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English Grammar

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  • 1. DOE - Enhanced Geothermal Systems Program Review Experimental and Analytical Research on Fracture Processes in Rock Herbert H. Einstein Massachusetts Institute of Technology Phone: 617-253-3598 Fax:617-253-6044 einstein@mit.edu
  • 2. Project Objectives Addresses GTP areas of interest No. 3: Fracture Formation and Growth and No. 4: Fracture Evaluation through Laboratory experimental and analytical work to • Understand fracture propagation and interaction • Create basis for methods allowing one to indirectly infer fracture mechanisms in the field.
  • 3. EGS Problem Importance: Fracture processes are central to much of geothermal energy extraction Technical issues: Propagation and coalescence of fractures are only partially understood, both under ambient temperature and under elevated (up to 150° C) temperature. Addresses EGS Technical Challenge: High-temperature rock usually is not extensively fractured. Effective geothermal energy extraction requires fractured rocks. The process of fracturing needs to be understood (effect of lithology, stress conditions, temperature, etc.)
  • 4. Project - Approach Tasks Task 1. Testing of different rock types and fracture geometries under different, mostly biaxial stress fields under ambient temperatures. Task 2. Conducting experiments like Task 1 but at elevated temperatures. Task 3. Extension of crack (fracture) initiation-, propagation- and coalescence criterion and incorporation in numerical models. Task 4. Initial steps toward modelling larger/smaller scale fractures. Task 5. Reporting and suggestions for further research.
  • 5. Experimental Work s flaw angle,b Internal flaw tip External flaw tip flaw angle,b 2a=12.7mm 2a=12.7mm ligament angle,a ligament length Internal flaw tip c s v 26 - 31 mm. 152.4 mm. Initial flaws s h s h External flaw tip 76.2 mm. s v (a) (b) Geometries of (Model) Rock Specimens. (a) Overall View. (b) Detail
  • 6. Experimental Work - Summary of Past Results Type Schematic path of Coalescence Mode of Coalescence Type Schematic path of Coalescence Mode of Coalescence I Shearing V Shearing + tension II Shearing + tension VI Shearing + tension III Shearing + tension VII Shearing + tension IV Tension VIII Shearing Types of Coalescence Patterns in Uniaxial Compression Based on Past Work
  • 7. Experimental Work – Recent Results Single Flaw gypsum marble Flaw Inclination = 45o
  • 8. Experimental Work – Recent Results Double Flaws Gypsum – Coplanar Flaw Inclination = 75o Gypsum – Stepped Flaw Inclination = 30o Bridging Angle = 60o
  • 9. Experimental Work – Recent Results Double Flaws Marble – Coplanar Flaw Inclination = 30o Marble – Stepped Flaw Inclination = 30o Bridging Angle = 60o
  • 10. Analytical Work - Crack Initiation and Propagation Criterion Core Region Propagation Criteria Mathematical Formulation Failure Envelope
  • 11. Analytical Work - Comparison of Simulation and Experiment Comparison between Numerical Prediction and Experiment for Coplaner-Open Flaw Geometry, Uniaxial Compression
  • 12. Analytical Work - Comparison of Simulation and Experiment Comparison between Numerical Prediction and Experiment for for Stepped, Non-Overlapping Open Flaws Uniaxial Compression
  • 13. Experimental Schematic
  • 14. Experimental Equipment
  • 15. Experimental Equipment
  • 16. Proposed Experimental Work Parameters to be Varied • Rock Type • Flaw Length (12.7, 25.4, 6.35 mm • Flaw Aperture (0.1 to 1 mm) • Flaw Geometry Key b1 – flaw inclination angle b2 – bridging angle L – ligament length a – half flaw length • Stress States (Uniaxial, Biaxial, Different Magnitudes)
  • 17. Other Aspects of Proposed Research • Measure Sonic Wave Velocity • Analysis - New propagation/coalescence criteria? • Details of Fracture Process • Scaling - Lab - Field - Seismic - Field - Rockbursts
  • 18. flaw inclination angle 30o; positive bridging angles 30o,60o PDWHULDO R R JSVXP FUDFN FUDFN FUDFNV FUDFNV FUDFNV PDUEOH FUDFN FUDFN
  • 19. Expected Accomplishments of Proposed Research • More complete picture of fracturing based on lab experiments including different scales • New analytical propagation/coalescence criteria if necessary • Some idea on relation to field
  • 20. Conclusions - GTP related objectives - better knowledge of fracture formation and growth and fracture evolution will be achieved - Will provide analytical tools for modeling fracturing process. - Will provide ideas on how fractures in the field are created

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