1. Stress- and Chemistry-Mediated Permeability Enhancement/Degradation in Stimulated Critically-Stressed Fractures DE-FG36-04GO14289, M001 October 1, 2004 – December 31, 2007 (3 years) Derek Elsworth, Penn State University, PI Avrami Grader (EGEE, PSU) Chris Marone (Geosciences, PSU) Phillip Halleck (EGEE, PSU), & Peter Rose, EGI, University of Utah

2. Towards the engineering of “EGS”: –Long-lived –Low-impedance –High heat flow Consistent understanding of the evolution of flow connections resulting from stimulation –Physical (effective stresses) –Chemical (dissolution/precipitation) Critical influences of: –Mechanical Influences [THM] –Chemical Influences [THC] Importance where fractures are “critically stressed” Resolve anomalous observations Purpose THMC

3. Objectives “… a consistent view of the thermal, hydraulic, mechanical, and chemical processes that influence permeability enhancement….and to be able to apply these principles to EGS reservoir development.” Hydro-MechanicalHydro-Chemical Constitutive Models Modeling/ Upscaling

4. Plan and Approach Hydro-MechanicalHydro-Chemical Constitutive Models Modeling/ Upscaling

5. Hydro-Chemical Reactor - Experimental Arrangement

7. Q mass X-ray CT Q fluid =Constant Apparatus

8. Typical Response [Polak et al., GRL, 2003]

9. Experiment Matrix

10. Hydro-Mechanical Reactor

11. Slide-Hold-Slide Friction Experiments Hold periods of 30 – 10 4 [sec]Hold periods of 30 – 10 4 [sec] @20 degree-C, peak coefficient is independent of hold periods @65 degree-C, peak value increases with increase of hold period

12. Experiment Matrix – Similar to Hydro-Chemical Reactor

13. Results 1.Hydro-Chemical Reactor 2.Hydro-Mechanical Reactor 3.Modeling

14. Fractured Limestone – Features of Response (predate this project) 0 hr 1462 hr 0 hr1462 hr

15. 1. Hydro-Chemical Reactor

16. Coso core 64-16 at 646 ft

17. Typical slice

18. Thresholded three- dimensional image of the fracture

19. Three- dimensional image of the large openings of the fracture

20. Three- dimensional combined image of the large aperture openings and the linking smaller apertures within the core.

21. 2. Hydro-Mechanical Reactor

22. Goal: Construct a numerical model to simulate permeability enhancement caused by hydraulic and chemical stimulation – ultimately apply to stimulation at Coso FLAC3D → [High Peclet Number Flows] → ToughReact → Fist step; Focus on a behavior of a single fracture ─ Mass transport within a fracture ─ Solve an advection-dispersion equation, complete with a reactive term ─ FEM, FDM ─ Accommodate a problem with high Peclet number (advection dominant) 3. Transport and Mechanical Modeling

23. Advection-dispersion equation with high Peclet number A Lagrangian-Eulerian Method Continuous injection

24. Replicate experimental result (Nova II) 1. Set initial aperture distribution 2. Apply I.C. and B.C. → Obtain velocity distr. in a fracture by solving Reynolds’ equation 3. Dissolution at contact area and free-face (reaction) → Obtain concentration distribution + Modify aperture distribution due to dissolution 4. Lagrangian-Eulerian method (Advection-diffusion) → Obtain concentration distribution within and out of domain Iteration

25. Replicate experimental result (Nova II) Numerical model is capable of replicating experiment though prescribed multiplier for dissolution rate constant is relatively large. Another mechanism instead of pressure solution may be active (mechanical creep?).

26. Replicate experimental result (Nova II) CT image Model prediction The model cannot perfectly represent experiment, but predict changes in aperture and contact area distribution with time

27. Impact/Merit Project recently initiated Providing meager data/information that are not well understood, and linking with improved understanding –Stress- and chemistry-mediated influences are potent –High temperatures where few data exist –Current understanding lacking Linkages and Dissemination –Closely tied to EGI parallel study incl. personnel transfer –Potential isotopic linkages for heat-flow areas (E. Sonnenthal) Products –Elsworth, D., and Yasuhara, H. (2005) Short timescale chemo- mechanical effects and their influence on the transport properties of fractured rock. Submitted for publication. Earth and Planetary Research Letters. 40 pp. –GRC Meeting September

28. Completion