Anisotropic Coal Research Articles

The influence of lamination and fracture on the velocity anisotropy of tectonic coals

The elastic anisotropy of coal formations is important for the prediction of unconventional coalbed methane reservoir performance and hydraulic fracturing. Although essential for a number of geophysical applications, the elastic anisotropy of coals is still poorly understood. Therefore, three sets of cylindrical coal samples are drilled parallel and perpendicular to the bedding plane, and the ultrasonic velocities under different confining pressures are measured in the laboratory. We combine experimental observations and modeling analysis to evaluate the elastic anisotropy of the coals. The results suggest that the velocity of coals parallel to the bedding plane is less sensitive to the confining pressure than that perpendicular to the bedding plane. The fracture densities of coals parallel to the bedding plane are much lower than those perpendicular to the bedding plane, and most of the fractures are closed when the pressure is higher than 15 MPa. The organic matter and clay content and their preferred orientation play dominant roles in the velocity anisotropy, and the fracture effect can significantly enhance it. Velocity anisotropy parameters of coals are positively correlated with the organic matter and clay content, and the preferred orientation of organic matter and clay plays a significant role in coal anisotropy. The results in this study are helpful in the characterization of fractures of coals and can provide an important rock physics basis for microseismic fracture monitoring and reservoir prediction.

Micromechanical property evolution and damage mechanism of coal subjected to ScCO2 treatment

Carbon dioxide (CO2) injection into deep coal seams holds considerable significance in both carbon mitigation and enhanced recovery of clean coalbed methane (CBM). The CO2-coal interaction leads to changes in the physicochemical properties, potentially impacting the stability of the target sequestration coal reservoir. This study applied the nanoindentation, scanning probe microscopy (SPM), X-ray diffraction (XRD), and electron probe X-ray Micro-Analyzer (EPMA) techniques to probe the micromechanical property change and damage mechanism of coal subjected to ScCO2 injection. The result shows that the load-displacement behavior of the tested coal changes with continuous ScCO2 treatment, and the mechanical strength significantly weakens after 4 days of ScCO2 treatment. The nanoindentation test indicates that the peak and creep displacements of tested coal increased by 168.79 % and 1046.32 % respectively with 4-day ScCO2 treatment, inferring that the coal deformation increases and changes from elastic to plastic after ScCO2 injection. As the ScCO2 treatment time increases, Young's modulus and hardness of coal exhibit an exponential decay trend and rapidly decrease by 77.77 %∼89.76 % and 68.37 %∼81.63 % respectively after the initial 3-day treatment, followed by a slow decrease with the ScCO2 duration prolonging. The XRD and EPMA results show that the carbonate minerals reacted preferentially with ScCO2, and silicate minerals experienced gradual dissolution, while the amorphous carbon fluctuated almost unchanged in the tested coal. Carbonate minerals in tested coal have decreased by 70.68 % within the first 1-day ScCO2 treatment, making the most significant contribution to the initial rapid weakening of coal mechanics. The mineral distribution is closely related to the mechanical anisotropy of coal, which is confirmed by the phenomenon that the coal anisotropy gradually weakens with the mineral reaction process. It is inferred that the mineral component and distribution of coal seams are important indicators for evaluating the intensity of physical and chemical reactions in ScCO2-injected reservoirs, which directly determines the mechanical damage behavior of sequestration reservoirs. This research provides basic support for site selection and safety assessment of carbon sequestration in deep coal seams.

Comparison of elastic properties of tectonic coals along the face and butt cleat directions

AbstractCoalbed methane is a hot spot for gas exploration at present, and fractures are widely developed in the coals. However, despite being essential for a number of geophysical applications such as reservoir prediction and hydraulic fracturing, the influence of fractures on the elastic properties and anisotropy of coals is still poorly understood. Therefore, three groups of cylindrical coals were drilled along the face and butt cleat directions to study the effects of pressure and fracture on the elastic properties and anisotropy of tectonic coals. The velocity along the face cleat direction is less sensitive to the pressure than that along the butt cleat direction due to the difference in compliant pore and fracture content. Coals display a strong directional dependence in petrophysical and elastic properties. The permeabilities and velocities of samples along the butt cleat direction are much lower than those along the face cleat direction. The fracture densities are quantitatively characterized by the theoretical model. The values along the face cleat direction are much smaller than those along the butt cleat direction. The influence of fractures on the elastic properties and anisotropy is comprehensively studied by theoretical and laboratory methods. The mineralogy and fracture effects are important factors to the elastic anisotropy of coals, the latter can be a dominant factor to the coal elastic anisotropy. The results can contribute to the understanding of the cleat system of coalbed methane reservoir and can provide a critical rock physics basis for micro‐seismic fracture monitoring and reservoir prediction.

Effect of cyclic loading-unloading on the mechanical anisotropy of coal under uniaxial compressive condition

Effect of cyclic loading-unloading on the mechanical anisotropy of coal under uniaxial compressive condition

Study on Gas Transport Pattern of Permeability Anisotropic Coal Seam under the Influence of Mining

Study on Gas Transport Pattern of Permeability Anisotropic Coal Seam under the Influence of Mining

Prediction of Fracture Initiation Pressure for Slotting-Directional Hydraulic Fracturing Based on the Anisotropy of Coal

Abstract The precise estimation of fracture initiation pressure is crucial for the effective implementation of slotting-directional hydraulic fracturing methods in coal seams. Nonetheless, current models fail to account for the impact of the morphology of the slotted borehole and the anisotropy of coal. To address this issue, a three-dimensional model was created in this study, which simplified the slotted borehole as an elliptical medium and the coal as an orthotropic medium. Laboratory experiments were conducted to validate the model, and the findings regarding the changes in fracture initiation pressure and deflection angle due to various factors were presented. The calculated outcomes of the proposed model align with the observed pattern of the experimental results, and the numerical discrepancy falls within the acceptable range of 7%, showcasing the precision of the proposed model. A rise in the horizontal stress difference and a decrease in the depth of the slots will result in an elevation of the fracture initiation pressure and deflection angle. In addition, the slotting angle will impact the distribution pattern of the fracture initiation pressure and deflection angle, underscoring the significance of these factors in the hydraulic fracturing of slotted boreholes.

Field pilot testing and reservoir simulation to evaluate processes controlling CO2 injection and associated in-situ fluid migration in deep coal

A key strategy for reducing global CO2 emissions is geologic sequestration of CO2. Deep, unmineable coal seams have been proposed as possible targets for CO2 injection and storage because of the global distribution of coal resources, and relative security of CO2 storage associated with CO2 adsorption. However, the current paradigm is that near-injector permeability loss caused by pure CO2 adsorption-induced coal swelling limits the effectiveness of this process making coal a less than an ideal target for CO2 injection/sequestration.In order to test the viability of CO2 injection and storage in deep coal seams, and evaluate processes controlling CO2 injection, migration and storage therein, a field pilot was implemented in the Mannville Coal (∼1500 m depth) of Alberta, Canada. An additional objective of the field pilot was to quantify the migration of water and natural gas away from the CO2 injection well, an important mechanism used to improve the performance of the pilot operator's enhanced hydrogen recovery (EHR™) process. To achieve these multiple objectives, a vertical injection well and closely-spaced offsetting vertical observation well were drilled, cored, and completed in the Mannville coal seams, and instrumented with pressure sensors to monitor both the coal and bounding zones during injection testing. Periodic fluid sampling at the observation well allowed for quantification of CO2 plume and fluid migration. Pre-CO2 injection/falloff testing was performed with water at variable injection rates/pressures to assess coal permeability/porosity changes without the complication of CO2-coal interaction. CO2 injection/falloff tests were then performed at increasing CO2 injection rates to assess the combined impact of injection pressure/CO2 rate and CO2-coal interaction (e.g., adsorption) on injectivity. All tests were history-matched with a numerical reservoir simulator; the calibrated model was used to study the key controls on CO2 storage, CO2 plume migration, in-situ fluid displacement, and injectivity.A key finding from simulation of the pilot results is that, for the Mannville coals in the study area, permeability increases caused by coal natural fracture dilation during injection outweigh permeability decreases caused by CO2 adsorption-induced coal swelling, resulting in an injectivity index increase with CO2 injection rate. In addition, the coals exhibit strong anisotropy in permeability and geomechanical properties; ignoring coal anisotropy in the simulation model causes coal swelling effects to be overestimated, resulting in an underestimate in CO2 plume extent.From the pilot results and analysis, it can be concluded that the deep Mannville coal could be a viable target for CO2 storage.

Influence of uniaxial strain loading on the adsorption-diffusion properties of binary components of CH4/CO2 in micropores of bituminous coal by macromolecular simulation

Heterogeneous and anisotropic coal tends to deform under strong mining disturbances, affecting the microscopic adsorption and diffusion of binary CH4-CO2 mixtures in micropores. Therefore, the grand canonical Monte Carlo and molecular dynamics methods were used to study the influence of the uniaxial strain loading on the adsorption-diffusion properties of binary components of CH4/CO2 in coal micropores. The results show that compression strain has significant effects on adsorption selectivity and thermodynamic factor compared to tension strain, and CH4 displacement efficiency is greater when the strain is less than −0.12. Both compression and tension strains have dual effects on promotion and inhibition of volumetric swelling of the gas-containing coal matrix, micropore development and gas diffusion. Furthermore, the diffusivity of CH4 is significantly larger than that of CO2, and Einstein diffusion is dominated by low-velocity motion. Results provide an important basis for the optimal coalbed methane recovery design in deformed coal reservoirs.

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.

Anisotropic Flow-Solid Coupling Model for Gas Extraction from Cis-Layer Boreholes and Its Application.

To investigate the effect of anisotropy of coal body on the gas extraction effect of cis-borehole, the anisotropy permeability model of coal based on structural anisotropy ratio and flow-solid coupling model were established at a working face of Zhongmacun mine Henan Province, China, as the research object, and COMSOL numerical simulation software was used. The results show that considering coal anisotropy, the gas pressure decreases more faster than that without coal anisotropy, and the farther away from the borehole, the smaller the difference between them. The extraction time was a logarithmic function of the effective extraction radius, the negative extraction pressure was an exponential function of the effective extraction radius, and the borehole diameter satisfies a power function relationship with the effective extraction radius. The variation of gas pressure with extraction time in different stratigraphic directions was analyzed, and gas pressure decreases faster in parallel stratigraphic directions and slower in vertical stratigraphic directions. Considering the complexity and safety of gas extraction at the working face, a 30% redundancy factor is added to determine the maximum magnitude and range of gas pressure drop when the spacing of cascade drill holes in a working face of Zhongmacun mine Henan Province, China, is 6 m, which can avoid the superposition of "blank zone" and ineffective extraction.

Characteristics of water migration during spontaneous imbibition in anisotropic coal

Spontaneous imbibition is a process of effectively wetting coal pores, which can improve wetting uniformity. Therefore, spontaneous imbibition is critical for outburst, dust and rockburst preventions. Due to the anisotropy of coal, water injected into the borehole has different wetting characteristics along different bedding directions. To reveal the characteristics of water migration in coal samples with different bedding directions during spontaneous imbibition, T2 spectrum and imaging images measured by nuclear magnetic resonance are comprehensively analyzed. The fracture presents the effects of huffing and puffing during spontaneous imbibition. However, the effects of huffing and puffing gradually decrease with increasing bedding angle. Imbibition equilibrium time increases with increasing bedding angles. The wetting effect of pores, liquidity and wetting uniformity of water gradually become poor with increasing bedding angles. Spontaneous imbibition process presents the Lucas-Washburn model, and the imbibition curve can be divided into spontaneous imbibition, transition and diffusion stages. The wetting time perpendicular to bedding direction, layout of water injection boreholes and gas drainage boreholes perpendicular to bedding direction should be fully considered to prevent wetting blank areas. This research reveals the wetting behaviors of anisotropic coal during imbibition and provides guidance for layout and optimization of coal seam water injection boreholes.

Biot's Coefficient and Permeability Evolution of Damaged Anisotropic Coal Subjected to True Triaxial Stress

Biot's Coefficient and Permeability Evolution of Damaged Anisotropic Coal Subjected to True Triaxial Stress

Dielectric anisotropy effects on the microwave-induced thermodynamic response of coal: Numerical simulations and experiments

Microwave heating technology is a promising solution in underground engineering, such as mining, coalbed methane extraction, and oil/gas production, that primarily depends on the conversion of microwave energy due to dielectric properties. However, inadequate understanding of the effects of dielectric properties on microwave irradiation limits further development of microwave techniques. In this study, based on the dielectric anisotropy of coal, an electromagnetic-thermomechanical coupled model was used to numerically investigate the thermodynamic response under microwave irradiation at 2.45 GHz and 1 kW for 180 s. We performed experimental verification via indoor test. Results show that the sensitivity of microwaves to the dielectric direction in the TE10 mode is x > z > y. In different dielectric anisotropy models, the temperature and maximum principal stress increase synchronously and linearly with increasing heating time, and the curve growth rate is positively correlated with the equivalent dielectric permittivity. There are also marked temperature and stress concentrations in the sample. The compressive stress is concentrated in the center of the sample, while the tensile stress is around the outer edge. For bedding coal, tensile fractures occur under microwave irradiation along the bedding, and shear fractures run through the bedding. With the increase of the input microwave energy, the failure mode of coal gradually expands from single fracture to fracture network.

Numerical Investigation of an Anisotropic Permeability Model for Bedded Coal Based on the Equivalent Fracture Aperture Coefficient

Permeability evaluation of bedded coal is always a difficult problem, thereby an improved permeability model is established based on the equivalent fracture aperture coefficient. The equivalent fracture aperture coefficient can reflect the anisotropy of coal in mechanical and seepage properties. This model is executed into COMSOL Multiphysics to perform numerical simulations. The evolutions of permeability, gas pressure, and cumulative gas production on three typical boundary conditions are investigated. Our simulation results show that this model is competent to represent evolution of anisotropic permeability in bedded coal during gas production, which is governed by the competitive effects of desorption and effective stress. Under the same boundary conditions, different equivalent fracture aperture coefficients lead to significantly different trends in anisotropic permeability evolution. Anisotropic permeability changes with gas pressure and time during production, and the magnitude of change depends on both the equivalent fracture aperture coefficient and applied boundary conditions. Our model shows that the bedding characteristics of permeability have a significant effect on gas production forecasts.

Modelling anisotropic adsorption-induced coal swelling and stress-dependent anisotropic permeability

To investigate the anisotropy of coal swelling, this study proposes an effective stress model for saturated, adsorptive fractured porous media by considering gas adsorption induced surface stress change on solid-fluid interface. The effective stress model can be used to capture the anisotropic swelling of coal combining anisotropic mechanical properties and to link with the anisotropic permeability. Direction dependent fracture compressibility is used to describe the evolution of anisotropic stress-dependent permeability behaviour. Particularly, the impact of gas adsorption on fracture compressibility is considered in the model. The proposed models were tested against experimental results and compared to relevant existing models available in literatures. The model predicts that the coal swelling in the direction perpendicular to the bedding plane, is greater than that in the parallel plane. Coal permeability in each direction can be affected by the stress changes in any directions. The permeability parallel to the bedding plane is more sensitive to change in stresses than in perpendicular to the bedding due to higher fracture compressibility. The cleat compressibility could increase with gas adsorption, especially for carbon dioxide. Permeability loss in the direction parallel to the bedding plane is more significant than that in the direction perpendicular to the bedding plane. The presented models provide a tool for quantifying gas adsorption-induced anisotropic coal swelling and permeability behaviours.

Prediction of interactive effects of CBM production, faulting stress regime, and fault in coal reservoir: Numerical simulation

The stress in coal reservoir is dynamic and redistributes during coalbed methane (CBM) production. It is abundantly evident in the literature that maximum and minimum horizontal stresses reduce with reservoir pressure depletion. This study aims to predict the change of Anderson's faulting (tectonic regimes) in anisotropic coal due to gas production. A series of fully coupled scenario-based numerical simulations are conducted to investigate the effects of anisotropic sorption and mechanical properties of coal reservoir and immediate underlying rock stratum, and faulting on stress redistribution and gas production. For this purpose, the common assumption of uniaxial strain condition and constant overburden stress together with the widely acceptable values for the mechanical properties of coal are used for the simulations. The simulation results show that initial faulting stress regime, anisotropy in coal, and sorption parameters such as modulus of solid expansion have a significant role in prediction of final stress regime. It was also observed that a permeable fault located parallel to a horizontal borehole may contribute to gas production for both cases of the borehole drilled in the horst and graben structures. The simulation results of this study offer implications on the impact of CBM production on stress redistribution, surface subsidence, and transition of faulting stress regime around horizontal boreholes and far-field.

Characteristics of Water Migration During Spontaneous Imbibition in Anisotropic Coal

Characteristics of Water Migration During Spontaneous Imbibition in Anisotropic Coal

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.

Characterization of anisotropic coal permeability with the effect of sorption-induced deformation and stress

Changes in permeability are related to coalbed methane (CBM) production activities, and to coal mining safety. However, inherent properties, i.g. anisotropy and heterogeneity make the gas flow mechanism more complex. In this work, an anisotropic coal permeability model with synergistic stress compression and gas adsorption was constructed to simulate gas flow behavior in all directions in coal. Changes in coal anisotropic permeability are restricted by the fracture deformation in coal in all directions. Wherein, the fracture deformation in coal is controlled by the effective volume strain coordinated by gas adsorption and stress. The influence of stress can be expressed by employing the generalized Hooke’s law. Then, based on the coal matrix bridge structure, an internal swelling coefficient has been proposed to assess the control of the matrix swelling on fracture width during the gas adsorption process to further obtain the fracture deformation caused by gas adsorption. The anisotropic coal permeability model was verified by laboratory test data under different effective stresses, gas pressures, with respect to their combined influences. Subsequently, based on the experimental conditions of uniaxial strain, the permeability model of anisotropic coal during gas depletion was further deduced, and changes in anisotropic coal permeability under uniaxial strain were also calculated. In addition, the sensitivity of the reduction rate of the elastic modulus, and the internal swelling coefficient was also studied, to reveal the matrix-fracture interaction mechanism.

Research on the Failure Evolution Process of Rock Mass Base on the Acoustic Emission Parameters

Fracture mechanics behavior and acoustic emission (AE) characteristics of fractured rock mass are related to underground engineering safety construction, disaster prediction, and early warning. In this study, the failure evolution characteristics of intact and fracture (e.g., single fracture, parallel fractures, cross fractures, and mixed fractures) coal were studied and contrasted with each other on the basis of the distribution of max amplitude of AE. The study revealed some meaningful results, where the value of b (i.e., the distribution characteristic of max amplitude of AE) could represent the failure evolution process of intact and fractured coal. The maximum amplitude distribution of AE events was characterized by Gaussian normal distribution, and the probability of the maximum amplitude of AE events corresponding to 35∼50 dB was the largest. In the stress range of 60∼80%, AE events and maximum amplitude increased rapidly, and the corresponding b value decreased. The energy of AE events showed a downward trend after reaching the maximum value at about 80% stress level. Under the same stress level, the more complex the fracture was, the larger the b value of coal–rock mass was, and the stronger the inhibition effect on the fracture expansion caused by the internal fracture distribution was. Due to the anisotropy of coal–rock mass with a single crack, the distribution of the b value was more discrete, while the anisotropy of coal–rock mass with mixed crack decreased, and the dispersion of the b value decreased. The deformation of cracked coal mainly caused by the adjustment of cracks during the initial loading b value experienced a trend of decreasing first, then increasing, and then decreasing in the loading process. When the load reached 0.8 times of the peak strength, the b value had a secondary decreasing trend, indicating the macroscopic failure of the sample, which could be used as a precursor criterion for the complete failure of coal–rock mass.