borehole collapse and instability becomes increasingly serious. Due to the high content of brittle minerals in deep shale, the complex situation of an underground collapse of a site is often manifested as borehole wall caving and falling. On the basis of the deep shale of China's Longmaxi formation, we developed a simulation experiment involving the complex loading mechanics of underground working conditions. The influence of loading and unloading confining pressure speeds and the sealing properties of drilling fluid on the mechanical properties of rock were analyzed. We established the dynamic changes in borehole formation, combining seepage and pore pressure, the effective stress field around the well, and a method of evaluating wellbore stability in deep shale gas horizontal wells. The results show that the deep shale of the Longmaxi formation has a high brittle mineral content and is prone to brittle fracturing along parallel bedding surfaces under downhole pressure. The effective liquid column pressure at the bottom of the wells change noticeably under different working conditions. Changes in bottom hole fluid column pressure are the cause of the complicated situation of well leakage and overflow in the horizontal sections of the Longmaxi formation. The effective support confining pressure of borehole wall rock differs with different tripping out speeds. The faster the tripping out is, the less the support confining pressure of the wellbore rock and the easier it is to induce the rock to break down along the bedding. The effective sealing property of drilling fluid also affects wellbore stability. A good sealing property of drilling fluid can improve the supporting effect of the fluid on borehole wall rock and improve its resistance strength. On the whole, the pressure of the bottom hole liquid column is effectively controlled during drilling in the Longmaxi formation; this can avoid complicated situations such as well leakage. The results of this study provide a theoretical basis for the study of wellbore stability in deep shale formations during the drilling process.

The first aspinothoracid arthrodire from the Late Devonian continental strata are found in western Junggar, Xinjiang

Arthrodira is the most widely distributed and diversified subgroup of Placodermi, which itself is the most diverse major group of the Devonian jawed vertebrates. Arthrodires were traditionally divided into the basal, paraphyletic “Dolichothoraci” and the highly nested monophyletic Brachythoraci, the latter of which displays reduction of body armor. Among brachythoracids, the Coccosteomorphi and Pachyosteomorphi further form a nested monophyletic Eubrachythoracidi. The sister group relationship between Coccosteomorphs and Pachyosteomorphs has been supported by most phylogenetic analyses, but the basal members and their interrelationships in these two groups are still unstable. Coccosteomorphs and pachyosteomorphs are the most diverse arthrodires in the Middle and Late Devonian. Among them, coccosteomorphs are found in strata of the lacustrine and littoral-neritic facies, while the pachyosteomorphs are mostly marine. Selenosteidae, one of the most well-known families in Pachyosteomorphi, are found in marine strata of Europe, North America, and North Africa. Junqing Wang found the nuchal plate of suspected Selenosteidae in the Xiejingsi Formation (early Famennian, Late Devonian) in Hunan Province, China, but due to the lack of identification features of the material, its exact systematic position awaits further determination. The Zhulumute Formation is a lithological unit in western Junggar, Xinjiang of northwest China and is mainly composed of conglomerates, pebbly sandstones and medium- to coarse-grained sandstones, with typical braided stream lithofacies. Channel-fill deposits are thick, with well-developed parallel bedding, scour structure, and large-scale trough, wedge and tabular cross-beddings. This part contains a large number of plant stem fossils in fragment. The fine-grained floodplain deposits are thin, with horizontal beddings and small sand ripple beddings; fine fragments of plant stems or leaves can be seen in the siltstones. These characteristics indicate that the Zhulumute Formation in the Bulongguoer area was formed in the braided stream environment. At the same time, according to the preserved plant fossil Leptophloeum rhombicum , its age is limited to the early Famennian. Here we report on a new selenosteid genus Bulongosteus , collected from the upper part of the Zhulumute Formation. The diagnostic characters of the new genus include: (1) A triangular nuchal plate with convex front edge, concave back edge and bilateral micro concave, and the side edge is overlaid with paranuchal plate; (2) the posterior margin of the paranuchal plate longer than that of the nuchal plate, and two sensory tubes were developed; (3) the preorbital plate enlarged to form the anterolateral margin of the orbit, with dense tumor spots on the surface; (4) the anterior edge of the marginal plate thickened to form the posterior margin of the orbit, and the three sensory tubes in the shape of “Y”; (5) a slender posterior ventrolateral plate. Pachyosteus , Rhinosteus , Melanosteus , and Walterosteu are similar to Bulongosteu s in Selenosteidae. In Rhinosteus , there are a few tumor spots on the surface of the bone and the distribution of the sensory tubes of the paranuchal plate is different from Bulongosteus . The posterior margin of Walterosteu is wider than Bulongosteus , and the distribution of the sensory tubes of the paranuchal plate is different. The characteristics of Pachyosteus and Melanosteus are the most similar to Bulongosteus , but the distribution of the sensory tubes in their paranuchal plate is different from Bulongosteus . And Pachyosteus has less tumor spots in the nuchal plate and has a trapezoid rostral plate. Melanosteus has some tumor spots in the preorbital and suborbital plate. Bulongosteu s is the first arthrodire reported from Xinjiang of northwest China, and also the first relatively complete aspinothoracid reported in China. Strikingly, Bulongosteu s is the first aspinothoracid arthrodire found in the continental strata in the world. The discovery of the new material significantly expands the geographical and ecological distribution of aspinothoracid arthrodires, and also shows that aspinothoracids, like many fishes in the Late Devonian, are euryhaline.

NMR study on pore structure and permeability of different layers of deep low-rank coal

ABSTRACT The difference in the development of coal pore system in bedding direction is of great significance to the exploration and development of coalbed methane and the prevention and control of gas disasters. In order to improve the understanding of the fluid storage and transportation rules in the development of horizontal wells for coalbed methane, the typical low-rank coals in western China are taken as the research object. Based on the NMR experiments, the effective pores, pore size distribution and pore connectivity of low-rank coals in parallel and vertical bedding directions are analyzed. Meanwhile, the reason for the difference in pore structure of parallel and vertical bedding coal is also explained. The results obtained are as follows: (1) All types of pores in low-rank coal are developed. The proportion of T 2 spectrum area of large pores in parallel and vertical bedding coal samples is 55% and 43%, respectively. After centrifugation, the T 2 spectral area of large pore peaks in parallel bedding coal samples reduced more than 85%, and the vertical bedding was reduced by 80%, indicating that the large pores in the coal sample were developed and the pores communicated with each other and had good connectivity, and most of the water escaped from the medium and large pores. (2) The total porosity, effective porosity, and movable fluid saturation of the parallel bedding are all larger than the vertical bedding, and the parallel bedding T 2 C is less than the vertical bedding at about 1 ms, indicating that the permeability of the parallel bedding is better than the vertical bedding. (3) The hole was squeezed more seriously in the vertical bedding direction, and the micro-cracks extended along the bedding direction. However, the poor continuity in the vertical bedding direction is the main reason for the difference in coal structure between the parallel bedding and vertical bedding directions.

Radial Permeability Measurements for Shale Using Variable Pressure Gradients

AbstractShale gas is becoming an important component of the global energy supply, with permeability a critical controlling factor for long‐term gas production. Obvious deviation may exist between helium permeability determined using small pressure gradient (SPG) methods and methane permeability obtained under actual field production with variable pressure gradients (VPG). In order to more accurately evaluate the matrix permeability of shale, a VPG method using real gas (rather than He) is established to render permeability measurements that are more representative of reservoir conditions and hence response. Dynamic methane production experiments were performed to measure permeability using the annular space in the shale cores. For each production stage, boundary pressure is maintained at a constant and the gas production with time is measured on the basis of volume change history in the measuring pump. A mathematical model explicitly accommodating gas desorption uses pseudo‐pressure and pseudo‐time to accommodate the effects of variations in pressure‐dependent PVT parameters. Analytical and semi‐analytical solutions to the model are obtained and discussed. These provide a convenient approach to estimate radial permeability in the core by nonlinear fitting to match the semi‐analytical solution with the recorded gas production data. Results indicate that the radial permeability of the shale determined using methane is in the range of 1×10−6 – 1×10−5 mD and decreases with a decrease in average pore pressure. This is contrary to the observed change in permeability estimated using helium. Bedding geometry has a significant influence on shale permeability with permeability in parallel bedding orientation larger than that in perpendicular bedding orientation. The superiority of the VPG method is confirmed by comparing permeability test results obtained from both VPG and SPG methods. Although several assumptions are used, the results obtained from the VPG method with reservoir gas are much closer to reality and may be directly used for actual gas production evaluation and prediction, through accommodating realistic pressure dependent impacts.

‘Halo-kinematic’ sequence stratigraphic analysis adjacent to salt diapirs in the deepwater contractional province, Liguro-Provençal Basin, Western Mediterranean Sea

Abstract This study investigates the coupled halokinetic and depositional processes of the contractional diapiric-minibasin province in the distal deepwater Liguro-Provencal basin. The Liguro-Provencal basin is situated above young oceanic crust developed during the NW-SE Miocene rifting and back-arc extension. The Messinian evaporite succession was deposited contemporaneously on extended continental crust and young oceanic crust. Post-Messinian postrift isostatic adjustments of the continental shelves influenced prograding sedimentary wedges, gravitational failure and gravity-driven deformation above salt, hence extensive salt diapirism is situated in a contractional domain in the deepwater basin overlying oceanic crust. The kinematic evolution of salt structures in the contractional province is analysed from preserved depositional patterns in successive genetic depocenters adjacent to salt diapirs. Within minibasin successions, four dominant depositional patterns have been classified as a layered, truncated-onlap wedge, onlap-wedge and truncated-hook. The truncated-onlap wedge sequences are early syn-kinematic depositional packages contemporaneous to salt growth and may converge towards the rising salt structure prior to diapirism. They are therefore categorised as the pre-diapiric sequence. The onlap wedge and truncated-hook styles form later syn-kinematic packages in diapiric growth phase and are labelled as halo-kinematic sequences. The layered sequence, maybe pre-kinematic and shows parallel beds all truncating on the diapir flank. This onlap wedge consists of on-lapping beds at the basal sequence boundary, stratal thinning or convergence towards the diapir flank and angular truncations on an erosional surface on the top depositional boundary proximal to the diapir flank. The truncated-hook sequence reflects an absence of beds on-lapping on the diapir roof within the sequence, beds truncate at the diapir flank and on an erosional/slip surface on top depositional boundary proximal to the diapir flank. The truncated-onlap wedge sequence reflects on-lapping beds in the middle of the sequence and older and younger beds that truncate on the top depositional boundary.

Analysis of Resistivity Anisotropy of Loaded Coal Samples

In this paper, an experimental study of the variation of resistivity of coal samples in different bedding directions at 1 MHz frequency was performed by establishing an experimental system for resistivity testing of coal under triaxial stress. The low‐pressure nitrogen gas adsorption (LP‐N2GA) experiment and scanning electron microscopy (SEM) were obtained to analyze the pore‐fracture structural characteristics of coal samples and the influence on resistivity anisotropy. Furthermore, the fundamental cause of anisotropy of coal resistivity is expounded systematically. The results show that the resistivity of loaded coal decreased first before increasing. The ionic conductance and the high degree of metamorphism slow down the decrease of resistivity. The distribution of pore and fracture structures is anisotropic. The connected pores and fractures are mainly distributed along the parallel bedding direction. The weak plane of bedding, diagenetic fractures, and plane fracture structures of parallel bedding result in the increase of fractures in the direction of vertical bedding, so increasing the potential barrier. Therefore, the resistivity in the vertical bedding direction is higher than that of the parallel bedding. Loading coal resistivity anisotropy degree is a dynamic change trend; the load increases anisotropy significantly under axial pressure, and the degree of anisotropy has a higher discreteness under confining pressure. It is mainly the randomness of the internal pore‐fracture compaction, closure, and development of the heterogeneous coal under the confining pressure; the more rapid the decline in this stage, the larger the stress damage degree.

Study on the lithofacies of the second member of Tengger formation in Jiergalangtu of Erlian Basin

Through analyzing coring well observation and in combination with drilling data log sheet and the analysis of the test data, combined with the regional sedimentary tectonic evolution background of the second member of Tengger formation in Jiergalangtu of Erlian basin, the results show that lithic arkose and feldspathic lithic sandstone are developed in the study area. There are abundant sedimentary structures, mainly including parallel bedding, laminated bedding and massive bedding, etc. According to the analysis of sedimentary characteristics, nine lithofacies types and four vertical assemblages are summarized, which respectively reflect different microfacies environments: subaqueous distributary channel subaqueous distributary channel interestuarine bar sheet sand.

Sedimentary facies and depositional processes of the Diexi Ancient Dammed Lake, Upper Minjiang River, China

Landslide events causing rivers to become dammed are common natural disasters in mountainous area. Based on the sedimentary facies analysis of 44 sections between proximal and distal parts of a dammed lake, this study reveals the sedimentary facies distribution of the Diexi Ancient Dammed Lake located in the upper Minjiang River, on the southeastern margin of the Tibet Plateau. Fluvial gravels and sand were deposited in the proximal part of the dammed lake. Rhythmically bedded silt and horizontally laminated silt to clay were deposited in the distal part of lake due to the deep and steady water conditions. After progradation of the fluvial system, flood sediments were deposited in the middle part of the lake. The flood carried a large amount of sediments into the lake, and the decelerating flow generated successively alternating layers of gravels and sand, followed by alternating layers of silt with low-angle cross-stratification, parallel bedding, and climbing ripples downstream. Gravel layers formed by four fluvial progradations were found based on the distribution of sedimentary facies in 44 sections. After the first fluvial progradation, lacustrine sediments at the highest point emerged from the water surface. The interval between the second and third fluvial progradation is relatively short, and only a terrace was formed, namely terrace VI. After the fourth fluvial progradation, terrace IV was formed. Although gravel layers formed by fluvial progradations corresponding to terraces II, III and V were not found, it can be determined that a terrace level does not necessarily correspond to a period of fluvial progradation. Therefore, the analysis of sedimentary facies upstream of landslide dams can be used as a supplementary method for inferring the evolution of the dammed lake.

Dynamic compression characteristics of layered rock mass of significant strength changes in adjacent layers

Abstract Layered rock mass of significant strength changes for adjacent layers is frequently observed in underground excavation, and dynamic loading is a prevalent scenario generated during excavation. In order to improve the driving efficiency and reduce engineering accidents, dynamic compression characteristics of this kind of rock mass should be understood. The dynamic properties of a layered composite rock mass are investigated through a series of rock tests and numerical simulations. The rock mass is artificially made of various proportions of sand, cement and water to control the distinct strength variations at various composite layers separated by parallel bedding planes. All rock specimens are prefabricated in a specially designed mould and then cut into 50 mm in diameter and 50 mm in height for split Hopkinson pressure bar (SHPB) dynamic compression testing. The test results reveal that increasing strain rate causes the increases of peak strength, σp, and the corresponding failure strain, ep, while the dynamic elastic modulus, Ed, remains almost unchanged. Interestingly, under the same strain rates, Ed of the composite rock specimen is found to decline first and then increase as the dip angle of bedding plane increases. The obtained rock failure patterns due to various dip angles lead to failure modes that could be classified into four categories from our dynamic tests. Also, a series of counterpart numerical simulations has been undertaken, showing that dynamic responses are in good agreement with those obtained from the SHPB tests. The numerical analysis enables us to look into the dynamic characteristics of the composite rock mass subjected to a broader range of strain rates and dip angles than these being tested.

Study on the anisotropic permeability in different rank coals under influences of supercritical CO2 adsorption and effective stress and its enlightenment for CO2 enhance coalbed methane recovery

The permeability of coal reservoir is dynamically changed during CO2 enhance coalbed methane recovery projects (CO2-ECBM) and is controlled by the two main influencing factors: effective stress and CO2 adsorption. Six coal core samples parallel and vertical bedding plane directions were drilled, which were then adopted to conduct the permeability tests under conditions of cyclic loading/unloading and supercritical CO2 saturation. Results show that the permeability is depended on the coal rank, the middle rank coal possesses the highest initial permeability because of its moderate compaction and largely generated cleats during the coalification. The permeability is anisotropic, manifesting in the permeability in parallel bedding plane direction is larger than that in the vertical bedding plane direction. The stress sensitivity coefficient and IPLRi in parallel bedding plane direction exceed that in vertical bedding plane direction, however, PALR exhibits the opposite law. Hereat, the fractures parallel to bedding plane are susceptible to the stress change and the cleats vertical to bedding plane are sensitive to the ScCO2 adsorption. In view of the irreversible permeability damage by the stress change and CO2 adsorption, experiments in this paper confirms that prefracturing can dramatically decrease the IPLRi, stress sensitivity coefficient and PALR of coal seams, which may be the potential and effective measure to enhence the injectivity of CO2 in coal seams.