Contents Preface Chapter 1 Introduction 1 1.1 Coal and Coal Measure Strata 1 1.1.1 Coal and Coal Seam Gas 1 1.1.2 Coal Measure Strata 7 1.2 Structure of Coal and Its Simplified Physical Model 13 1.2.1 Pores and Fractures in Coal 13 1.2.2 Simplified Physical Model of Coal 20 1.3 Comparison of Coal and Rock 22 1.3.1 Comparison of Structure and Strength 22 1.3.2 Comparison of Adsorption Properties 23 1.3.3 Comparison of Gas Outburst and Rock Burst Disasters 25 1.4 Research Contents of Coal Mechanics 26 1.4.1 Definition of Coal Mechanics 26 1.4.2 Development of Coal Mechanics 27 1.4.3 Structure of This Book 31 References 32 Chapter 2 Coal Formation and Metamorphism 41 2.1 Coal Forming Process 41 2.2 Coalification 43 2.2.1 Coal Diagenesis 43 2.2.2 Coal Metamorphism 43 2.3 The Maceral Composition and Metamorphic Type of Coal 44 2.3.1 The Maceral Composition of Coal 44 2.3.2 Metamorphism Types of Coal 46 2.4 Gas Generation in Coal 49 2.4.1 Gas Generation During the Biochemical Coalification Period 50 2.4.2 Gas Generation During the Coal Metamorphism Period 50 2.4.3 Coal Seam Gas Composition 53 2.4.4 Occurrence Status of Coal Seam Gas 56 2.5 Effect of Magma Intrusion on Coal Metamorphism and Gas Occurrence 59 2.5.1 Ways of Magma Intrusion into Coal Seams 59 2.5.2 Thermal Temperature Field Analysis of Magma 60 2.5.3 The Influence of Magma on Coal Metamorphism 64 2.5.4 The Influence of Magma on Gas Occurrence 65 Reference 72 Chapter 3 Basic Physical Properties and Characteristics of Coal Pores and Fractures 75 3.1 The Basic Physical Properties of Coal 75 3.1.1 Moisture of Coal 76 3.1.2 Ash in Coal 78 3.1.3 Volatiles of Coal 80 3.1.4 Density of Coal 81 3.1.5 Hardness of Coal 82 3.2 Coal Pore Features 85 3.2.1 Classification of Coal Pore 85 3.2.2 Coal Pore Characterization Methods 87 3.2.3 Pore Structure Characterization 93 3.3 Coal Fracture Features 97 3.3.1 Classification of Coal Fracture 97 3.3.2 Fracture Distribution Characteristics in Coal 97 3.4 The Matrix Characteristics of Coal 100 3.4.1 Definition of Coal Matrix 100 3.4.2 Coal Matrix Scale 101 References 104 Chapter 4 Gas Adsorption-Desorption Properties of Coal 107 4.1 Gas adsorption Properties of Coal 107 4.1.1 Gas Adsorption Mechanism in Coal 107 4.1.2 Gas Adsorption Law of Coal 108 4.1.3 Main Factors Affecting Gas Adsorption Properties of Coal 110 4.1.4 Test Methods of Gas Adsorption Properties 118 4.2 Gas Desorption Properties of Coal 123 4.2.1 Gas Desorption Mechanism 123 4.2.2 Main Factors Affecting Gas Desorption Properties 124 4.2.3 Gas Desorption Models of Coal 129 4.2.4 Test Methods of Gas Desorption Properties 132 4.2.5 Application of Gas Desorption Properties of Coal in Gas Content Determination 133 4.3 Gas Diffusion in Coal 137 4.3.1 Physical Process of Gas Diffusion in Coal 137 4.3.2 Mathematical Models of Coal Gas Diffusion 139 4.3.3 The Desorption Index of Drilling Cuttings (K1) 141 References 143 Chapter 5 Foundation of Coal Mechanics 146 5.1 Stress State 146 5.1.1 Concept of Stress 146 5.1.2 Specification of Stress State at a Point 148 5.1.3 Plane Stress State and the Mohr Stress Circle 150 5.1.4 Principal Stresses and Their Directions 153 5.2 Strain State 156 5.2.1 Displacement and Strain Concepts 156 5.2.2 Geometric Equation 158 5.2.3 Principal Strain and Volumetric Strain 160 5.3 Strength Criterion 162 5.3.1 Mohr-Coulomb Strength Criterion 162 5.3.2 Drucker-Prager Strength Criterion 164 5.3.3 Griffith’s Strength Criterion 165 5.4 The Effective Stress 166 References 168 Chapter 6 Strength and Deformation Characteristics of Coal Containing Gas 170 6.1 Mechanical Tests of Coal Containing Gas 170 6.1.1 Testing Facility and Experimental Principles 171 6.1.2 Basic Requirements for Strength Tests of Coal Containing Gas 173 6.1.3 Coal Sample Preparation Methods 174 6.2 Strength Characteristics of Coal Containing Gas 180 6.2.1 Conventional Compression Tests of Coal 180 6.2.2 Pre-peak Confining Pressure Unloading Tests of Coal Containing Gas 186 6.2.3 Analysis of the Strength Characteristics of Coal Containing Gas 190 6.3 Deformation Characteristics of Coal Containing Gas 201 6.3.1 Analysis of the Deformation Characteristics of Coal Containing Gas 201 6.3.2 Influence of Gas on Coal Deformation 207 6.4 Constitutive Equation of Coal Containing Gas 209 6.4.1 Analysis of the Constitutive Relationship Characteristics of Coal 209 6.4.2 Linear Elastic Stage 213 6.4.3 Nonlinear Elastoplastic Stage 215 6.4.4 Ideal Plastic Stage 216 6.4.5 Strain Softening Stage 216 6.5 Macroscopic Damage Features of Coal Containing Gas 216 6.6 Failure Mechanism and Strength Theory of Coal Containing Gas 220 6.6.1 C. D. Martin Fracture Strain Model 220 6.6.2 Damage Dilatation Progressive Failure Mechanism of Coal 222 6.6.3 Damage Dilatation Yield Criterion of Coal 225 References 230 Chapter 7 Seepage Properties and Permeability Evolution Model of Coal 235 7.1 Basic Concepts and Theoretical Basis of Coal Permeability 235 7.1.1 Basic Concepts of Coal Permeability 235 7.1.2 Theoretical Basis of Permeability Evolution 240 7.2 Typical Models of Permeability Evolution 245 7.2.1 Palmer–Mansoori Model 249 7.2.2 Shi–Durucan Model 251 7.2.3 Robertson–Christiansen Model 253 7.2.4 Evaluation of Classical Permeability Models 256 7.3 Instrument, Principle, and Method for Testing Coal Permeability 261 7.4 Impacts of Effective Stress and Sorption-Induced Deformation on Coal Permeability 264 7.4.1 Sorption-Induced Deformation of the Coal Matrix 264 7.4.2 Impact of Effective Stress on Coal Permeability 268 7.4.3 Impact of Sorption-Induced Deformation on Coal Permeability 269 7.4.4 Competitive Effect of Sorption-Induced Deformation and Effective Stress 270 7.5 Effect of Stress Perturbation on Coal Permeability 275 7.5.1 Evolution of Coal Permeability During Loading 275 7.5.2 Evolution of Coal Permeability During Unloading 282 7.5.3 Analysis of the Differences in Permeability Evolution Between Loading and Unloading 287 7.5.4 Relationships Between Coal Body Damage Evolution and Permeability During Unloading 291 7.6 Permeability Evolution Model of Gas Migration in Coal in a Stress Disturbed Area 308 7.6.1 Basic Ideas of Model Construction 308 7.6.2 Permeability Evolution Model of Coal Under the Action of Mining Stress Disturbance 309 7.6.3 Permeability Evolution Model of Coal Under the Action of Gas–Solid Coupling 311 7.6.4 Permeability Evolution Model of Coal Considering the Actions of Mining Stress Disturbance and Gas–Solid Coupling 317 References 317 Chapter 8 Geostress and Coal Seam Gas Occurrence 325 8.1 Basic Concept of Geostress 325 8.1.1 Concept of Geostress 325 8.1.2 Simplified Model of Geostress 326 8.2 In Situ Measurement Methods for Geostress 329 8.2.1 Fundamental Principles 329 8.2.2 Hydraulic Fracturing Method 331 8.2.3 Stress Relief Method 336 8.3 Evolution Rule of Geostress with Burial Depth 342 8.3.1 Measurement Results of Geostress 342 8.3.2 Evolution Rules of Geostress with Burial Depth 342 8.4 Tectonic Stress of Coal-Bearing Strata 347 8.4.1 Tectonic Stress 347 8.4.2 Influence of Tectonics on Coal Seam Occurrence 348 8.4.3 Dynamic Metamorphism of Tectonic Stress on Coal 349 8.5 Variation of Gas Parameters with Burial Depth 351 8.5.1 Variation of Gas Pressure with Burial Depth 351 8.5.2 Variation of Gas Content with Burial Depth 355 8.5.3 Variation of Permeability With Burial Depth 356 References 359 Chapter 9 Flow Theory of Gas in Coal Seams 365 9.1 Fundamental Laws of Gas Migration 365 9.1.1 Gas Migration Process in Coal Seams and Its Basic Assumptions 365 9.1.2 Classification of the Gas Flow Field in Coal Seams 369 9.2 Theory of Gas Diffusion in Coal Matrixes 373 9.3 Theory of Gas Seepage 376 9.4 Gas–Coal Coupling Model 378 9.4.1 Model Establishment of Gas-Coal Coupling 379 9.4.2 Boundary and Initial Conditions 381 9.5 Evolution Characteristics of Pore Pressure and Coal Permeability During Adsorption Equilibration 382 9.5.1 Introduction to the Physical Model 383 9.5.2 Variation of Gas Pressures with Equilibration Time 387 9.5.3 Permeability Variation with Equilibration Time 390 9.6 Characteristics of the Gas Flow Field During Extraction 392 9.6.1 Introduction to the Physical Model 393 9.6.2 Gas Pressure Distribution Characteristics During Gas Extraction 395 9.6.3 Daily Gas Drainage Volume of an Original Coal Seam 396 References 397 Chapter 10 The Application of Coal Mechanics in the Stress Relief Gas Extraction 402 10.1 Technology Synopsis of the Stress Relief Gas Extraction 402 10.2 Evolution Regulations of the Mining-Induced Stress and Expansion Deformation 410 10.2.1 Evolution Regulations of the Mining-Induced Stress 410 10.2.2 The Expansion Deformation 417 10.3 Temporal Evolution Regulation of the Permeability 422 10.3.1 The Basic Method for Forecasting the Permeability Evolution 422 10.3.2 Temporal Evolution Regulation of the Volumetric Stress and the Permeability 424 10.4 Stress Relief Gas Extraction and the Gas Flow Characteristics 428 10.4.1 The Initial Boundary Conditions in the Stress Relief Gas Extraction Model 428 10.4.2 Stress Relief Gas Extraction Analysis 429 10.4.3 Daily Gas Extraction Volume in Unloaded Coal Seam 430 10.5 Stress Relief Gas Extraction Method 434 10.5.1 Technology Synopsis of the Stress Relief Gas Extraction Method 436 10.5.2 Field Investigation of the Stress Relief Gas Extraction Effort 439 References 444 Chapter 11 Application of Coal Mechanics in Coal and Gas Outbursts 447 11.1 Introduction to Coal/Gas Outbursts 447 11.1.1 General Laws of Coal/Gas Outbursts 447 11.1.2 Research Methods 453 11.2 Mechanical Mechanism of Coal/Gas Outbursts 457 11.2.1 Hypothesis of Coal/Gas Outbursts 457 11.2.2 Division of Outburst Process 458 11.3 Coal/Gas Outburst Preparation and Trigger 459 11.3.1 Theoretical Model 459 11.3.2 Instability Criterion for the Coal/Gas Outburst Trigger 463 11.3.3 Numerical Model Calculation 467 11.3.4 Influencing Factors of the Outburst Trigger 469 11.3.5 Influence of Mechanical Properties on Outburst Tendency 474 11.4 Coal/Gas Outburst Development 480 11.4.1 Analysis of the Coal/Gas Outburst Development Process 480 11.4.2 Verification of the Outburst Development Process 484 11.5 Effect of Rapid Desorbed Gas on Transporting Coal 490 11.5.1 Energy Demand for Transporting Coal in the Development Stage of Outbursts 490 11.5.2 Source of Rapid Desorbed Gas 495 References 502
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