Applied Concepts in Fractured Reservoirs

John C. Lorenz and Scott P. Cooper, are Senior Geologists at FractureStudies LLC, in Edgewood, NM.

• Foreword xiPreface xiiiAcknowledgements xvIntroduction xviiPart 1 Understanding Natural Fractures: Fracture Types, Dimensions, and Origin 11.1 Introduction 11.2 Nomenclature and Fracture-Classification Systems 11.2.1 Introduction 11.2.2 Other Classification Systems 31.2.3 Classifications for Fractures in Outcrops and Cores 41.2.4 Expulsion Fractures and Natural Hydraulic Fractures 51.2.5 Other Fracture Terminology 51.2.6 Sets, Systems, Domains, and Systematic Fractures 71.3 Fracture Characteristics and Dimensions 81.3.1 Introduction 81.3.2 Fracture Distribution Patterns 81.3.3 Fractography 101.3.4 Fracture Dip Angles 131.3.5 Fracture Distributions 131.3.6 Fracture Heights and Terminations 161.3.7 Fracture Lengths 181.3.8 Fracture Widths, Apertures, and Mineralization 191.3.9 Fracture Spacing 221.3.10 Fracture Strike 271.3.10.1 Fracture Orientations Relative to the In Situ Stresses 281.3.11 Discussion 281.4 The Mechanics of Fracturing Rock in Extension and Shear 291.4.1 Introduction 291.4.2 Origins of Geologic Stress Systems 311.4.2.1 Stresses in a Tectonically Quiescent Basin 311.4.2.2 Other Potential Sources of Horizontally Isotropic Stress 321.4.2.3 Stresses in a Tectonically Active Basin 321.4.3 Rock Susceptibility to Fracture: Basic Concepts 351.4.3.1 Introduction 351.4.3.2 Intrinsic Controls on Fracture Susceptibility 381.4.3.3 Extrinsic Controls on Fracture Susceptibility 391.4.3.4 How Rock Breaks: Grain-Scale Cracking, Yield, and Failure 411.4.3.5 Extrapolation to the Subsurface 431.4.4 Interplay Between Developing Fractures and the In Situ Stresses 441.4.5 The Importance of Pore Pressure 451.4.5.1 Introduction 451.4.5.2 The Relationship between Pore Pressure and Stress 451.4.5.3 Biot’s Coefficient 471.4.5.4 Mohr Diagrams and Pore Pressure 471.4.5.5 Pore Pressure Makes Rock Weak and Brittle 471.4.5.6 Sources of Pore Pressure 501.4.5.7 Alternate Theories 511.4.6 Summary 521.5 Other Fracture Types 531.5.1 Introduction 531.5.2 Deformation-Band Shear Fractures, Compaction Bands, and Dilation Bands 531.5.2.1 General Characteristics 531.5.2.2 Dimensions and Distributions 531.5.2.3 Origin 541.5.3 Faults and Fractures 551.5.4 Microfractures 561.5.5 Stylolites and Associated Extension Fractures 591.5.6 Bed-Parallel Shear Fractures 591.5.7 Beef-Filled Fractures 621.5.8 Ptygmatically Folded Fractures 631.5.9 Alteration of Fracture Systems by Dissolution 64Appendix 1.A The Relationship Between Pore Pressure and the In Situ Effective Stresses 66Introduction 66Vertical Stress 67Horizontal Stress 67Effective Vertical Stress 67Effective Horizontal Stress 68Stress Differential 68Part 2 Measuring and Analyzing Fractures in Reservoirs 712.1 Introduction 712.1.1 Reasons to Take Core 722.1.2 Analyses 732.1.3 Fracture Data Sources 732.1.4 Quantitative vs. Semi-Quantitative Data 732.1.5 Timing of a Fracture Study 732.1.6 Need for Experience 742.1.7 Other Data Sources 742.2 Planning a Core Program for Fracture Analysis 742.2.1 Introduction 742.2.2 Core Diameter and Length 742.2.3 Substituting Sidewall Core Samples 742.2.4 Orienting a Core 742.2.5 Drilling Parameters 752.2.6 Trip Time for Core Recovery 752.2.7 Collecting Data on Site 752.2.8 Running an Image Log 762.2.9 Back-to-Back Cores 762.2.10 On-Site Processing 762.2.11 CT Scans 772.2.12 Removing Core from the Barrel 772.2.13 Core-Jam Prevention Measures 772.2.14 Maximizing and Documenting Core Continuity 772.2.15 Slabbing Protocol 772.2.16 Scheduling Fracture Logging and other Core Processes 782.3 Logging Core for Fractures 782.3.1 Wash the Core! 782.3.2 Use all the Core and Remove it from the Core Boxes 792.3.3 Laying Out Intervals of Core for Fracture Logging 792.3.4 Core-Logging Toolkit 802.3.5 Recording Data 812.3.6 Making and Using a Master Orientation Line 822.3.7 Differentiating Natural from Induced Fractures 832.4 Taking, Measuring and Analyzing Fracture Data 842.4.1 Fracture Type 842.4.2 Fracture Depths: Intensity and Density 862.4.3 Fracture Dip Angles 882.4.3.1 Measuring Dip Angles 882.4.3.2 Using Dip Angles 892.4.4 Fracture Distributions 902.4.5 Fracture Heights and Terminations 912.4.6 Fracture Widths, Apertures, and Mineralization 942.4.7 Fracture Spacings 982.4.7.1 Spacings from Horizontal Core 992.4.7.2 Spacings from Vertical Core 1032.4.7.3 Converting Vertical Observations to Horizontal Fracture Spacings 1032.4.7.4 Spacings of Inclined and Shear Fractures 1052.4.7.5 Uses of Spacings 1052.4.8 Measuring and Using Fracture Strikes 1052.4.8.1 Measuring Fracture Strikes in Vertical Core 1062.4.8.2 Measuring Fracture Strikes in Deviated or Horizontal Cores 1092.5 New Core vs. Archived Core 1102.6 Oriented Core 1122.6.1 Other Ways of Orienting a Core 1162.7 Using CT Scans 1182.8 Fracture Data from Image Logs 1192.9 Comparing Fracture Data from Outcrops, Core, and Logs 1222.9.1 Introduction 1222.9.2 Large-Scale Outcrop Studies 1232.9.3 Local Outcrop Studies 1232.9.3.1 Raton Basin 1232.9.3.2 Rifle Gap 1252.9.3.3 San Ysidro 1272.10 Fracture Data from 3D Seismic Surveys 1282.11 Fracture Data Acquired by LiDAR 1302.12 Fracture Data from Engineering Tests 1322.13 Case Studies in Estimating Fracture Effectiveness from Core 1332.13.1 Introduction 1332.13.2 Case Study 1: Archived Vertical, Unoriented Core 1332.13.3 Case Study 2: New, Un-Slabbed Horizontal Core 1342.13.3.1 Introduction 1342.13.3.2 Fracture Effectiveness 1372.13.3.3 System Effectiveness and Permeability Anisotropy 1372.13.4 Case Study 3: New, Slabbed, Vertical Core 1392.13.4.1 Introduction 1392.13.4.2 Calculating Effectiveness 1392.13.4.3 Description of the High-Angle Extension Fractures 141Appendix 2.A Workflow and List of Tests, Data 144Appendix 2.B Core-Handling, Marking, Sampling, and Analysis Protocol for Core Studies Focused on Natural Fractures 144Appendix 2.C Slabbing Recommendations for Horizontal Core 146Part 3 Effects of Natural Fractures on Reservoirs 1493.1 Introduction 1493.2 Classification 1493.3 The Permeability Behavior of Individual Fractures 1503.3.1 Introduction 1503.3.2 Three Categories of Fracture Effects 1503.3.3 Stylolites 1543.3.4 Microfractures 1543.4 The Effects of Fracture Systems 1563.4.1 Introduction 1563.4.2 Fracture-Controlled Permeability Anisotropy 1573.4.2.1 Case Study: The Midale Field 1573.4.2.2 Case Study: The Rulison Field 1583.4.2.3 Case Study: The Spraberry Formation 1583.4.3 Fracture-Controlled Sweet Spots 1623.5 The Sensitivity of Fracture Permeability to Changing Stress 1643.5.1 Stress-Sensitive Extension Fractures 1643.5.1.1 Case Study: The Bulo Bulo Field 1673.5.2 Stress-Sensitive Shear Fractures 1693.5.3 Damage Due to Production-Related Scale 1713.6 Fracture Volumetrics 1723.6.1 Introduction 1723.6.2 Fracture Volume/Fracture Porosity 1733.6.3 Fracture Permeability 1743.6.4 Transfer Function 1763.6.5 Fracture Surface Areas 1763.7 Effects of Fractures on Drilling and Coring 1773.8 Completions: The Interaction Between Natural and Hydraulic Fractures 1783.8.1 Early Conceptual Models 1783.8.2 Direct Evidence of the Characteristics of Hydraulic Fractures 1793.8.3 The Developing Hydraulic-Fracture Model 1823.8.4 Nuclear Stimulations 184References 187Index 205