Basic Observations of Permeability Evolution – EGS and SGRs Key Issues in EGS and SGRs

1. Effects of Solid-State and Pore-Fluid Chemistry and Stress on Permeability EvolutionDerek Elsworth (Penn State) and Josh Taron (USGS) • Basic Observations of Permeability Evolution – EGS and SGRs • Key Issues in EGS and SGRs • Spectrum of Behaviors EGS to SGR • Homogeneous Permeability Flow Modes • Diagenesis • Permeability Evolution • Basin Evolution • Stimulation and Production • Scaling Relations in Rocks and Proppants • Reinforcing Feedbacks • Induced Seismicity • Mineralogical Transformations – Seismic -vs- Aseismic • First- and Second-Order Frictional Effects • Key Issues

2. Basic Observations of Permeability Evolution Resource • Hydrothermal (US:104 EJ) • EGS (US:107 EJ; 100 GW in 50y) Challenges Prospecting (characterization) Accessing (drilling) Creating reservoir Sustaining reservoir Environmental issues Observation Stress-sensitive reservoirs T H M C all influence via effective stress Effective stresses influence Permeability Reactive surface area Induced seismicity Understanding T H M C is key: Size of relative effects of THMC(B) Timing of effects Migration within reservoir Using them to engineer the reservoir Permeability Reactive surface area Induced seismicity

3. Key Questions in EGS and SGRs Needs • Fluid availability • Native or introduced • H20/CO2 working fluids? • Fluid transmission • Permeability microD to mD? • Distributed permeability • Thermal efficiency • Large heat transfer area • Small conduction length • Long-lived • Maintain mD and HT-area • Chemistry • Environment • Induced seismicity • Fugitive fluids • Ubiquitous [Ingebritsen and Manning, various, in Manga et al., 2012]

4. Contrasts Between EGS & SGRs

5. Thermal Drawdown EGS –vs- SGRs In-Reservoir Water Temperature Distributions: Rock Temp (in reservoir) Thermal Output: Water Temp (at outlet)

6. Thermal Recovery at Field Scale Spherical Reservoir Model Parallel Flow Model [Elsworth, JGR, 1989] [Gringarten and Witherspoon, Geothermics,1974] Spacing, s, is small Trock [Note: not linear in log-time] Spacing, s, is large Dimensionless temperature Tinjection Dimensionless time Dimensionless time [Elsworth, JVGR, 1990]

7. What Does This Mean? • This makes the case that: • Permeability needs to be large enough to allow Mdot_sufficient without: • 1. Fracturing reservoir during production • 2. Large pump costs • Beyond that – issues of heterogeneity are imp: • 1. No feedbacks (Rick) • 2. Reinforcing feedbacks (Kate/Paul/Golder/ Gringarten) • Diagenesis contributes to this: • 1. Initial basin evolution [k0,n0] • 2. Reservoir stimulation/development [k,n=f(t)] • 3. Reinforcing feedbacks [k,n=f(x,t)] for THMC

8. Effects of Solid-State and Pore-Fluid Chemistry and Stress on Permeability EvolutionDerek Elsworth (Penn State) and Josh Taron (USGS) • Basic Observations of Permeability Evolution – EGS and SGRs • Key Issues in EGS and SGRs • Spectrum of Behaviors EGS to SGR • Homogeneous Permeability Flow Modes • Diagenesis • Permeability Evolution • Basin Evolution • Stimulation and Production • Scaling Relations in Rocks and Proppants • Reinforcing Feedbacks • Induced Seismicity • Mineralogical Transformations – Seismic -vs- Aseismic • First- and Second-Order Frictional Effects • Key Issues

9. Controls on Reservoir Evolution Many processes of vital importance to EGS/SGR are defined by coupled THMC processes. Thermal sweep/fluid residence time Short circuiting Induced seismicity Prolonged sustainability of fluid transmission Fractures dominate the fluid transfer system Transmission characterized by: History of mineral deposition Chemo-mechanical creep at contacting asperities Mechanical compaction Shear dilation and the reactivation of relic fractures

10. Typical Response of Fractures (Dissolution) [Polak et al., GRL, 2003]

11. Typical Response of Fractures (Precipitation) Experimental arrangement Precipitation Thermal gradient along fracture [Dobson et al., 2001]

12. s D b s grain grain dissolution dissolution dissolution precipitation precipitation precipitation Time Time diffusion diffusion diffusion grain grain s s s s Time Db Dissolution Processes Approaches to Determine dk or db

13. Component Model • Interface Dissolution • Interface Diffusion • Pore Precipitation [Yasuhara et al., JGR, 2003]

14. Matching Compaction Data [Experimental data from Elias and Hajash, 1992]

15. System Evolution at 35-70 MPa and 150°C Observation Extension 70 MPa and 150°C 35 MPa and 150°C [Experimental data from Elias and Hajash, 1992] [Yasuhara et al., JGR, 2003]

16. Timescales of Evolution of Granular Systems at 35 MPa and 75-150°C 75°C 150°C [Yasuhara et al., JGR, 2003]

17. Permeability Evolution in Granular Systems at 35 MPa and 75-300°C 75°C 150°C 300°C [Yasuhara et al., JGR, 2003]

18. Fracture/ProppantDiagenesis

19. Do we understand the mechanisms? Various mechanisms – appear complex but include: • Dissolution/precipitation • Solid and aqueous chemical transformations • Fluid/chemical assisted strength loss of proppant and proppant collapse Observation Experiment Characterization Analysis [Dae Sung Lee et al., 2009]

20. THMC/HPHT Continuum Models • THMC-S – Linked codes Spatial Permeability Evolution Temporal Permeability Evolution

21. dc (a) Asperity contacts Local contact area, Alc (b) (c) Constraint on Fracture Apertures and Fluid Concentrations Increasing fracture closure

22. Modeling Results - Novaculite K+~x300 [Yasuhara et al., JGR, 2004]

23. Projected Response of FractureDefine projected behavior for varied temperatures….and mean stressmagnitudes [Yasuhara et al., JGR, 2004]

24. Reactive - Hydrodynamic Controls Pe < 1 Dispersion dominated – Perturbations damped Peclet No. (Pe) Pe > 1 Advection dominated – Perturbations enhanced Damkohler No. (Da) Da << 1 Reaction slow - Undersaturated along fracture – Perturbations damped Da larger << 1 – Reaction faster Saturated along fracture – Perturbations enhanced PeDa No. (Removes <q>) [Sherwood No.]

25. Reactive Hydrodynamics: Role of Damkohler Number (PeDa) High PeDa 15 cm x 10cm Voxel = 1 mm Aperture: Black (0)-White(0.25mm) Low PeDa Time [Detwiler and Rajaram, WRR, 2007]

26. Effects of Solid-State and Pore-Fluid Chemistry and Stress on Permeability EvolutionDerek Elsworth (Penn State) and Josh Taron (USGS) • Basic Observations of Permeability Evolution – EGS and SGRs • Key Issues in EGS and SGRs • Spectrum of Behaviors EGS to SGR • Homogeneous Permeability Flow Modes • Diagenesis • Permeability Evolution • Basin Evolution • Stimulation and Production • Scaling Relations in Rocks and Proppants • Reinforcing Feedbacks • Induced Seismicity • Mineralogical Transformations – Seismic -vs- Aseismic • First- and Second-Order Frictional Effects • Key Issues

27. Triggered Seismicity – Key Questions THMC Model: Principal trigger - change in (effective) stress regime: Fluid pressure Thermal stress Chemical creep How do these processes contribute to: Rates and event size (frequency-magnitude) Spatial distribution Time history (migration) How can this information be used to: Evaluate seismicity Manage/manipulate seismicity Link seismicity to permeability evolution Reservoir Conditions:

28. Observations of Induced Seismicity (Basel) [Goertz-Allmann et al, 2011] [Shapiro and Dinske, 2009]

29. r-t Plot - Fluid and Thermal Fronts and Induced Seismicity [Izadi and Elsworth, in review, 2013]

30. Fault Reactivation (and Control) Controls on Magnitude and Timing: kfault & kmedium [10-16 – 10-12 m2] Injection temperature dT [50C – 250C] Stress field obliquity [45-60 degrees] Permeability & Magnitude Timing Injection well Fault [Gan and Elsworth, in review, 2013]

31. Seismic –vs- Aseismic Events Duration (s) [secs -> years] [Peng and Gomberg, Nature Geosc., 2010] Seismic Moment (N.m) [Magnitude]

32. Approaches – Rate-State versus Brittle Behavior • Rate-State Brittle Low velocity High velocity Low velocity System Stiffness (Stored Energy) a ln(v/v0) Coefficient of friction µ0 (a-b)ln(v/v0) DC Displacement -b ln(v/v0) Failure Criterion (Trigger)

33. Seismic –vs- Aseismic Events Friction Velocity Strengthening (stable slip) Velocity Weakening (unstable slip) Stability (a-b) [Ikari et al., Geology, 2011]

34. Scale Effects in Hydrology – Space and Time • Remote earthquakes trigger dynamic changes in permeability • Unusual record transits ~8y • Sharp rise in permeability followed by slow “healing” to background • Scales of observations: • Field scale • Laboratory scale • Missing intermediate scale with control Permeability Permeability [Elkhoury et al., Nature, 2006]

35. Role of Wear Products Sample Holder Shear-Permeability Evolution Dissolution Products Sample [Faoro et al., JGR, 2009]

36. Key Questions in EGS and SGRs • Needs • Fluid availability • Native or introduced – fluid/geochemical compatibility • H20/CO2 working fluids? – arid envts. • Fluid transmission • Permeability microD to milliD? – high enough? • Distributed permeability • Characterizing location and magnitude • Defining mechanisms of perm evolution (chem/mech/thermal) • Well configurations for sweep efficiency and isolating short-circuits • Thermal efficiency • Large heat transfer area – better for SGRs than EGS? • Small conduction length – better for SGRs than EGS? • Long-lived • Maintain mD and HT-area – better understanding diagenetic effects? • Chemistry - complex • Environment • Induced seismicity - Event size (max)/timing/processes (THMCB) • Fugitive fluids – Fluid loss on production and environment – seal integrity • Ubiquitous