1. Effects of Solid-State and Pore-Fluid Chemistry and Stress on Permeability Evolution Derek 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
EGS (Order of Mag.)
Property
ESRs (Order of Mag)
Fractured-non-porous
General
Porous-fractured
<<1%,<1%
Porosity, n0 -> nstim
~10-30%, ~same
microD -> mD
Permeability, k0 -> kstim
>mD -> >mD
106
Kf/kmatrix
106 ->1
10-100m
Heat transfer length, s
1m -> 1cm
>>100/1. >100/1
*Heatsolid/Heatfluid
~10/1-2/1, same ?
Chemistry
V. Strong
TM Perm. Feedbacks
Less strong
Moderate, late time
TC Perm. Feedbacks
Strong?

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

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

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 Evolution Derek 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)
Change in temperature only – ~3MPa
Change in aperture with each temp increment
But net dissolution
Magnitude of perm change is 12-4um. About x100 decrease – for dissolution
Suggests M-C coupling – matters where dissolution occurs – at bridging asperities or free face
[Polak et al., GRL, 2003] 10

11. Typical Response of Fractures (Precipitation)
Experimental arrangement
Precipitation
Thermal gradient along fracture
Change in temperature only – ~3MPa
Change in aperture with each temp increment
But net dissolution
Magnitude of perm change is 12-4um. About x100 decrease – for dissolution
Suggests M-C coupling – matters where dissolution occurs – at bridging asperities or free face
[Dobson et al., 2001] 11

12. Dissolution Processes Approaches to Determine dk or db
precipitation
diffusion
dissolution
grain
Time D b s s
Time 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
[Yasuhara et al., JGR, 2003]

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

18. Fracture/Proppant Diagenesis

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
Use these data, and stress compaction data to constrain expected response
Strain gives compaction and loss of porosity
Porosity gives perm change with similar outcomes to before – higher temp gives faster dissolution and faster perm loss
Characterization
Analysis
[Dae Sung Lee et al., 2009]

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

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

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

23. Projected Response of Fracture Define projected behavior for varied temperatures ….and mean stress magnitudes
[Yasuhara et al., JGR, 2004]

24. Reactive - Hydrodynamic Controls
Peclet No. (Pe)
Pe < 1 Dispersion dominated – Perturbations damped
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 Evolution Derek 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
Parameters utilized in simulation k0
Permeability[m2]
10-17 Pp
Pore Pressure[Mpa]
14.8
Pinj
Fluid Pressure[Mpa]
17.8
Tres
Reservoir Temperature[°c]
250
Tinj
Fluid Temperature[°c] 70 S
Fracture Spacing[m]
10 to 500 Q:
Flow rate
t :
Time h:
Thickness ϕ:
Porosity b:
Aperture
[Izadi and Elsworth, in review, 2013]

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

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

32. Approaches – Rate-State versus Brittle Behavior
System Stiffness
(Stored Energy)
Rate-State Brittle µ0
(a-b)ln(v/v0)
a ln(v/v0) DC
Low velocity
High velocity
Displacement
Coefficient of friction
-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
Permeability
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
Also influence of time
Permeability is not a static property, but evolves with time.
These data show five-fold jump due to far-field small eqrthquakes, to reset permeability, and time-dependnet permeability loss with time
Seen in laboratory, and now in field – opportunity now at intermediate scale. DUSEL offers this chance.
Permeability
[Elkhoury et al., Nature, 2006]

35. Role of Wear Products Sample Holder Shear-Permeability Evolution
Dissolution Products
[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