Bedding Plane Direction Research Articles

Experimental investigation on mechanical behavior, energy evolution and gas permeability of anisotropic phyllite subjected to triaxial compression and cyclic loading

The bedding plane direction significantly affects the mechanical properties, deformation and gas permeability evolution of rock mass in gas storage engineering. In this paper, for phyllite, a series of triaxial compression with cyclic loading and gas permeability apparatus are conducted to investigate such directional effect. The peak stress, Young’s modulus, energy evolution, failure mode and gas permeability coefficient are analyzed. For triaxial compression test, with increasing inclination angles, the deformation parameters (E, E50, μ) and characteristic stress parameters (σcc, σci, σcd) both show a U-shape pattern. For triaxial cyclic loading test, the peak stress of specimen first decreases and then increases, i.e., the peak stresses at the inclination angles of 0°, 30°, 45°, 60° and 90° are 118.18, 84.35, 64.18, 42.54 and 84.57 MPa, respectively. The energy evolutions of specimens at different inclination angles are similar, but increase with increasing cyclic number. i.e., when β=60°, with the cycle number increasing from 1 to 3, the elastic strain energy, the dissipated strain energy and the total strain energy increase by 343.55%, 175.08%, and 259.87%, respectively. In addition, the failure mode of phyllite specimens is analyzed and summarized as three distinct types, i.e., tensile-shear failure, shear-sliding failure and split-shear failure. The relationships between the initial gas permeability and confining pressure, deviatoric stress show positive correlations. The failure of layered phyllite is most likely to occur at a bedding plane inclination angle of 45° to 60°.

Wellbore stability analysis of inclined wells in transversely isotropic formations accounting for hydraulic-mechanical coupling

As the main hydrocarbon reservoirs, sedimentary formations are usually anisotropic in elasticity, strength, and permeability. Traditional wellbore stability analysis assumes that the surrounding rocks are isotropic and elastic, whereas some analytical and numerical methods assume that the surrounding rocks are anisotropic in both elasticity and strength. Since the anisotropy of permeability is usually ignored, the wellbore instability mechanism in anisotropic formations may not be identified. Therefore, based on the poroelasticity theory and the assumption of generalized plane strain, this study proposed a pseudo-3D coupled hydraulic-mechanical finite element model by considering the anisotropies of elasticity, strength, and permeability. Taking the L203-x well as an example, the elastic and permeability anisotropies and the wellbore trajectory and stability in any bedding plane directions were analyzed parametrically. Results showed that the model simultaneously considering the strength and elastic anisotropies was in line with the actual hole enlargement rate of the L203-x well. With the increased anisotropy index of the elastic parameters, the anisotropic induced pore pressure is opposite to that of isotropic situation on the 2D cross-sectional plane of the wellbore, and the effective stress increased and was deflected at a certain angle. The effective hoop stress was biased toward the direction of higher stiffness, and the shear failure area that grew smaller gradually decreased. With the increased anisotropy index of permeability, the pore pressure rapidly increased along the direction parallel to the bedding plane, the effective radial stress of shape became rectangular along the bedding direction, the high-stress concentration zone of effective hoop stress changed from four to two, and the shear failure area grew smaller. The relative importance order of the anisotropy influences on the wellbore stability was thus: strength anisotropy > elastic parameter anisotropy > permeability anisotropy. A comparison of the well inclination and bedding plane orientation showed that, when the relative angle between the well axis and bedding normal was small, the influence of the bedding plane declined. In practical drilling engineering, avoiding drilling parallel to bedding planes is conducive to borehole stability.

Energy Evolution Characteristics of Water-Saturated and Dry Anisotropic Coal under True Triaxial Stresses

During deep underground coal mining, water-injection-related engineering methods are generally carried out to reduce the hazards of coal dynamic disasters. The energy evolution characteristics of coal can better describe the deformation and failure processes, as it is more consistent with the in situ behavior of underground mining-induced coal. In this study, experimental efforts have been paid to the energy evolution characteristics of water-saturated and dry anisotropic coal under true triaxial stresses. The effects of water saturation, intermediate stress, and anisotropic weak planes of coal on the true triaxial energy evolution were systematically evaluated. The results show that the overall energy is weakened due to the water adsorption for water-saturated coal samples. The water-weakening effect on the overall energy of water-saturated coal is more pronounced when perpendicular to the bedding plane direction than in the other two cleat directions. The accumulation elastic energy anisotropy index of dry and water-saturated coal samples is higher than 100.00%. Both accumulation and residual elastic energy of dry and water-saturated coal samples show an increasing-then-decreasing trend with intermediate stress increase. The results obtained in this study help understand the in situ behavior of coal during deep underground mining and control coal dynamic disasters.

Estimating stress percolation patterns in hydraulic fractured Gondwana coal using Raman spectroscopy

The present article discusses the importance of combining the laboratory hydraulic fracturing (HF) method with Raman spectroscopy to estimate stress percolation patterns. In order to demonstrate this idea, Gondwana coal (GC) samples of India are employed. The GC samples are taken perpendicular and parallel to the bedding plane direction for the studies. Raman spectroscopy characterization is performed on GC samples to predict the nature of (i.e., tensile or compressive) stress percolation patterns on the fractured samples. Based on the shift in the D-band of Raman spectra the nature of stress percolation patterns is predicted. The proposed method calculates onset and shift in the plastic yielding during HF depending on the nature of stress. A relation between the D-band shift of Raman spectra against the radial crack propagation is estimated considering the geochemical variations for mode I fracture. In general, the fracture modes refer to the decomposition of the crack tip stresses, if the applied stress is orthogonal to the local plane of the crack surface it is referred to as mode I fracture. It was observed that, in perpendicular directions, the GC sample possessed a tensile stress percolation pattern in contrast to the parallel direction.

Progressive damage and fracture of biaxially-confined anisotropic coal under repeated impact loads

Coal masses in underground mines are in multiaxial stress states after excavation, and they are frequently subjected to dynamic loads particularly from continuous mining-induced seismic events. The changes of in-situ stress conditions and external dynamic loads may induce coal bursts, which involves the violent breaking of coal masses. Meanwhile, the coal bursting process may be influenced by bedding planes that are considered as unique structures in coal masses, which raises uncertainties of coal bursting mechanisms. This paper aims to reveal the progressive damage and burst proneness of anisotropic coal under coupled static stress and repeated dynamic loads. The tested coal specimens have five bedding plane orientations (β = 0°, 30°, 45°, 60°, and 90°), where β is the angle between the bedding plane and dynamic loading direction. A triaxial Hopkinson bar (Tri-HB) system is utilised to apply the biaxial static stress on the specimen and then followed by dynamic loading, and the stress and strain can be recorded by the system accordingly. After the loading test, ultrasonic wave velocity is measured to examine the degree of attenuation, and the quantification of microcrack characteristics is conducted by using the synchrotron-based X-ray computed tomography (CT). Then, the specimen is loaded again under the same biaxial static stress and dynamic load, and measured by ultrasonic sensors and X-ray CT. This process will be repeated until the final failure of the specimen. We found that both dynamic peak stress and P-wave velocity show a decreasing trend due to damage accumulation with increasing number of dynamic tests. The bearing capacities of anisotropic coal are mainly governed by bedding angles, for example, the specimen with β = 30° undergoes the lowest impact numbers, but the one with β = 90° can undergo the highest numbers. The crack initiation and propagation are progressively revealed by two-dimensional (2D) X-ray slices and 3D fracture network reconstructions. As impact number increases, both 2D and 3D fractal dimensions of cracks increase exponentially, whereas dynamic peak stress and P-wave velocity are negatively correlated with them.

Impact of bedding plane direction and type of plastic microparticles on stiffness of inherently anisotropic gap-graded soils: Index, wave propagation and micromechanical-based interpretations

Sand-clay mixtures are often encountered in natural deposits and compacted soils in earth structures in the form of gap-graded soils. They commonly display anisotropic fabric which markedly influences their wave propagation and mechanical properties. In this study, a comprehensive experimental program is carried out to thoroughly examine the influence of the inclined bedding plane and clay type on the small strain shear modulus (Gmax) of granular soils mixed with various portions of low and high plasticity fines, controlling in this way the double diffuse layer of the microparticles. To this end, cylindrical samples containing different fines contents are prepared with their corresponding optimum water contents and maximum dry densities in a standard proctor mold. Undisturbed specimens are then extracted by the continuous coring method and wave propagation tests with bender elements are applied at different angles with respect to the bedding plane (α-direction), evaluating in this way shear wave velocity and Gmax. The test results suggested that α-direction has a significant influence on Gmax of sand-clay mixtures and this influence is amplified at lower fines contents and higher confining pressures. Mixtures of sand with high plasticity clay are observed to have lower degree of anisotropy compared to those containing low plasticity clay, specifically at lower confining pressures. The contribution of fines in the load carrying structure of the solid skeleton decreases for high plasticity clays; the phenomenon which is primarily attributed to the thick diffuse double layer of the microparticles. This trend becomes more pronounced at higher contents of fines inclusion where the sand-in-fine structure dominates. A previously proposed expression for sands is rectified and extended so as to develop a Gmax model of sand-cohesive clay mixtures taking into account the bedding plane direction. Micromechanical-based interpretations are further elaborated considering the intervention of the microparticles on the contact response of sand grains altering their normal and tangential contact behavior, so as to supplement the findings, from the wave propagation tests, with multi-scale insights.

Constitutive model for monotonic and cyclic loading on anisotropic clays

In the present work, a constitutive model for anisotropic clays is proposed and evaluated under monotonic and cyclic loading. To that end, the hypoplastic model for anisotropic clays by Mašín is extended to consider the effects observed on cyclic loading. The extension follows from the intergranular strain anisotropy (ISA) approach by Fuentes and Triantafyllidis. Similar to the intergranular strain (IS) theory by Niemunis and Herle, the ISA extension enhances the model in many aspects, such as the increase of the stiffness and reduction of the plastic strain rate on cyclic loading. Different monotonic and cyclic tests are simulated to evaluate the model's capabilities with the use of samples having vertical and horizontal bedding planes. Simulations obtained with the proposed model and the three-surface kinematic hardening model with transverse isotropic stiffness according to the work of Graham and Houlsby in 1983 (hereafter denoted A3-SKH) are directly compared and analysed. Some additional simulations with the same hypoplastic model extended through the conventional IS theory are also included. The results indicate that a better agreement of the pore pressure accumulation, and some effects emerging from the sample anisotropy, such as ‘inclined’ q–p cycles under undrained conditions, and the dependency of the pore pressure accumulation on the bedding plane direction, are provided by the proposed model. On the other hand, the A3-SKH model estimates with more accuracy the vertical strain accumulation on samples with vertical and horizontal bedding planes.

An anisotropic viscoplasticity model for shale based on layered microstructure homogenization

AbstractViscoplastic deformation of shale is frequently observed in many subsurface applications. Many studies have suggested that this viscoplastic behavior is anisotropic—specifically, transversely isotropic—and closely linked to the layered composite structure at the microscale. In this work, we develop a two‐scale constitutive model for shale in which anisotropic viscoplastic behavior naturally emerges from semianalytical homogenization of a bilayer microstructure. The microstructure is modeled as a composite of soft layers, representing a ductile matrix formed by clay and organics, and hard layers, corresponding to a brittle matrix composed of stiff minerals. This layered microstructure renders the macroscopic behavior anisotropic, even when the individual layers are modeled with isotropic constitutive laws. Using a common correlation between clay and organic content and magnitude of creep, we apply a viscoplastic modified Cam‐Clay plasticity model to the soft layers, while treating the hard layers as a linear elastic material to minimize the number of calibration parameters. We then describe the implementation of the proposed model in a standard material update subroutine. The model is validated with laboratory creep data on samples from three gas shale formations. We also demonstrate the computational behavior of the proposed model through simulation of time‐dependent borehole closure in a shale formation with different bedding plane directions.

Strength Control Factors of Chlorite Schist under Schistose Structure

In tunnel engineering, it is important to understand the influence of schistose structure on the failure strength of chlorite schist. To explore the strength control factors of chlorite schist, this paper firstly analyzes the mineral composition and meso structure of chlorite schist of different weathering states. The results show that the mineral composition of chlorite schist is changed during the weathering process, and that chlorite is an anisotropic rock mass. Next, a series of uniaxial compressive tests were conducted on chlorite schist samples with different bedding angles (the angle between bedding plane and loading direction; θ=0°, 15°, 30°, 45°, 60°, 75°, and 90°), moisture conditions (dry and saturated), and weathering states (strongly weathered and weakly weathered). Based on the test data, the authors discussed the change laws of the rock strength with bedding angle, weathering state, and moisture condition. The main results are as follows: Chlorite schist is a low-anisotropy rock mass, whose compressive strength exhibited a V-shaped trend with the growing bedding angle; the schistose structure is the internal cause of the deformation and the anisotropic or transversely isotropic strength of the schist; the schistose structure is reshaped and further damaged by external factors (e.g. water softening and weathering effects) in engineering. The research findings help to improve the rock stability and support design in tunnel engineering.

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.

Study on the damage evolution of secondary tunnel lining in layered rock stratum

Concrete cracking is prone to occur in secondary linings during tunnel operation. In this article, the Zhegu mountain tunnel, which is mainly situated in weak phyllite stratum, was taken as an example to study this issue. Firstly, a field investigation was conducted to reveal the cracking status of the tunnel lining. Then, similarity model tests were adopted to analyze the mechanical responses of linings in layered rock strata with different inclination angles. Finally, the damage Mohr-Coulomb criterion with a statistical distribution feature is put forward to investigate the damage patterns of linings. The results show that: (1) the anisotropic behavior of phyllite is the main factor that causes the asymmetric cracking in secondary lining. (2) The distribution of inner force and displacement along tunnel perimeter has distinct asymmetric features, and their increasing trends with loading steps contain three stages, e.g., elastically, plastically, and unsteadily deformed stage. (3) The failure processes of linings situated in layered strata with different inclination angles are similar with three pronounced period, that is, initial damage period, damage period, and instability period.(4) For layered stratum, the serious damage sections of lining are distributed in certain range with the tangent of tunnel contour being parallel to the direction of bedding planes, and the line connecting the central points of serious damage sections on both sides deflects toward the direction of minor principal stress to a certain extent.(5) The existence of void leads to a more concentrated distribution of serious damage area in lining than that of lining with no void behind it.

Effect of rock brittleness on propagation of hydraulic fractures in shale reservoirs with bedding‐planes

AbstractHydraulic fracturing forms complex hydraulic fracture networks (HFNs) in shale reservoirs and significantly improves the permeability of shale reservoirs. Although rock brittleness is a major factor in determining whether a shale reservoir can be fractured, the relationship between HFNs and rock brittleness remains unclear. To investigate this relationship in a shale reservoir with bedding planes, this paper presents a series of hydraulic fracturing simulations based on a hydromechanically coupled discrete element model. In addition, we analyzed the sensitivity of the difference in rock brittleness to bedding‐plane angle and density. The parameters used in the model were verified by comparing the simulated results with experimental results and a theoretical equation. The results showed that breakdown pressure and injection pressure increased with increasing rock brittleness. The tensile hydraulic fracture of a shale reservoir (THFSR) was always the most abundant type of hydraulic fracture (HF)—almost 2.5 times the sum of the other three types of HFs. The distribution of areas with higher fluid pressure deviated from the direction of the maximum principal stress when the angle between the bedding plane and maximum principal stress directions was large. Upon increasing this bedding‐plane angle, the breakdown pressure and rock brittleness index first decreased and then increased. However, regardless of bedding angle, the relative proportions of the various types of HFs remained essentially constant, and the seepage area expanded in the direction of the maximum principal stress. Increased bedding‐plane density resulted in a gradual increase in the total number of HFs, with significantly fewer of the THFSR type, and the large seepage areas connected with each other. This study thus provides useful information for preparing strategies for hydraulic fracturing.

The Influence of Bedding Planes and Permeability Coefficient on Fracture Propagation of Horizontal Wells in Stratification Shale Reservoirs

The complexity of hydraulic fractures (HF) significantly affects the success of reservoir reconstruction. The existence of a bedding plane (BP) in shale impacts the extension of a fracture. For shale reservoirs, in order to investigate the interaction mechanisms of HF and BPs under the action of coupled stress-flow, we simulate the processes of hydraulic fracturing under different conditions, such as the stress difference, permeability coefficients, BP angles, BP spacing, and BP mechanical properties using the rock failure process analysis code (RFPA2D-Flow). Simulation results showed that HF spread outward around the borehole, while the permeability coefficient is uniformly distributed at the model without a BP or stress difference. The HF of the formation without a BP presented a pinnate distribution pattern, and the main direction of the extension is affected by both the ground stress and the permeability coefficient. When there is no stress difference in the model, the fracture extends along the direction of the larger permeability coefficient. In this study, the in situ stress has a greater influence on the extension direction of the main fracture when using the model with stress differences of 6 MPa. As the BP angle increases, the propagation of fractures gradually deviates from the BP direction. The initiation pressure and total breakdown pressure of the models at low permeability coefficients are higher than those under high permeability coefficients. In addition, the initiation pressure and total breakdown pressure of the models are also different. The larger the BP spacing, the higher the compressive strength of the BP, and a larger reduction ratio (the ratio of the strength parameters of the BP to the strength parameters of the matrix) leads to a smaller impact of the BP on fracture initiation and propagation. The elastic modulus has no effect on the failure mode of the model. When HF make contact with the BP, they tend to extend along the BP. Under the same in situ stress condition, the presence of a BP makes the morphology of HF more complex during the process of propagation, which makes it easier to achieve the purpose of stimulated reservoir volume (SRV) fracturing and increased production.

Experimental study of stress anisotropy and noncoaxiality of dense sand subjected to monotonic and cyclic loading

Abstract The noncoaxiality of the principal stress direction and plastic principal strain increment has been broadly recognized as an influencing parameter for design of soil structures. Here we performed a series of systematic hollow cylinder experiments to study the effects of stress anisotropy on the noncoaxiality of dense Babolsar and Toyoura sands. A total of 25 undrained torsional shear tests were carried out under constant mean confining pressure, and fixed principal stress directions, α. We investigated the stress-strain behavior of dense sands for different α-directions, and cyclic stress ratio, CSR, under monotonic and cyclic loading conditions. The results show that the noncoaxiality value depends on the CSR, the level of plastic strain, and α-direction. Independent of the principal stress direction, maximum noncoaxiality value was observed at peak shear strength when there is most interlock between sand particles. Minimum noncoaxiality was recorded in α = 45° tests, in which the direction of maximum shear stress coincided with the horizontal bedding plane direction (weakest plane).

Using pressure pulse decay experiments and a novel multi-physics shale transport model to study the role of Klinkenberg effect and effective stress on the apparent permeability of shales

The confining pressure imposed on a shale formation has a significant impact on the apparent permeability of the rock. Gas flow in low-permeability shales differs significantly from liquid flow because of the Klinkenberg effect, which results from gas molecule slip at the wall surfaces inside the nanopores. This effect causes the increase of apparent permeability (i.e., the measured permeability). In this study, cores extracted from four U.S. shale formations were tested using a pulse decay permeameter (PDP) under varying combinations of confining and pore pressures. The Klinkenberg coefficient was calculated to interpret the change in the measured apparent permeability as a function of pore pressure and effective stress. Next, based on the various combinations of confining and pore pressures, the actual values of the Biot coefficient were calculated by data fitting. Moreover, the samples were cored in the directions parallel to and perpendicular to the shale bedding planes to unravel the role of bedding plane direction on the apparent permeability. Furthermore, a novel, multi-physics shale transport (MPST) model was developed to account for the coupled multi-physics processes of geomechanics, fluid dynamics, and Klinkenberg effect for gas transport in shales. In the MPST model, pore pressure and effective stress are the two independent input variables, and the measured apparent permeability is the model output. The MPST model was then used to fit the PDP experimental data, and the successful data fitting confirmed that the MPST model captures the critical multi-physics processes that regulate the apparent permeability.

Mechanical Properties and Failure Behavior of Dry and Water-Saturated Anisotropic Coal Under True-Triaxial Loading Conditions

In underground coal mining, coal failure generally occurs due to the relatively weak strength of the coal and the high applied mining-induced stresses. The outer complex geological conditions (i.e., tectonic structure and water intrusion) and internal structural anisotropy of the coal introduce uncertainty in predicting its mechanical properties and failure behavior. In this study, laboratory investigations of the mechanical properties and failure behavior of dry and water-saturated anisotropic coal samples subjected to different true-triaxial loading stresses were conducted. The effects of water weakening, intermediate stress, and structural anisotropy on the mechanical properties and failure behavior of the coal were systematically studied. The results indicate that the presence of water significantly reduced the strength, elastic modulus, and strength anisotropy of the coal. The maximum stress at failure first increased and then decreased with increasing intermediate stress. The residual strength-to-peak strength ratios and failure plane angles of the coal showed a linear increase with increasing intermediate stress. When the coal samples were loaded in the bedding plane direction, the brittleness of the coal was higher than when they were loaded in the other two cleat plane directions. In addition, when the coal samples were loaded in the butt cleat plane direction, the brittleness of the coal decreased with increasing intermediate stress. Two typical failure modes of the dry and water-saturated coal samples were observed: shear and mixed splitting and shear failures. The dominant failure mode of the coal also varied with the loading direction relative to the weakness planes, which could be well recognized and predicted by the acoustic emission (AE) characteristic curves. To further reveal the fracture mechanism, the microcrack patterns of the coal were further identified based on the AE parameters.

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.

A Test Device System for Cracking Mud Shale under Hydraulic Fracturing

The exploitation of shale gas has always been a major problem, and the solution to this problem must start with the study of its nature. A kind of simulation device and monitoring method of mud shale cracking was designed and assembled by ourselves. First of all, made with two layers of the general principle and four layers of 60 ° prefabricated crack two groups of similar material sample of the shale, then in turn for each block of multistage hydraulic fracturing test, and the use of strain gauge for strain monitoring. Results show that under the condition of no native prefabricated crack, hydraulic direction tend to toward weak bedding plane direction of crack extension until she reached the weak bedding surface, and then the main cracks along the weak bedding surface directly, at the same time there are prefabricated crack in native and weak stratification plane, in stage and the area between the prefabricated crack priority area map cracking formation, the results for the exploitation of shale gas has important guiding significance.

Geomechanical characterization of gondwana shale across nano-micro-meso scales

We study multiphase and multiscale mechanical properties of the Gondwana shale samples obtained from the Raniganj Formation of central and eastern India. The mechanical properties of black and gray shale samples cored perpendicular to the bedding plane direction are analyzed across nano-to-micro scales using the nanoindentation method. We characterize the multiphase structure of the two shales in terms of three mechanical phases–quartz/feldspar/pyrite, organic matter, and kaolinite–and use that to explain the heterogeneous elastoplastic behavior observed during nanoindentation. We quantify the fracture toughness of black and gray shales at microscale using our modified energy method, which relates the energy released during a pop-in event to a radial fracture model. We extend the deconvolution approach over multiple properties to estimate the uncertainty in multiphase structure and phase volume fractions. We used the viscosity-toughness scaling analysis to study the difference in hydraulic fracturing behavior of the black and gray shales. We show the influence of nano- and micro-scale multiphase mineralogical properties on characteristic length, time and pressure scales of fracture propagation at mesoscale.

Characterisation and evaluation of shockwave generation in water conditions for coal fracturing

We investigated the effect of circuit parameters on shockwaves generated by electrohydraulic discharge (EHD), and the application of EHD to stimulation of coal seam gas reservoirs. The discharge circuit analyses indicate that the deposited electrical energy used for shockwave generation is not simply determined by charging voltage or capacitance. The discharge efficiency is also affected by mutual interactions among inductance, resistance and capacitance as well as real-time current change. Furthermore, the relation between charging voltage and peak pressure strength revealed that voltage at breakdown point, ranging from 9 kV to 28 kV, was more closely associated to shock strength rather than the initial charge voltage. Generally, a higher breakdown voltage tends to lead to a stronger shockwave. Finally the EHD fracturing stimulation conducted on coal cubes showed the coal was effectively fractured with enormous cracks and voids. By using three-dimension computed-tomography (CT), the fracture extension and propagation in the coals are found to be obvious and extensive. Though the existing fracture greatly affects the new fracture development in vertical direction, the horizontal fractures parallel with the bedding plane direction are more likely to occur and even expand across the entire tested coal core. The dramatic coal permeability improvement (over a hundred times) after EHD also validates the cutting-through fractures, as does the enhancement of pore and porosity observed by morphology and structural characterisation via scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP).