Cleats are systematic, natural fractures in coal seams. They account for most of the permeability and much of the porosity of coalbed methane reservoirs and can have a significant effect on the success of hydraulic fr...Cleats are systematic, natural fractures in coal seams. They account for most of the permeability and much of the porosity of coalbed methane reservoirs and can have a significant effect on the success of hydraulic fracturing stimulation. Laboratory hydraulic fracturing experiments were conducted on coal blocks under true tri-axial stress to simulate fracturing stimulation of coal seams. Fractures were initiated by injecting a water gel with luminous yellow fluorescent dye into an open hole section of a wellbore. The impact of cleats on initiation and propagation of hydraulic fractures in coal seams is discussed. Three types of hydraulic fracture initiation and propagation pattern were observed in this study： 1） The hydraulic fracture initiated and then grew along the cleat. 2） The hydraulic fracture initiated along a butt cleat or a fracture （natural or induced by drilling） oriented roughly in the minimum horizontal stress direction, then turned to propagate along the first face cleat that it encountered or gradually turned towards the maximum horizontal stress direction. 3） The hydraulic fracture initiated perpendicular to the minimum stress and, when it encountered a face cleat, tended to propagate along the cleats if the extension direction does not deviate greatly （〈20° as determined in this paper） from the maximum horizontal stress direction. When a coal seam is hydraulically fractured, the resulting fracture network is controlled by the combined effect of several factors： cleats determine the initiation and extension path of the fracture, the in-situ stress state dominates the main direction of the fracture zone and bedding planes impede fracture height growth.展开更多
The coalbed methane(CBM)reservoirs in the areas of eastern Yunnan and western Guizhou are characterized by developed cleats and fractures and low fracturing pressures,so cementing slurry(“slurry”for short)can invade...The coalbed methane(CBM)reservoirs in the areas of eastern Yunnan and western Guizhou are characterized by developed cleats and fractures and low fracturing pressures,so cementing slurry(“slurry”for short)can invade into CBM reservoir easily,resulting in reservoir damage and abnormal increase of reservoir transformation fracturing pressure.In order to reveal the damage mechanisms of slurry to this type of coal reservoirs,we analyzed the physical and chemical properties and potential damage modes of coal rocks.Then,the development situations of fractures and pores before and after the coal core samples were internally contaminated and the invasion and plugging situations of slurry in fractures and pores were analyzed intuitively by means of CT scanning and scanning electron microscope(SEM),and the percentage of slurry and fractures in coal core volume was calculated.In this way,a method to quantitatively evaluate the damage of slurry to coal reservoirs was established.And the following research results were obtained.First,under the effect of differential pressure,slurry and its filtrate invade into coal reservoirs along the fractures.The invasion degree varies with the development degree of fractures and pores.The more developed the fractures and pores,the higher the invasion degree.Second,the cement products formed after the slurry in the reservoirs gets cemented and solidified fill the fractures and pores tightly and cover the surface of coal core samples densely,so CBM flowing channels are blocked severely.Consequently,the permeability of coal core samples decreases and the compressive strength of coal rocks increase,leading to the abnormal increase of subsequent fracturing pressure and impacting the fracturing stimulation effects.Third,the effect of slurry filtrate on the alkali sensitivity and velocity sensitivity of coal rocks is much less than the damage degree of slurry invasion to coal rocks.In conclusion,this newly developed quantitative evaluation method for the damage of slurry to coal reservoirs is of guiding significance to improving the cement job quality of coal reservoirs and ensuring the efficient CBM development.展开更多
Coal mass consists of matrices and cleats,which exhibits significant difference in mechanical properties,such as uniaxial compressive strength and Young’s modulus.Understanding this difference is critical for a numbe...Coal mass consists of matrices and cleats,which exhibits significant difference in mechanical properties,such as uniaxial compressive strength and Young’s modulus.Understanding this difference is critical for a number of engineering applications,such as assessing the stability of cleated coal seam gas wellbores,underground exca-vation stability in coal seams,and estimating cleat aperture response during gas extraction and surface response to reservoir depletion.The conventional method of measuring coal mechanical properties using strain gauges or displacement transducers is impractical and unreliable as it only captures the value for the installed point.This study explores the use of a two-dimensional Digital Image Correlation(2D-DIC)method to quantify the areal deformation of coal matrix and cleat regions and their contribution to the bulk mechanical properties of coal.Cyclic uniaxial compression tests were performed on coal specimens from the Goonyella Middle Seam,Australia.The results from the DIC technique were initially validated against strain gauge and Advanced Video Exten-someter(AVE)measurements,showing minimal percentage differences:5%with the strain gauge;16.6%with the coal cleat region,12.03%with the coal matrix region,and 9.28%with the coal bulk region compared to AVE.These results demonstrate that DIC is a reliable and accurate method for measuring coal deformation.Comparative analysis of cleat,matrix,and overall coal surface regions revealed distinct variations in Young’s modulus,with ratios of E_(cleat):E_(matrix):E_(overall)=0.24:1.60:1.00.The calculated cleat and matrix moduli are 143.6 MPa and 1785.3 MPa respectively.The contributions of E_(matrix)and E_(cleat)to the overall Young’s modulus(E_(overall))were quantified,revealing that the matrix accounts for 56%(A=0.56)and the cleat for 44%(1-A=0.44)of the overall modulus.The compressibility of the cleat shows six times that of the coal matrix(C_(cleat):C_(matrix):C_(overall)=4.24:0.62:1.00),highlighting the critical role of cleats in coal deformation and stress-induced permeability changes.Furthermore,Poisson’s ratios computed from the DIC for the tested coal samples range from 0.19 to 0.33,showing strong agreement with reported values in the literature.By integrating DIC analysis with traditional mechanical testing,this study offers a robust approach to evaluating full-field deformation mechanisms in fractured materials.These findings advance the understanding of coal’s mechanical properties,which in turn supports more accurate geotechnical modeling,optimizes mining design,and enhances coal seam gas extraction strategies.展开更多
Reliable forecasting of coal seam gas production and gas injectivity(e.g.,CO_(2) or air)requires an accurate understanding of coal’s anisotropic permeability,which governs the directional flow of gas.Although the ani...Reliable forecasting of coal seam gas production and gas injectivity(e.g.,CO_(2) or air)requires an accurate understanding of coal’s anisotropic permeability,which governs the directional flow of gas.Although the anisotropic nature of coal permeability is well recognized,little attention has been paid to how this ratio evolves with changes in effective stress or with the injection of gases that have different affinities to coal.In this work,more than 600 permeability tests were conducted on eight cubic Australian coal samples using He,N_(2) and CO_(2) gases under varying effective stresses,providing a comprehensive dataset that allows the combined effects of effective stress and gas adsorption on permeability anisotropy to be robustly assessed on the same samples.The results demonstrated that all coal samples exhibited evident permeability anisotropy,with ratios ranging from 1.11 to 6.55.For the first time,quantitative relationships between the anisotropy ratio,effective stress,and initial permeability were established for each of the three injection gases,highlighting how gas adsorption and effective stress changes both anisotropic permeability magnitude and ratio.These findings provide new insights into the directional flow behavior of gases in coal seams,with implications for underground compressed air energy storage and CO_(2) sequestration.展开更多
Coalbed methane(CBM)is an important unconventional natural gas.Exploitation of multilayered CBM reservoir is still facing the challenge of low production rate.Radial borehole fracturing,which integrates radial jet dri...Coalbed methane(CBM)is an important unconventional natural gas.Exploitation of multilayered CBM reservoir is still facing the challenge of low production rate.Radial borehole fracturing,which integrates radial jet drilling and hydraulic fracturing,is expected to create complex fracture networks in multilayers and enhance CBM recovery.The main purpose of this paper is to investigate the mechanisms and efficacy of radial borehole fracturing in increasing CBM production in multiple layers.First,a two-phase flow and multi-scale 3 D fracture network including radial laterals,hydraulic fractures and face/butt cleats model is established,and embedded discrete fracture model(EDFM)is applied to handle the complex fracture networks.Then,effects of natural-fracture nonuniform distribution are investigated to show the advantages of targeted stimulation for radial borehole fracturing.Finally,two field CBM wells located in eastern Yunnan-western Guizhou,China were presented to illuminate the stimulation efficiency by radial borehole fracturing.The results indicated that compared with vertical well fracturing,radial borehole fracturing can achieve higher gas/water daily production rate and cumulative gas/water production,approximately 2 times higher.Targeted communications to cleats and sweet spots and flexibility in designing radial borehole parameters in different layers so as to increase fracture-network complexity and connectivity are the major reasons for production enhancement of radial borehole fracturing.Furthermore,the integration of geology-engineering is vital for the decision of radial borehole fracturing designing scheme.The key findings of this paper could provide useful insights towards understanding the capability of radial borehole fracturing in developing CBM and coal-measure gas in multiple-thin layers.展开更多
A series of experiments were pertbrmed to determine rock mechanical parameters related to hydraulic fracturing of coal. The effect of confining pressure and pore pressure on the strength of coal was stt, died. Experim...A series of experiments were pertbrmed to determine rock mechanical parameters related to hydraulic fracturing of coal. The effect of confining pressure and pore pressure on the strength of coal was stt, died. Experimental results show that the coal seam in the study areas has a relatively low elastic modulus, high Poisson＇s ratio, high fragility and is easily broken and compressed. The coal seam is considered as a transversely isotropic medium, since the physical properties in the direction of bedding plane and orthogonal to the bedding plane vary markedly. Based on the generalized plane strain model, stress distribution for an arbitrarily orientated wellbore in the coal seam was determined. In a horizontal well, hydraulic fracturing was lbund to initiate in the coal seam mass due to tensile failure, or from cleats due to shear or tensile failure. For those coal seams with abundant natural cleats, hydraulic fracture initiation can be induced by any of these mechanisms. In this study, hydraulic fracture initiation criteria tbr a horizontal well in a coal seam were established.展开更多
To explore the geological characteristics and exploration potential of the Carboniferous Benxi Formation coal rock gas in the Ordos Basin,this paper presents a systematic research on the coal rock distribution,coal ro...To explore the geological characteristics and exploration potential of the Carboniferous Benxi Formation coal rock gas in the Ordos Basin,this paper presents a systematic research on the coal rock distribution,coal rock reservoirs,coal rock quality,and coal rock gas features,resources and enrichment.Coal rock gas is a high-quality resource distinct from coalbed methane,and it has unique features in terms of burial depth,gas source,reservoir,gas content,and carbon isotopic composition.The Benxi Formation coal rocks cover an area of 16×104km^(2),with thicknesses ranging from 2 m to 25 m,primarily consisting of bright and semi-bright coals with primitive structures and low volatile and ash contents,indicating a good coal quality.The medium-to-high rank coal rocks have the total organic carbon(TOC)content ranging from 33.49%to 86.11%,averaging75.16%.They have a high degree of thermal evolution(Roof 1.2%-2.8%),and a high gas-generating capacity.They also have high stable carbon isotopic values(δ13C1of-37.6‰to-16‰;δ13C2of-21.7‰to-14.3‰).Deep coal rocks develop matrix pores such as gas bubble pores,organic pores,and inorganic mineral pores,which,together with cleats and fractures,form good reservoir spaces.The coal rock reservoirs exhibit the porosity of 0.54%-10.67%(averaging 5.42%)and the permeability of(0.001-14.600)×10^(-3)μm^(2)(averaging 2.32×10^(-3)μm^(2)).Vertically,there are five types of coal rock gas accumulation and dissipation combinations,among which the coal rock-mudstone gas accumulation combination and the coal rock-limestone gas accumulation combination are the most important,with good sealing conditions and high peak values of total hydrocarbon in gas logging.A model of coal rock gas accumulation has been constructed,which includes widespread distribution of medium-to-high rank coal rocks continually generating gas,matrix pores and cleats/fractures in coal rocks acting as large-scale reservoir spaces,tight cap rocks providing sealing,source-reservoir integration,and five types of efficient enrichment patterns(lateral pinchout complex,lenses,low-amplitude structures,nose-like structures,and lithologically self-sealing).According to the geological characteristics of coal rock gas,the Benxi Formation is divided into 8 plays,and the estimated coal rock gas resources with a buried depth of more than 2000 m are more than 12.33×10^(12)m^(3).The above understandings guide the deployment of risk exploration.Two wells drilled accordingly obtained an industrial gas flow,driving the further deployment of exploratory and appraisal wells.Substantial breakthroughs have been achieved,with the possible reserves over a trillion cubic meters and the proved reserves over a hundred billion cubic meters,which is of great significance for the reserves increase and efficient development of natural gas in China.展开更多
Fracture propagation mechanisms in coalbed methane(CBM) reservoirs are very complex due to the development of the internal cleat system. In this paper, the characteristics of initiation and propagation of hydraulic fr...Fracture propagation mechanisms in coalbed methane(CBM) reservoirs are very complex due to the development of the internal cleat system. In this paper, the characteristics of initiation and propagation of hydraulic fractures in coal specimens at different angles between the face cleat and the maximum horizontal principal stress were investigated with hydraulic fracturing tests. The results indicate that the interactions between the hydraulic fractures and the cleat system have a major effect on fracture networks. "Step-like’’ fractures were formed in most experiments due to the existence of discontinuous butt cleats. The hydraulic fractures were more likely to divert or propagate along the butt cleat with an increase in the angles and a decrease in the horizontal principal stress difference. An increase in the injection rate and a decrease in the fracturing fluid viscosity were more conducive to fracture networks. In addition, the influence on fracture propagation of the residual coal fines in the wellbore was also studied. The existence of coal fines was an obstacle in fracturing, and no effective connection can be formed between fractures. The experimental investigation revealed the fracture propagation mechanisms and can provide guidance for hydraulic fracturing design of CBM reservoirs.展开更多
Dimensions and mutual relationships amongst fracture(cleat)parameters such as maximum aperture width,average aperture width,length,spacing etc.control the connectivity within a fracture network and the fluid flow in a...Dimensions and mutual relationships amongst fracture(cleat)parameters such as maximum aperture width,average aperture width,length,spacing etc.control the connectivity within a fracture network and the fluid flow in a coal seam as the matrix permeability,here,is negligible.In this paper,we document cleat size distributions and investigate length–aperture relationships from coals of Raniganj coalfield in Eastern India.This coalfield has a proven extractable reserve of six billion tons of coal and holds immense potential to be one of the largest coal bed methane fields serving India’s growing energy needs.Here,cleat length(L)correlates with corresponding maximum aperture width(Dmax)in a power-law function with an exponent of 0.84(DmaxαL0.84)instead of the commonly observed exponents of 1 or 0.5 applicable for other natural‘opening-mode’fractures.The conventional wisdom pertains that laminar fluid flow(Q)through an isolated,smooth-walled,parallel-plate fracture,embedded in an impermeable matrix,is directly proportional to the cube of its aperture width(b,equivalent to Davg;cubic law:Qαb3).This assumes a linear relationship between length and fracture aperture.However,the modified relationship between cleat length and average aperture width changes the cubic law applicable for Raniganj coal seam and now fluid flow correlates with aperture width in a power-law function with an exponent of 4.25(Qαb4.25)instead of 3(cube).Such simplifications will come handy for the modeling and estimation of fluid flow as it will reduce the effort of cleat length measurement which is anyway difficult and can be misleading due to the risk of undersampling.展开更多
Cleat system of coal reservoir is one of the main migrating passage of coalbed methane (CBM). The development of cleat system has important influence on both the preservation of CBM in geological history and surface C...Cleat system of coal reservoir is one of the main migrating passage of coalbed methane (CBM). The development of cleat system has important influence on both the preservation of CBM in geological history and surface CBM exploitation. The relationship among cleat porosity, net confine pressure, rock mechanics, coal seam’s occurrence and other factors of coal reservoir is established and simulated based on the energy conservation law. The result indicates that the net confine pressure and buried depth of coalbed are the major control factors of cleat porosity. The extensive stress and abnormal high reservoir pressure can make cleats open thus increase tbe cleat porosity; while the overburden pressure and compressive stress make cleats close and decrease the cleat porosity. The influence of occurrence (dip and dip angle) of coalbed on cleat porosity depends on the change of the above mentioned factors. It is also affected by rock mechanics parameters to some extent, while water-gas saturation and reservoir temperature have little effect on cleat porosity. The above conclusions are of great significance in geological exploration and surface exploitation region determination of CBM.展开更多
After a general analysis regarding the concept of coal ＂cleat system＂, its genetic origin and practical applications to coalbed methane （CBM） commercial production and to C02 geological sequestration projects, the...After a general analysis regarding the concept of coal ＂cleat system＂, its genetic origin and practical applications to coalbed methane （CBM） commercial production and to C02 geological sequestration projects, the authors have developed a method to answer, quickly and accurately in accordance with the industrial practice and needs, the following yet unanswered questions： （1） how to define the spatial orientation of the different classes of cleats presented in a coal seam and （2） how to determine the frequency of their connectivites. The new available and presented techniques to answer these questions have a strong computer based tool （geographic information system, GIS）, able to build a complete georeferentiated database, which will allow to three-dimensionally locate the laboratory samples in the coalfield. It will also allow to better understand the coal cleat system and consequently to recognize the best pathways to gas flow through the coal seam. Such knowledge is considered crucial for understanding what is likely to be the most efficient opening of cleat network, then allowing the injection with the right spatial orientation, of pressurized fluids in order to directly drain the maximum amount of gas flow to a CBM exploitation well. The method is also applicable to the CO2 geological sequestration technologies and operations corresponding to the injection of CO2 sequestered from industrial plants in coal seams of abandoned coal mines or deep coal seams.展开更多
Cleats are the dominant micro-fracture network controlling the macro-mechanical behavior of coal.Improved understanding of the spatial characteristics of cleat networks is therefore important to the coal mining indust...Cleats are the dominant micro-fracture network controlling the macro-mechanical behavior of coal.Improved understanding of the spatial characteristics of cleat networks is therefore important to the coal mining industry.Discrete fracture networks(DFNs)are increasingly used in engineering analyses to spatially model fractures at various scales.The reliability of coal DFNs largely depends on the confidence in the input cleat statistics.Estimates of these parameters can be made from image-based three-dimensional(3D)characterization of coal cleats using X-ray micro-computed tomography(m CT).One key step in this process,after cleat extraction,is the separation of individual cleats,without which the cleats are a connected network and statistics for different cleat sets cannot be measured.In this paper,a feature extraction-based image processing method is introduced to identify and separate distinct cleat groups from 3D X-ray m CT images.Kernels(filters)representing explicit cleat features of coal are built and cleat separation is successfully achieved by convolutional operations on 3D coal images.The new method is applied to a coal specimen with 80 mm in diameter and 100 mm in length acquired from an Anglo American Steelmaking Coal mine in the Bowen Basin,Queensland,Australia.It is demonstrated that the new method produces reliable cleat separation capable of defining individual cleats and preserving 3D topology after separation.Bedding-parallel fractures are also identified and separated,which has his-torically been challenging to delineate and rarely reported.A variety of cleat/fracture statistics is measured which not only can quantitatively characterize the cleat/fracture system but also can be used for DFN modeling.Finally,variability and heterogeneity with respect to the core axis are investigated.Significant heterogeneity is observed and suggests that the representative elementary volume(REV)of the cleat groups for engineering purposes may be a complex problem requiring careful consideration.展开更多
The ride vibration of a tractor is affected mostly by the stiffness and damping coefficient of the seat suspension,cabin suspension,cabin rubber mounts,and rubber tires.However,in the case of rubber tractor tires,the ...The ride vibration of a tractor is affected mostly by the stiffness and damping coefficient of the seat suspension,cabin suspension,cabin rubber mounts,and rubber tires.However,in the case of rubber tractor tires,the stiffnesses and damping coefficients have not been researched adequately thus far,and it is not simple to measure these characteristics.In this study,a method for measuring and analyzing the stiffnesses and damping coefficients of rubber tractor tires,which were the input parameters for the tractor ride vibration simulation,was proposed.The cleat test,proposed in this study,did not require separate and complicated test equipment,unlike the conventional methods.The test was conducted simply by measuring acceleration under the driving conditions of the vehicle without detaching tires from the vehicle body or setting up additional test equipment.Based on the ground-vertical acceleration data obtained,the stiffness was calculated using the logarithmic decrement method,and the damping coefficient was calculated using least squares exponential curve fitting.The result of the cleat test indicated that the front tires had stiffnesses of 486.08-570.69 kN/m and damping coefficients of 4.02-4.52 kN·s/m;the rear tires had stiffnesses of 409.42-483.79 kN/m and damping coefficients of 2.21-2.67 kN·s/m.During the test,40 mm height cleats were installed on the track and the speed of the tractor was set to 7 and 10 km/h,which were the most common speeds during the operation.This study is meaningful in that it has presented a new method that improves the practicality of results,reduces cost,and simplifies the test process for measuring the stiffnesses and damping coefficients of rubber tractor tires.展开更多
基金support from the National Natural Science Foundation of China (Grant Nos. 51274216 and 51322404)
文摘Cleats are systematic, natural fractures in coal seams. They account for most of the permeability and much of the porosity of coalbed methane reservoirs and can have a significant effect on the success of hydraulic fracturing stimulation. Laboratory hydraulic fracturing experiments were conducted on coal blocks under true tri-axial stress to simulate fracturing stimulation of coal seams. Fractures were initiated by injecting a water gel with luminous yellow fluorescent dye into an open hole section of a wellbore. The impact of cleats on initiation and propagation of hydraulic fractures in coal seams is discussed. Three types of hydraulic fracture initiation and propagation pattern were observed in this study： 1） The hydraulic fracture initiated and then grew along the cleat. 2） The hydraulic fracture initiated along a butt cleat or a fracture （natural or induced by drilling） oriented roughly in the minimum horizontal stress direction, then turned to propagate along the first face cleat that it encountered or gradually turned towards the maximum horizontal stress direction. 3） The hydraulic fracture initiated perpendicular to the minimum stress and, when it encountered a face cleat, tended to propagate along the cleats if the extension direction does not deviate greatly （〈20° as determined in this paper） from the maximum horizontal stress direction. When a coal seam is hydraulically fractured, the resulting fracture network is controlled by the combined effect of several factors： cleats determine the initiation and extension path of the fracture, the in-situ stress state dominates the main direction of the fracture zone and bedding planes impede fracture height growth.
基金supported by the National Major Science&Technology Project“Development of large oil/gas fields and CBM”(No.:2016ZX05044)the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China(No.:IRT_14R58)。
文摘The coalbed methane(CBM)reservoirs in the areas of eastern Yunnan and western Guizhou are characterized by developed cleats and fractures and low fracturing pressures,so cementing slurry(“slurry”for short)can invade into CBM reservoir easily,resulting in reservoir damage and abnormal increase of reservoir transformation fracturing pressure.In order to reveal the damage mechanisms of slurry to this type of coal reservoirs,we analyzed the physical and chemical properties and potential damage modes of coal rocks.Then,the development situations of fractures and pores before and after the coal core samples were internally contaminated and the invasion and plugging situations of slurry in fractures and pores were analyzed intuitively by means of CT scanning and scanning electron microscope(SEM),and the percentage of slurry and fractures in coal core volume was calculated.In this way,a method to quantitatively evaluate the damage of slurry to coal reservoirs was established.And the following research results were obtained.First,under the effect of differential pressure,slurry and its filtrate invade into coal reservoirs along the fractures.The invasion degree varies with the development degree of fractures and pores.The more developed the fractures and pores,the higher the invasion degree.Second,the cement products formed after the slurry in the reservoirs gets cemented and solidified fill the fractures and pores tightly and cover the surface of coal core samples densely,so CBM flowing channels are blocked severely.Consequently,the permeability of coal core samples decreases and the compressive strength of coal rocks increase,leading to the abnormal increase of subsequent fracturing pressure and impacting the fracturing stimulation effects.Third,the effect of slurry filtrate on the alkali sensitivity and velocity sensitivity of coal rocks is much less than the damage degree of slurry invasion to coal rocks.In conclusion,this newly developed quantitative evaluation method for the damage of slurry to coal reservoirs is of guiding significance to improving the cement job quality of coal reservoirs and ensuring the efficient CBM development.
文摘Coal mass consists of matrices and cleats,which exhibits significant difference in mechanical properties,such as uniaxial compressive strength and Young’s modulus.Understanding this difference is critical for a number of engineering applications,such as assessing the stability of cleated coal seam gas wellbores,underground exca-vation stability in coal seams,and estimating cleat aperture response during gas extraction and surface response to reservoir depletion.The conventional method of measuring coal mechanical properties using strain gauges or displacement transducers is impractical and unreliable as it only captures the value for the installed point.This study explores the use of a two-dimensional Digital Image Correlation(2D-DIC)method to quantify the areal deformation of coal matrix and cleat regions and their contribution to the bulk mechanical properties of coal.Cyclic uniaxial compression tests were performed on coal specimens from the Goonyella Middle Seam,Australia.The results from the DIC technique were initially validated against strain gauge and Advanced Video Exten-someter(AVE)measurements,showing minimal percentage differences:5%with the strain gauge;16.6%with the coal cleat region,12.03%with the coal matrix region,and 9.28%with the coal bulk region compared to AVE.These results demonstrate that DIC is a reliable and accurate method for measuring coal deformation.Comparative analysis of cleat,matrix,and overall coal surface regions revealed distinct variations in Young’s modulus,with ratios of E_(cleat):E_(matrix):E_(overall)=0.24:1.60:1.00.The calculated cleat and matrix moduli are 143.6 MPa and 1785.3 MPa respectively.The contributions of E_(matrix)and E_(cleat)to the overall Young’s modulus(E_(overall))were quantified,revealing that the matrix accounts for 56%(A=0.56)and the cleat for 44%(1-A=0.44)of the overall modulus.The compressibility of the cleat shows six times that of the coal matrix(C_(cleat):C_(matrix):C_(overall)=4.24:0.62:1.00),highlighting the critical role of cleats in coal deformation and stress-induced permeability changes.Furthermore,Poisson’s ratios computed from the DIC for the tested coal samples range from 0.19 to 0.33,showing strong agreement with reported values in the literature.By integrating DIC analysis with traditional mechanical testing,this study offers a robust approach to evaluating full-field deformation mechanisms in fractured materials.These findings advance the understanding of coal’s mechanical properties,which in turn supports more accurate geotechnical modeling,optimizes mining design,and enhances coal seam gas extraction strategies.
基金funded by industry members APLNG,Arrow Energy,and Santos through The Gas and Energy Transition Research Centre in The University of Queensland.
文摘Reliable forecasting of coal seam gas production and gas injectivity(e.g.,CO_(2) or air)requires an accurate understanding of coal’s anisotropic permeability,which governs the directional flow of gas.Although the anisotropic nature of coal permeability is well recognized,little attention has been paid to how this ratio evolves with changes in effective stress or with the injection of gases that have different affinities to coal.In this work,more than 600 permeability tests were conducted on eight cubic Australian coal samples using He,N_(2) and CO_(2) gases under varying effective stresses,providing a comprehensive dataset that allows the combined effects of effective stress and gas adsorption on permeability anisotropy to be robustly assessed on the same samples.The results demonstrated that all coal samples exhibited evident permeability anisotropy,with ratios ranging from 1.11 to 6.55.For the first time,quantitative relationships between the anisotropy ratio,effective stress,and initial permeability were established for each of the three injection gases,highlighting how gas adsorption and effective stress changes both anisotropic permeability magnitude and ratio.These findings provide new insights into the directional flow behavior of gases in coal seams,with implications for underground compressed air energy storage and CO_(2) sequestration.
基金supported by the National Natural Science Foundation of China(National R&D Program for Major Research Instruments,51827804)Youth Program of National Natural Science Foundation of China(52004299)National Science Foundation for Distinguished Young Scholars(51725404)
文摘Coalbed methane(CBM)is an important unconventional natural gas.Exploitation of multilayered CBM reservoir is still facing the challenge of low production rate.Radial borehole fracturing,which integrates radial jet drilling and hydraulic fracturing,is expected to create complex fracture networks in multilayers and enhance CBM recovery.The main purpose of this paper is to investigate the mechanisms and efficacy of radial borehole fracturing in increasing CBM production in multiple layers.First,a two-phase flow and multi-scale 3 D fracture network including radial laterals,hydraulic fractures and face/butt cleats model is established,and embedded discrete fracture model(EDFM)is applied to handle the complex fracture networks.Then,effects of natural-fracture nonuniform distribution are investigated to show the advantages of targeted stimulation for radial borehole fracturing.Finally,two field CBM wells located in eastern Yunnan-western Guizhou,China were presented to illuminate the stimulation efficiency by radial borehole fracturing.The results indicated that compared with vertical well fracturing,radial borehole fracturing can achieve higher gas/water daily production rate and cumulative gas/water production,approximately 2 times higher.Targeted communications to cleats and sweet spots and flexibility in designing radial borehole parameters in different layers so as to increase fracture-network complexity and connectivity are the major reasons for production enhancement of radial borehole fracturing.Furthermore,the integration of geology-engineering is vital for the decision of radial borehole fracturing designing scheme.The key findings of this paper could provide useful insights towards understanding the capability of radial borehole fracturing in developing CBM and coal-measure gas in multiple-thin layers.
基金the financial support from the National Natural Science Foundation of China(No.51204195,No.51074171 and No.51274216)
文摘A series of experiments were pertbrmed to determine rock mechanical parameters related to hydraulic fracturing of coal. The effect of confining pressure and pore pressure on the strength of coal was stt, died. Experimental results show that the coal seam in the study areas has a relatively low elastic modulus, high Poisson＇s ratio, high fragility and is easily broken and compressed. The coal seam is considered as a transversely isotropic medium, since the physical properties in the direction of bedding plane and orthogonal to the bedding plane vary markedly. Based on the generalized plane strain model, stress distribution for an arbitrarily orientated wellbore in the coal seam was determined. In a horizontal well, hydraulic fracturing was lbund to initiate in the coal seam mass due to tensile failure, or from cleats due to shear or tensile failure. For those coal seams with abundant natural cleats, hydraulic fracture initiation can be induced by any of these mechanisms. In this study, hydraulic fracture initiation criteria tbr a horizontal well in a coal seam were established.
基金Supported by the PetroChina Science and Technology Major Project(2023ZZ18-03)Changqing Oilfield Major Science and Technology Project(2023DZZ01)。
文摘To explore the geological characteristics and exploration potential of the Carboniferous Benxi Formation coal rock gas in the Ordos Basin,this paper presents a systematic research on the coal rock distribution,coal rock reservoirs,coal rock quality,and coal rock gas features,resources and enrichment.Coal rock gas is a high-quality resource distinct from coalbed methane,and it has unique features in terms of burial depth,gas source,reservoir,gas content,and carbon isotopic composition.The Benxi Formation coal rocks cover an area of 16×104km^(2),with thicknesses ranging from 2 m to 25 m,primarily consisting of bright and semi-bright coals with primitive structures and low volatile and ash contents,indicating a good coal quality.The medium-to-high rank coal rocks have the total organic carbon(TOC)content ranging from 33.49%to 86.11%,averaging75.16%.They have a high degree of thermal evolution(Roof 1.2%-2.8%),and a high gas-generating capacity.They also have high stable carbon isotopic values(δ13C1of-37.6‰to-16‰;δ13C2of-21.7‰to-14.3‰).Deep coal rocks develop matrix pores such as gas bubble pores,organic pores,and inorganic mineral pores,which,together with cleats and fractures,form good reservoir spaces.The coal rock reservoirs exhibit the porosity of 0.54%-10.67%(averaging 5.42%)and the permeability of(0.001-14.600)×10^(-3)μm^(2)(averaging 2.32×10^(-3)μm^(2)).Vertically,there are five types of coal rock gas accumulation and dissipation combinations,among which the coal rock-mudstone gas accumulation combination and the coal rock-limestone gas accumulation combination are the most important,with good sealing conditions and high peak values of total hydrocarbon in gas logging.A model of coal rock gas accumulation has been constructed,which includes widespread distribution of medium-to-high rank coal rocks continually generating gas,matrix pores and cleats/fractures in coal rocks acting as large-scale reservoir spaces,tight cap rocks providing sealing,source-reservoir integration,and five types of efficient enrichment patterns(lateral pinchout complex,lenses,low-amplitude structures,nose-like structures,and lithologically self-sealing).According to the geological characteristics of coal rock gas,the Benxi Formation is divided into 8 plays,and the estimated coal rock gas resources with a buried depth of more than 2000 m are more than 12.33×10^(12)m^(3).The above understandings guide the deployment of risk exploration.Two wells drilled accordingly obtained an industrial gas flow,driving the further deployment of exploratory and appraisal wells.Substantial breakthroughs have been achieved,with the possible reserves over a trillion cubic meters and the proved reserves over a hundred billion cubic meters,which is of great significance for the reserves increase and efficient development of natural gas in China.
基金funded by the National Science and Technology Major Project of China (2016ZX05046004-003)Northeast Petroleum University Innovation Foundation for Postgraduate (YJSCX2017-010NEPU and YJSCX2017-009NEPU)
文摘Fracture propagation mechanisms in coalbed methane(CBM) reservoirs are very complex due to the development of the internal cleat system. In this paper, the characteristics of initiation and propagation of hydraulic fractures in coal specimens at different angles between the face cleat and the maximum horizontal principal stress were investigated with hydraulic fracturing tests. The results indicate that the interactions between the hydraulic fractures and the cleat system have a major effect on fracture networks. "Step-like’’ fractures were formed in most experiments due to the existence of discontinuous butt cleats. The hydraulic fractures were more likely to divert or propagate along the butt cleat with an increase in the angles and a decrease in the horizontal principal stress difference. An increase in the injection rate and a decrease in the fracturing fluid viscosity were more conducive to fracture networks. In addition, the influence on fracture propagation of the residual coal fines in the wellbore was also studied. The existence of coal fines was an obstacle in fracturing, and no effective connection can be formed between fractures. The experimental investigation revealed the fracture propagation mechanisms and can provide guidance for hydraulic fracturing design of CBM reservoirs.
基金This is part of KB's doctoral research.This work is also a part of the pilot project for a project proposal(Project No:154)submitted to The Ministry of Earth Sciences(MoES)Government of India.The authors are grateful to the Indian Institute of Science Education and Research Bhopal(IISERB)for funding the necessary equipment and providing logistical support+3 种基金We thank Coal India Limited(CIL)for their support and access to coal mines of Raniganj and Barakar coal fieldsThe authors are also thankful to Prof.S.K.Tandon of the Department of Earth and Environmental Sciences,USER Bhopal(Grant No.INST/EES/2016037)three anonymous referees and the Editor for their valuable comments that helped immensely to improve the manuscriptThe authors have no conflict of interest while publishing this study.
文摘Dimensions and mutual relationships amongst fracture(cleat)parameters such as maximum aperture width,average aperture width,length,spacing etc.control the connectivity within a fracture network and the fluid flow in a coal seam as the matrix permeability,here,is negligible.In this paper,we document cleat size distributions and investigate length–aperture relationships from coals of Raniganj coalfield in Eastern India.This coalfield has a proven extractable reserve of six billion tons of coal and holds immense potential to be one of the largest coal bed methane fields serving India’s growing energy needs.Here,cleat length(L)correlates with corresponding maximum aperture width(Dmax)in a power-law function with an exponent of 0.84(DmaxαL0.84)instead of the commonly observed exponents of 1 or 0.5 applicable for other natural‘opening-mode’fractures.The conventional wisdom pertains that laminar fluid flow(Q)through an isolated,smooth-walled,parallel-plate fracture,embedded in an impermeable matrix,is directly proportional to the cube of its aperture width(b,equivalent to Davg;cubic law:Qαb3).This assumes a linear relationship between length and fracture aperture.However,the modified relationship between cleat length and average aperture width changes the cubic law applicable for Raniganj coal seam and now fluid flow correlates with aperture width in a power-law function with an exponent of 4.25(Qαb4.25)instead of 3(cube).Such simplifications will come handy for the modeling and estimation of fluid flow as it will reduce the effort of cleat length measurement which is anyway difficult and can be misleading due to the risk of undersampling.
文摘Cleat system of coal reservoir is one of the main migrating passage of coalbed methane (CBM). The development of cleat system has important influence on both the preservation of CBM in geological history and surface CBM exploitation. The relationship among cleat porosity, net confine pressure, rock mechanics, coal seam’s occurrence and other factors of coal reservoir is established and simulated based on the energy conservation law. The result indicates that the net confine pressure and buried depth of coalbed are the major control factors of cleat porosity. The extensive stress and abnormal high reservoir pressure can make cleats open thus increase tbe cleat porosity; while the overburden pressure and compressive stress make cleats close and decrease the cleat porosity. The influence of occurrence (dip and dip angle) of coalbed on cleat porosity depends on the change of the above mentioned factors. It is also affected by rock mechanics parameters to some extent, while water-gas saturation and reservoir temperature have little effect on cleat porosity. The above conclusions are of great significance in geological exploration and surface exploitation region determination of CBM.
基金Fundao Fernando Pessoa/Fernando Pessoa University for supporting this investigation in the scope of the GIAGEB-Global Change, Energy, Environment and Bioengineering Research Unit
文摘After a general analysis regarding the concept of coal ＂cleat system＂, its genetic origin and practical applications to coalbed methane （CBM） commercial production and to C02 geological sequestration projects, the authors have developed a method to answer, quickly and accurately in accordance with the industrial practice and needs, the following yet unanswered questions： （1） how to define the spatial orientation of the different classes of cleats presented in a coal seam and （2） how to determine the frequency of their connectivites. The new available and presented techniques to answer these questions have a strong computer based tool （geographic information system, GIS）, able to build a complete georeferentiated database, which will allow to three-dimensionally locate the laboratory samples in the coalfield. It will also allow to better understand the coal cleat system and consequently to recognize the best pathways to gas flow through the coal seam. Such knowledge is considered crucial for understanding what is likely to be the most efficient opening of cleat network, then allowing the injection with the right spatial orientation, of pressurized fluids in order to directly drain the maximum amount of gas flow to a CBM exploitation well. The method is also applicable to the CO2 geological sequestration technologies and operations corresponding to the injection of CO2 sequestered from industrial plants in coal seams of abandoned coal mines or deep coal seams.
文摘Cleats are the dominant micro-fracture network controlling the macro-mechanical behavior of coal.Improved understanding of the spatial characteristics of cleat networks is therefore important to the coal mining industry.Discrete fracture networks(DFNs)are increasingly used in engineering analyses to spatially model fractures at various scales.The reliability of coal DFNs largely depends on the confidence in the input cleat statistics.Estimates of these parameters can be made from image-based three-dimensional(3D)characterization of coal cleats using X-ray micro-computed tomography(m CT).One key step in this process,after cleat extraction,is the separation of individual cleats,without which the cleats are a connected network and statistics for different cleat sets cannot be measured.In this paper,a feature extraction-based image processing method is introduced to identify and separate distinct cleat groups from 3D X-ray m CT images.Kernels(filters)representing explicit cleat features of coal are built and cleat separation is successfully achieved by convolutional operations on 3D coal images.The new method is applied to a coal specimen with 80 mm in diameter and 100 mm in length acquired from an Anglo American Steelmaking Coal mine in the Bowen Basin,Queensland,Australia.It is demonstrated that the new method produces reliable cleat separation capable of defining individual cleats and preserving 3D topology after separation.Bedding-parallel fractures are also identified and separated,which has his-torically been challenging to delineate and rarely reported.A variety of cleat/fracture statistics is measured which not only can quantitatively characterize the cleat/fracture system but also can be used for DFN modeling.Finally,variability and heterogeneity with respect to the core axis are investigated.Significant heterogeneity is observed and suggests that the representative elementary volume(REV)of the cleat groups for engineering purposes may be a complex problem requiring careful consideration.
基金This research was supported by the Basic Science Research Program through the National Research Foundation of Korea(NRF)funded by the Ministry of Education(Grant No.2019R1F1A1059712).
文摘The ride vibration of a tractor is affected mostly by the stiffness and damping coefficient of the seat suspension,cabin suspension,cabin rubber mounts,and rubber tires.However,in the case of rubber tractor tires,the stiffnesses and damping coefficients have not been researched adequately thus far,and it is not simple to measure these characteristics.In this study,a method for measuring and analyzing the stiffnesses and damping coefficients of rubber tractor tires,which were the input parameters for the tractor ride vibration simulation,was proposed.The cleat test,proposed in this study,did not require separate and complicated test equipment,unlike the conventional methods.The test was conducted simply by measuring acceleration under the driving conditions of the vehicle without detaching tires from the vehicle body or setting up additional test equipment.Based on the ground-vertical acceleration data obtained,the stiffness was calculated using the logarithmic decrement method,and the damping coefficient was calculated using least squares exponential curve fitting.The result of the cleat test indicated that the front tires had stiffnesses of 486.08-570.69 kN/m and damping coefficients of 4.02-4.52 kN·s/m;the rear tires had stiffnesses of 409.42-483.79 kN/m and damping coefficients of 2.21-2.67 kN·s/m.During the test,40 mm height cleats were installed on the track and the speed of the tractor was set to 7 and 10 km/h,which were the most common speeds during the operation.This study is meaningful in that it has presented a new method that improves the practicality of results,reduces cost,and simplifies the test process for measuring the stiffnesses and damping coefficients of rubber tractor tires.
