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

• 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

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-Mechanical Hydro-Chemical

Constitutive Models

Modeling/Upscaling

Plan and Approach

Hydro-Mechanical Hydro-Chemical

Constitutive Models

Modeling/Upscaling

Hydro-Chemical Reactor - Experimental Arrangement

        

        

        

        

Qmass

X-ray CT

Qfluid=Constant

Apparatus

Typical Response

        

        

        

        

[Polak et al., GRL, 2003]

Experiment Matrix

Hydro-Mechanical Reactor

Slide-Hold-Slide Friction ExperimentsSlide-Hold-Slide Friction Experiments

0.55

0.60

0.65

0.70

0.75

10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5

Coe

ffic

ien

t of

fric

tion

Shear displacement [mm]

30 s100 s

300 s

1000 s3000 s

10000 sT = 20 oC0.55

0.60

0.65

0.70

0.75

18.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5C

oeff

icie

nt o

f fr

ictio

n

Shear displacement [mm]

30 s100 s

300 s

1000 s

3000 s

10000 sT = 65 oC

•Hold periods of 30 – 10Hold periods of 30 – 1044 [sec][sec]

@20 degree-C, peak coefficient is independent of hold @20 degree-C, peak coefficient is independent of hold periodsperiods

@65 degree-C, peak value increases with increase of hold @65 degree-C, peak value increases with increase of hold periodperiod

Experiment Matrix – Similar to Hydro-Chemical Reactor

Results

1. Hydro-Chemical Reactor

2. Hydro-Mechanical Reactor

3. Modeling

Fractured Limestone – Features of Response (predate this project)

        

        

        

        

0 hr

1462 hr

0 hr 1462 hr

1. Hydro-Chemical Reactor

Coso core 64-16 at 646 ft

Typical slice

Thresholdedthree-dimensional image of the fracture

Three-dimensional image of the large openings of the fracture

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

2. Hydro-Mechanical Reactor

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

Advection-dispersion equation with high Peclet number

A Lagrangian-Eulerian Method

Continuous injection

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2

Concentration (Analytical)Concentration (Eulerian)Concentration (Lagrangian-Eulerian)

Con

cent

ratio

n

Distance

Element Pe = 105

V =1.0C

0 = 1.0

l =0.02t =1.0

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

012

3

p

b

RT

AkV

dt

dM ecagmPS

4

3 2

eq

ieqe

FF

C

CCkA

dt

dM 2It

era

tion

Replicate experimental result (Nova II)

0.0

5.0

10.0

15.0

20.0

0 400 800 1200 1600 2000 2400 2800 3200

ExperimentPrediction

Ape

rtur

e [

m]

Time [hr]

PS:1.2 x 106

FF:200 FF:20 FF:12

Element size: 2.0 x 2.0mm2

Time step: t = 100 sec

0.0

2.0

4.0

6.0

8.0

10.0

0 400 800 1200 1600 2000 2400 2800 3200

ExperimentPrediction

Si c

once

ntra

tion

[ppm

]

Time [hr]

• 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?).

Replicate experimental result (Nova II)

CT image

<Aperture and contact area distribution (after experiment)>

Model prediction

The model cannot perfectly represent experiment, but predict changes in aperture and contact area distribution with time

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

Completion