Contents Chapter 1 Introduction 1 1.1 Research background and significances 1 1.2 Unstable dynamic propagation behaviours of the multistage hydrofracturing fracture network 7 1.2.1 Deflection of fractures under the initiation sequence and perforation cluster spacing and well spacing 9 1.2.2 Disturbance of the stress field in the propagation process of multiple fractures 15 1.3 Mechanisms of induced deflection of fluid-driven fractures 16 1.3.1 Stress shadow effects between multiple fractures 17 1.3.2 Controlling factors of stress shadows 21 1.4 Numerical analysis of continuous stress field and discontinuous fracture 25 1.4.1 Continuum-discontinuum numerical methods and models 26 1.4.2 Simulation of stress-dependent unstable dynamic propagation of fractures 32 1.5 Challenges and perspectives 37 1.6 Conclusions 38 References 39 Chapter 2 Dual bilinear cohesive zone model for fluid-driven propagation of multiscale tensile and shear fractures 48 2.1 Introduction 48 2.2 Governing partial differential equations for hydrofracturing 52 2.2.1 Governing equations of solid deformation 53 2.2.2 Governing equations of fluid flow in fractured porous media 55 2.3 Dual bilinear cohesive zone model 56 2.4 Numerical discretization 58 2.4.1 Finite element discretization for solid 58 2.4.2 Finite volume discretization for fluid 59 2.5 Detection and separation of discrete elements 60 2.6 Global algorithm and procedure 62 2.7 Results and discussion 63 2.7.1 Verification of fracture propagation through analytical solutions in KGD and PKN models 63 2.7.2 Laboratory scales: dynamic propagation of small-size hydraulic fractures 67 2.7.3 Engineering scales: dynamic propagation of large-size hydraulic fractures 68 2.7.4 Distribution of tensile and shear fractures in hydrofracturing process 73 2.8 Conclusions 75 References 76 Chapter 3 Multi-thread parallel computation method for dynamic propagation of hydraulic fracture networks 80 3.1 Introduction 80 3.2 Governing partial differential equations and fracture criteria for hydrofracturing 83 3.3 Multi-thread parallel computation scheme for solid and fluid analysis 83 3.4 Global algorithm and procedure 84 3.5 Results and discussion 86 3.5.1 Example 1: Verification for multi-thread parallel computation solutions of hydraulic fracture propagation 86 3.5.2 Example 2: Multi-thread parallel computation efficiency for fluid-driven fracture propagation 90 3.5.3 Example 3: Parallel computation using multi-type elements and meshes 93 3.5.4 Example 4: Dynamic propagation behaviours of fractures under in-situ stresses and external fluid drive 96 3.6 Conclusions 99 References 100 Chapter 4 Heterogeneous continuum-discontinuum computation method for dynamic diversion and penetration of hydraulic fractures contacting multi-layers and granules 105 4.1 Introduction 105 4.2 Governing partial differential equations and fracture criteria in fractured porous media 109 4.3 Combined finite element-discrete element-finite volume method and algorithm for multi-materials 110 4.4 Numerical models of tight heterogeneous reservoirs with bedding and granules 112 4.5 Results and discussions of dynamic propagation behaviours of hydraulic fractures in the multilayered reservoir 118 4.5.1 A typical example implementation of fracture propagation and pore pressure in the heterogeneous tight reservoir with beddings 118 4.5.2 Influence of bedding deviation angle and geomaterial properties on hydraulic fracture propagation 120 4.5.3 Quantitative final length and propagation states of hydrofracturing networks in heterogeneous tight reservoirs with beddings 126 4.6 Results and discussions of dynamic propagation behaviours of hydraulic fractures in embedded multi-granule reservoir 127 4.6.1 A typical example implementation of fracture propagation and pore pressure in the heterogeneous tight reservoir with granules 127 4.6.2 Influence of granule distribution and geomaterial properties on hydraulic fracture 129 4.6.3 Quantitative final length and propagation states of hydrofracturing networks in heterogeneous tight reservoirs with granules 132 4.7 Conclusions 134 References 135 Chapter 5 Dynamic propagation and intersection of hydraulic fractures and pre-existing natural fractures involving the sensitivity factors 139 5.1 Introduction 139 5.2 Combined finite element-discrete element method for hydrofracturing in fractured reservoirs 142 5.2.1 Geomechanical equations in hydrofracturing and gas production 142 5.2.2 Leak-off of fracturing fluid 144 5.2.3 Discrete fracture network model 144 5.2.4 Numerical discretization 145 5.3 Numerical models of fractured reservoir embedded discrete fracture networks 146 5.3.2 Cases study for typical pre-existing natural fractures 147 5.4 Results and discussion 151 5.4.1 Sensitivity factors of pre-existing natural fractures 151 5.4.2 Quantitative length and volume of fracture networks 155 5.4.3 Gas production in enhanced permeability fractured reservoirs 160 5.5 Conclusions 165 References 166 Chapter 6 Unstable propagation of multiple hydraulic fractures and stress shadow effects in multilayered reservoirs 171 6.1 Introduction 171 6.2 Numerical methods for hydro-mechanical coupling and bedded interfaces 173 6.2.1 Geomechanical equations 173 6.2.2 Characterization technology for bedded interfaces in multilayered reservoirs 176 6.2.3 Local remeshing and microseismicity analysis 177 6.3 Numerical models of multilayered reservoirs 177 6.4 Results and discussion 181 6.4.1 Dynamic unstable propagation of multiple hydraulic fractures 181 6.4.2 Evolution of stress field and injected fluid volume 184 6.4.3 Influences of in-situ stress and bedded interfaces in multilayered reservoirs 190 6.4.4 Microseismic event distributions and magnitudes induced by unstable fractures 192 6.5 Conclusions 196 References 197 Chapter 7 Unstable propagation of multiple hydraulic fractures and shear stress disturbance in multi-well hydrofracturing 202 7.1 Introduction 202 7.2 Combined finite element-discrete element method considering thermal-hydro-mechanical coupling 205 7.3 Numerical models and cases of multiple horizontal wells 208 7.4 Results and analysis of unstable propagation of hydraulic fractures with variable well spacings 211 7.4.1 Mesh refinement and thermal diffusion in fracture propagation process 211 7.4.2 Fracture network propagation and shear stress shadows 212 7.4.3 Quantitative analysis of propagation length and volume 219 7.5 Results and analysis of unstable propagation of hydraulic fractures with variable initiation sequences 222 7.5.1 Fracture network propagation and shear stress shadows 222 7.5.2 Quantitative analysis of propagation length and volume 231 7.6 Conclusions 234 References 235 Chapter 8 Unstable propagation of multiple three-dimensional hydraulic fractures and shear stress disturbance in heterogeneous reservoirs 239 8.1 Introduction 239 8.2 Combined finite element-discrete element method for hydrofracturing 244 8.2.1 Geomechanical equations in hydrofracturing considering hydro-mechanical coupling 244 8.2.2 Leak-off of fracturing fluid 246 8.2.3 Poroelastic effective medium model 247 8.2.4 Numerical discretization 248 8.3 Three-dimensional numerical models of multistage hydrofracturing 249 8.3.1 Three-dimensional geometrical and finite element models 249 8.3.2 Cases study for varying fracturing scenarios, perforation cluster spaces, and heterogeneous properties 250 8.4 Results and discussion 255 8.4.1 Spatial propagation morphology of fracturing fracture network 255 8.4.2 Disturbance of stress field during dynamic propagation of multiple hydraulic fractures 264 8.4.3 Quantitative area and volume of three-dimensional fracturing fracture networks 275 8.5 Conclusions 279 References 280 Chapter 9 Unstable propagation of multiple three-dimensional hydraulic fractures and shear stress disturbance considering thermal diffusion 285 9.2 Governing equation for hydrofracturing by considering thermal-hydro- mechanical coupling 287 9.2.1 Solid deformation of rock matrix 287 9.2.2 Fluid flow in the rock matrix and fracture networks 287 9.2.3 Thermal diffusion 289 9.3 Heat transfer and mesh adaptive analysis of the fracture propagation process 289 9.3.1 Heat transfer between the finite element nodes 289 9.3.2 Fracture criteria 290 9.3.3 Local mesh refinement 291 9.4 Three-dimensional numerical model with multiple perforations for parallel hydraulic fractures 292 9.5 Results and analysis of unstable dynamic propagation of the three-dimensional parallel hydraulic fractures 295 9.5.1 Perforation cluster spacing 295 9.5.2 Sequential, simultaneous, and alternate fracturing 298 9.5.3 Thermal diffusion effects 304 9.6 Conclusions 306 References 307 Chapter 10 Summary and prospect 311 10.1 Summary 311 10.2 Prospect 315 Abstract 317 编后记 318
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