Fracturing in Deep Boreholes: Stress, Structural and Lithology-controlled Fracture Initiation and Propagation in Deep Geothermal Boreholes in the ... North Alpine Foreland Basin (Springer Theses)

The development of the base-load capable, climate-friendly, and practically inexhaustible source of "geothermal energy" represents an important pillar of the energy supply of the future. If it were possible to expand geothermal energy production accordingly, Germany could generate 100% of its energy in a climate-neutral manner by 2050. The joint research project "Dolomitkluft," funded by the German Federal Ministry for Economic Affairs and Energy from 2016 to 2018, aims to establish a new and improved reservoir model for the Upper Jurassic carbonates of the Northern Alpine Foreland Basin for deep geothermal energy. Emerged from this project, the dissertation by Mr. Stockinger geomechanically and numerically characterizes the deep geothermal reservoir in carbonate rocks--limestones and dolomites--of the Upper Jurassic in the Northern Alpine Foreland Basin in over 4000 m depth. This book specifically addresses fracture initiation, propagation, and hydraulic conductivity around a borehole and their controlling factors such as the in situ stress, the existing discontinuity network, and the geomechanical rock properties. Mr. Stockinger has thus successfully addressed the most important aspects for the retrievability of deep geothermal energy at its point of origin--namely the (deep) borehole Supervisor’s Foreword Abstract Acknowledgements Contents Symbols | Notations | Abbreviations Stress Terms Abbreviations Defined by This Dissertation Rock Mechanical Parameters List of Figures List of Tables 1 Introduction 1.1 Deep Geothermal Energy in the North Alpine Foreland Basin 1.2 Motivation and Research Goals 1.3 Scope of this Thesis and Link to the “Dolomitkluft” Research Report 1.4 Geology of the Study Area 1.4.1 Evolution of the North Alpine Foreland Basin 1.4.2 Geology of the Upper Jurassic Formation References 2 Rock Mechanical Basics 2.1 Deformation of Rocks 2.1.1 Elastic Deformations 2.1.2 Plastic Deformations 2.1.3 Elastic and Plastic Deformation in Uniaxial Compression 2.2 Stresses 2.2.1 Stress acting on a Plane and Stresses acting at a Point 2.2.2 Stress Regimes and the Mechanism of Faulting 2.2.3 Effective Stress 2.2.4 Frictional Faulting Limitations 2.3 Borehole Stability in Vertical and Deviated Wells 2.3.1 Elastic and Plastic Deformation 2.3.2 Borehole Failure 2.4 Mechanical Data from the Weißjura-Group 2.4.1 Rock Mechanical Properties 2.4.2 Stress Regimes and Stress Indicators in the Weißjura-Group of the Molasse Basin 2.5 Application of Numerical Models 2.5.1 Overview of Numerical Methods 2.5.2 Irazu FEMDEM (Geomechanica) References 3 The BMWi Project “Dolomitkluft” and the Study Site 3.1 Technical Execution of the Wells GEN-1 and GEN-1ST-A1 3.2 Well Trajectories of GEN-1 and GEN-1ST-A1 3.3 Data from “Dolomitkluft” 3.3.1 GEN-1 3.3.2 GEN-1ST-A1 3.3.3 Testing at the Well Site 3.3.4 HMI Logs of GEN-1 and GEN-1ST-A1 3.3.5 Findings on Stress Regimes 3.3.6 Temperature of the Reservoir References 4 Sampling and Methodology 4.1 Sampling 4.1.1 Sampling from Outcrops 4.1.2 Acquisition and Evaluation of Drill Cores and Well Logging Data 4.2 Laboratory Work 4.2.1 Sample Preparation 4.2.2 Non-destructive Testing 4.2.3 Destructive Testing 4.3 Stress Rotation 4.3.1 General Approach 4.3.2 Mathematical Stress Rotation 4.3.3 Regulations Concerning the Stress Rotation 4.3.4 Step by Step Rotation and Implementation in ©Python 4.3.5 Advantages to previously Applied Methods 4.4 Numerical Modelling with Irazu (©Geomechanica) 4.4.1 Model Setup 4.4.2 Input Parameters 4.4.3 Follow-Up Steps to the Final Model 4.4.4 Scenarios 4.5 Limitations and Assumptions 4.5.1 Anisotropy and Inhomogeneities 4.5.2 Poroelasticity 4.5.3 Overpressure 4.5.4 Well Logging Data 4.5.5 Temperature Effects References 5 Results 5.1 Rock Mechanical Properties of the Analog Rocks 5.1.1 Dynamic Rock Properties of the Analog Rocks 5.1.2 Static Rock Properties of the Analog Rocks 5.1.3 Indirect Tensile Strength σt of the Analog Rocks 5.1.4 Calculated and Derived Parameters from Analog Rocks 5.1.5 Density, Isotropy, Porosity and Grainsize of the Analog Rocks 5.2 Drill Cores 5.2.1 Geological and Rock Mechanical Description of the in situ Rock Cores 5.2.2 Overcored Specimens and their Rock Mechanical Parameters 5.2.3 Properties and Parameters derived from US Velocities 5.2.4 US Measurements on the in situ Rock Cores 5.3 Stress- and Structurally-Controlled Phenomena 5.3.1 Indicators and Quantifiers for in situ Stresses on Rock Cores 5.3.2 Stress- and Structurally-Controlled Phenomena in HMI Logs 5.4 Fracture Networks from Core Runs, HMI Logs and 360° Core Images 5.4.1 Joint and Fracture Traces recorded on Cores in Core Boxes 5.4.2 Fracture Traces recorded on the HMI Log of GEN-1ST-A1 5.4.3 360° Core Images 5.5 Stresses acting along the Drill Path 5.5.1 Sections of resulting Stresses along different Well Trajectories 5.5.2 Resulting Stresses of different Stress Regimes on the Drill Trajectories 5.5.3 Tangential (Hoop) and Radial Stresses at the Borehole Wall 5.5.4 Potential for Borehole Failure 5.6 Fracture Initiation and Propagation from Numerical Modeling 5.6.1 Results for the Model Setup 5.6.2 Results from Numerical Modeling References 6 Discussion 6.1 Evaluation of Rock Mechanical Parameters 6.1.1 Dependability on the Parameters of the Tested Rocks 6.1.2 Matching Attributes of Analog Samples with in situ Rock Cores 6.2 Conclusions on the Rock Mass from in situ Drill Cores 6.2.1 Time-Dependent Effects 6.2.2 Scale-Related Effects 6.2.3 Stress-Related Effects 6.3 Fracture Network 6.3.1 Bedding 6.3.2 Fracture Network from the HMI Logs 6.3.3 Fracture Network from the Drill Cores 6.3.4 Comparison of the in situ Fracture Network with Literature 6.4 Determining and Quantifying the in situ Stress Field 6.4.1 Approvals and Contradictions for possible Stress Regimes 6.4.2 Quantifying the in situ Stress Field 6.5 Conclusions from Numerical Models 6.5.1 Reliability of Input Parameters for Numerical Models 6.5.2 Comparison of the Elastic (Empirical) Approaches with the Numerical Models 6.5.3 Key Findings from Numerical Models References 7 Conclusions, Implementation and Outlook 7.1 Analog Rocks versus in situ Rocks 7.2 Borehole Stability and Weakening of the Rock Mass 7.3 Core Drilling 7.4 Productivity of the Wells and Permeability of the Rock Mass 7.5 Characteristic Types of Fracturing around the Borehole 7.6 Recommendations for Future Research References Appendices Appendix A: Fully Applicable Python Code for Stress Rotation Appendix B: Rock Mechanical Properties of the Analog Rocks References