Coupled Thermo-Hydro-Mechanics in Fractured Rocks: Modelling Status and Data Needs Chin-Fu Tsang Lawrence Berkeley National Laboratory USA Imperial College

Coupled Thermo-Hydro-Mechanics in Fractured Rocks: Modelling Status and

Data Needs

Chin-Fu Tsang

Lawrence Berkeley National Laboratory USAImperial College London UKUppsala University Sweden

Talk Outline• Introduction to coupled thermo-Hydro-mechanical

(THM) processes in geological systems • Two examples of major field studies

– Study of FEBEX Experiment– Study of YM Heater Test

• More recent Studies

– Coupled THM processes in CO2 injection storage

– Coupled THM processes in methane production from gas hydrates

• General lessons learned, challenges and data needs

Introduction• A number of geoscience problems require the understanding and

modeling of the effects of coupled processes involving

Temperature gradient (T) Hydrologic flow (H)

Mechanical deformation (M) Chemical reactions (C)

in fractured rock and bentonite (and argillaceous clay) systems

• One major example is the concern over their role in the performance of a radioactive waste geologic repository

• However, the problem is of wider interest, such as

— geothermal energy extraction,— earthquakes induced by fluid injection,— fluid injection into deep petroleum reservoirs,— disposal of solid waste— sequestration of CO2 and

— gas production from methane hydrates

Scientific ChallengesThe processes, T, H, M and C have widely different characteristic time and spatial scales

– Thermal effects (T) in rock material has relatively large time and spatial scales

– Mechanical effects (M) have a short time scale, responses can propagate with the speed of sound (Deformability is controlled mainly by the presence of large discontinuities, such as faults and shear zones)

– Groundwater flow and transport (H, C) are sensitive to small-scale heterogeneities and characterized by long time periods for flow and solute transport

Numerical Modeling Challenges

• Numerically, T, H, M and C processes are usually modeled by different techniques, such as

— finite-element methods (FEM)

— discrete-element methods (DEM)

— finite-difference methods (FDM)

— discrete fracture network (DFN) methods

• In addition, many of the coupled processes are nonlinear and anisotropic

• Constitutive equations typically contain different parameter sets, with uncertain parameter values

• To combine all these processes into a coupled numerical model is a major challenge

Recent International Projectsfor studying THM(C) Processes

• DECOVALEX Project (1992-2007)– Gone through 4 phases

– About 10 countries

– Many publications

D-2011 Project (2008-2011)

• THERESA Project

• TIMODAZ Project

• Others







• General lessons learned: challenges and limitations

Two Examples of THM Field Studies

• Study of FEBEX THM(C) experiment by ENRESA at Grimsel– 5-year heating with temperature to 100°C– Bentonite clay with fractured crystalline rock– Unsaturated bentonite and fractured rock

unsaturated near tunnel wall (due to ventilation)

• Study of coupled THMC (C=chemistry) test at Yucca Mountain, USA– 8-year experiment with 4-year heating and 4-year

cooling– Temperature up to 200°C– Unsaturated fractured porous medium (tuff)

Heater (diameter 0.97)

Bentonite blocks

Steel liner

Granite

Heaters Bentonite

ConcretePlug

4.54

1.0

4.54 4.34

17.4 2.7

(m)

Access tunnelGranite

2.27

FEBEX (FULL-SCALE ENGINEERING BARRIERS EXPERIMENT) AT GRIMSEL TEST SITE IN SWITZERLAND

(1997-2002 with followup tests)

Multi-national project coordinated by ENRESA (Spain)

Max heater temperature = 100 °C

THM PROCESSES OF FEBEX(after Gens, 2003)

Model of FEBEX In Situ Test

• Fully coupled THM

• Liquid water and vapor flow with evaporation-condensation

• Heat transfer with conduction and convection

• Thermal expansion

• Mechanical elastoplastic deformation

• Moisture swelling and shrinkage

• Porosity and permeability changes caused by deformation

ROCMAS: A 3D finite element program for analysis of coupled THM processes in unsaturated/saturated geological media

(Rutqvist et al, International Journal of Rock Mechanics, 2001)

Processes modeled:Finite element grid:

X

Y

Z

66 m

56 m

SEQUENCE FOR THM MODELING OF FEBEX

1) Calibration of rock mass properties against measurements during and after excavation of the drift

2) Back-analyses of bentonite properties from laboratory tests

3) Prediction of THM behavior during the heater test (1000 days)

Laboratory Experiments for Back-analyses of Coupled THM Properties of Bentonite

• Thermal Conduction Tests

• Suction Tests (water retention)

• Isothermal Infiltration Tests (relative permeability)

• Thermal Vapor Diffusion Tests (diffusion constants)

• Thermal Expansion Tests

• Permeability vs Porosity for Gas Flow and Liquid Flow

• Oedometric Tests with Controlled Suction (swelling behavior)

TH

TM

HM

ExperimentProcess

T

H

TH Properties of Bentonite Buffer Material

Water retention: Van Genuchten’s P0 = 35 MPa ( = 2.910-8 1/Pa), m = 0.45

Intrinsic permeability: k = 2.0×10-21 m2

Relative permeability: kr = S3

Tortuosity factor: = 0.8

Thermal conductivity: = 0.6 to 1.3 W/(m C)

Specific heat (solid part) Cs = 138T + 732.5 J/(kg C)

Thermal diffusion factor ftv = 2

(ftv enhances vapor diffusion under thermal gradient ,Philip and De Vries, 1957)

Bentonite PropertiesBack-analysis of state surface parameters from laboratory tests

dz zv dde

de 01

gzyxM P 3

1

Measured:

z

s

M z

Numerical modeling of lab tests to back-calculate a, b, c, d

sdscbae MM loglogloglog

Relation between state surface and measured quantities:

Oedometric test with controlled suction (s)

State surface:

Bentonite Properties

Back-analysis of state surface parameters from laboratory tests

Swelling under constant external load

z

1) Apply external load on unsaturad sample

z

2) Saturate sample under constant external load and measure strain

Moisture

S

dz

Si 50%

Saturation

Bentonite PropertiesBack-analysis of state surface parameters from laboratory tests


S

dz

z = 0.5 MPa

Suction (MPa)

Ve

rtic

als

tra

in,d

z

10-1 100 101 102 103

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

50 % saturation

100 % saturation

98 % saturation

Experiment

Modeling

Suction (MPa)

Ve

rtic

als

tra

in,d

z

10-1 100 101 102 103

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Bentonite Properties

Back-analysis of state surface parameters from laboratory tests


z = 0.5 MPa

z = 1 MPa

z = 5 MPa

z = 10 MPa

S

dz

z

Collapsing behavior

Experiment

Modeling

Prediction of THM behavior at FEBEX heater test

Calculation sequence:1 Pre-heating) Starts 8 months before the heat is turned-on to take into

account the wetting of the benonite during experimental setup.

2 Heating) The heater power is increased step-wise during the first 53 days of heating. Then the power of each heater is individually controlled by a constant heater temperature of 100 C

TIME (day)

HE

AT

ER

PO

WE

R(W

)

0 500 10000

500

1000

1500

2000

2500

3000

1200 W

2000 W

Constant temperaturepower control mode

Pre-heating Heating

TIME (Days)

TE

MP

ER

AT

UR

E(o

C)

0 100 200 300 400 500 600 700 800 900 100010

20

30

40

50

SF21:1(7 cm into rock)

SKI MODELSKB MODEL

MEASURED

COMPARISON OF MODEL RESULTS WITH FIELD MEASUREMENTS

TIME (Days)

RE

LA

TIV

EH

UM

IDIT

Y(%

)0 100 200 300 400 500 600 700 800 900 1000

0102030405060708090

100

SKB MODELMEASURED

SKI MODEL

HC

TIME (Days)

ST

RE

SS

(MP

a)

0 100 200 300 400 500 600 700 800 900 10000

1

2

3

4

5

SKB MODEL

E2G1(at rock wall)

MEASURED E2G1

SKI MODEL

MEASURED E2G2

E2G2

•The figure shows examples of comparison of •temperature in the rock•water saturation and•stress in the buffer

•Good agreement between simulated and measured results, especially for temperature and water content.

•Measured delay in swelling stress during the first several months was probably caused by the existence of gaps between the bentonite blocks.

Yucca Mountain DST Thermal Test

• Heat released from radioactive decay of the waste gives rise to T H C M processes within the unsaturated rock mass, which would impact the transport of radionuclides

• Test objective is to acquire a more in-depth understanding of the coupled processes

• Drift Scale Test, 60 m scale, ~190 kW for 4 years (max temperature 200oC on drift wall)

• Close integration of detailed numerical modeling along with testing program

TH Coupled ProcessesHeat Transfer

– Conduction, convection– Counterflow of liquid and

vapor - heat pipe

Moisture Redistribution– Fast transport of vapor in

fractures– Condensation– Imbibition into matrix– Gravity drainage in

fractures– Drying front moves out

from heat source with time– As heat output declines,

dryout zone contracts

THM Coupled ProcessesProcesses• Thermal expansion

and thermally induced stresses– Open or close

fractures– Change porosity– Change

permeabilityImpact• Altering flow paths and

effect on water seepage into drifts

• Far field flow and transport

THC Coupled ProcessesChemical evolution of waters, gases and minerals coupled to TH – Drying concentrates aqueous species

in remaining liquid phase– Pure water in condensation zones

promotes dissolution of minerals

Reaction rates– Generally increase with elevated

temperatures

pH affected by – CO2 degassing and transport

Mineral dissolution and precipitation– Changes porosity and permeability– Alters chemistry of water that could

contact waste package if seepage into drift occurs

Drift Scale Test (DST)

Heat Turned on: Dec 3, 1997Heat turned off: Jan 14, 2002Cooling phase monitoring and testing until April 2006

Pre-Test Characterization

Passive Monitoring during Heating and Cooling

•Temperature•Displacement •Strain•Humidity•Pressure •Acoustic Emission (microfracturing)

Periodic Active Testing during Heating and Cooling

Air Permeability Gas SamplingWater sampling

LaboratoryT-PropertiesH-PropertiesM-PropertiesMIN/PETPore water

FieldRock ClassificationFracture MappingBorehole VideosAir Permeability

GPRNeutron logERT

} Matrix Liquid Saturation

Probing THMC Processes-Drift Scale TestProbing THMC Processes-Drift Scale Test

Comparing Model Results with MeasurementsComparing Model Results with MeasurementsClose integration of sophisticated and detailed numerical

modeling with measurements

– Pre-heat characterization to measure site-specific properties important for coupled processes

– Model predictions prior to commencement of test

– Use of early test results to discriminate alternative conceptual models applied in pre-test simulations

Numerical Tools

– TH: TOUGH2

– THC: TOUGHREACT

– THM:TOUGHFLAC

Model predictions compared to data: selected examples

– THC: CO2 evolution, dissolution and precipitation

– TH, THM: Fracture air permeability evolution from moisture redistribution and stress-induced fracture aperture changes

74:4

57:4

75:4

58:375:3

76:3

76:459:3

59:4

X (m)

Z(m

)

-25 -20 -15 -10 -5 0 5 10 15 20 25-15

-10

-5

0

5

10

15

20

25

1.51.31.10.90.70.50.30.1

Fk (-)

Near Drift

Mid

Away from Drift

DETAILED ANAYSIS OF GROUPS OF AIR-PERMEABILITY DATA TO DISTINGUISH TM FROM FULL THM EFFECTS

TM only

TM only

TIME (Months)

PE

RM

EA

BIL

ITY

CO

RR

.FA

CT

OR

0 10 20 30 40 50 60 700

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 Simulated

THM (Total)

TM (Intrinsic Permeability)

TH (Relative Permeability)

Measured

76:3

76:459:359:4

Measured

75:3

75:458:3

TIME (Months)P

ER

ME

AB

ILIT

YC

OR

R.F

AC

TO

R

0 10 20 30 40 50 60 700

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 Simulated

THM (Total)



TIME (Months)

PE

RM

EA

BIL

ITY

CO

RR

.FA

CT

OR

0 10 20 30 40 50 60 700

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Simulated

THM (Total)



Measured

74:4

57:4

Measured and Modeled CO2 over time

Gas Sampling

Degassing of CO2 from pore water with elevated temperature: thus CO2 concentration in gas samples initially increases with temperature

As temperature exceeds boiling, rock mass dries out and CO2 concentration drops

Model Predictions

Successfully captured trend of CO2 field data without calibration

Changes in CO2 partial pressure play important role in pH and water chemistry and subsequent mineral alteration







• General lessons learned: challenges and limitations

CO2 Injection Into a Brine Aquifer

Fault slip when > (n - P)tan

Hydraulic fracture when P > h

Hydraulic properties is a function of effective stress: = f(’) k = f(’)Pc = f(k, )

Effects of stress change on the performance of an injection site

Expansion of aquifer rock:

= f(’,E)

0 5 10 15 20 2515

20

25

30

35

40

45

pressureLithostatic

Current study

INJE

CT

ION

PR

ES

SU

RE

,P(M

Pa

)

TIME (Years)

H Calculation with k = k0

HM Calculation with k = k(')

Model Simulation of Injection Operation

CO2 Injection Pressure (constant rate injection)

-

-800 m

200 m

200 m

Fault Zone (10 m wide)

75 m

Storage Aquifer

Caprock

Caprock

Deep Aquifer

Deep Aquifer

200 m10

0 m

100

m

-1600 m

Geomechanical Effect of CO2 Injection

▪ 2D Multi-Layer System at Depth with Faulted Caprock▪ Failure Criteria for Fault Slip and Hydraulic Fracturing▪ Stress-Induced Hydrological Property Changes

Hydromechanical Changes After 30 Years

0.0

1

0.010.05

0.0

5

0.1

0.1 0.1

0.1

0.2

0.4

0.40.5 0.5

0.5

0.5

0.6 0.6

Distance from Injection Point (m)

De

pth

(m)

-3000 -2000 -1000 0 1000 2000 3000

-1500

-1400

-1300

-1200

-1100

-1000

-900

0.60.50.40.30.20.10.050.01

Caprock

FaultCaprock

Fault

Distance from Injectin Point (m)

De

pth

(m)

-3000 -2000 -1000 0 1000 2000 3000

-1500

-1400

-1300

-1200

-1100

-1000

-900

-800

-700

-6001086420

'zz (MPa)

▪ Faults allow for significant leakage from storage aquifer▪ Upflow is partially diverted sideways into middle and upper aquifer▪ Significant pressure and stress changes in upper aquifers, causing

hydromechanical changes

CO2 SaturationReduction in Vertical Compressive Stress

Potential for Fault Slip

▪ Potential for injection-induced slip reactivation may be largest in upper layers, depending on initial stress field

▪ For fault stress regime (h= 1.5v), reactivation occurs on subhorizontal faults. Faulting limited to the region of strongly increased fluid pressure.

▪ For fault stress regime (h= 0.7v), reactivation occurs on subvertical faults. Faulting predominantly in the upper zones of multi-aquifer system.


De

pth

(m)

-3000 -2000 -1000 0 1000 2000 3000

-1500

-1400

-1300

-1200

-1100

-1000

-900

-800

-700

-600210

-1

Psm (MPa)


De

pth

(m)

-3000 -2000 -1000 0 1000 2000 3000

-1500

-1400

-1300

-1200

-1100

-1000

-900

-800

-700

-600210

-1

Psm (MPa)

Fault Stress Regime (h= 1.5v)

Fault Stress Regime (h= 0.7v)

Observations on CO2 Geomechanical Effects

A general reduction in the effective mean stress (due to pressure increase and local stresses) induces strongly coupled hydromechanical changes in the lower part of the caprock.

Shear reactivation in the lower part of the caprock could take place at an injection pressure below the lithostatic pressure.

For multi-layer systems, the potential for injection-induced slip reactivation may be largest in upper layers, depending on initial stress field

The type of stress regime (e.g., compressive or extensional types) is a key parameter that determines whether fracturing and shear slip are likely to take place along subhorizontal or subvertical fractures.

Once the CO2 fluid leaks to the upper part of the caprock (for example through a permeable fault) the upward CO2 migration is accelerated because of the combined effects of relative permeability and viscosity changes, as well as changes in intrinsic permeability caused by pressure-induced hydromechanical effects.

"Burning ice". Methane, released by heating, burns; water drips.Inset: clathrate structure (University of Göttingen, GZG. Abt. Kristallographie).Source: USGS

• A solid form of water that contains a large amount of methane within its crystal structure (a clathrate hydrate)

• One liter of methane clathrate solid contains about 168 liters of methane gas (at STP)

• Methane hydrates occur naturally in hydrate-bearing sediments (HBS) offshore in shallow depths below the ocean floor and onshore beneath the permafrost

• If economically producible, it could contribute significantly to future energy supplies

• Several production methods, including depressurization, thermal methods, and inhibitor injection, are being considered for extraction of gas from HBS

Methane Hydrate

Geomechanical Performance During Gas Production

• Deposits that are suitable targets for production often involves unconsolidated sediments that are usually characterized by limited shear strength

• The geomechanical response of hydrate-bearing sediments and potential wellbore instability and casing deformation are serious concerns that need to be addressed and understood before industrial gas production from hydrate deposits can be developed

• During production, the dissociation of the solid hydrates (a strong cementing agent) can significantly degrade the structural stability of hydrate-bearing sediments

• the evolution of pressure, temperature, saturation distribution, and salt concentration in hydrate-bearing systems undergoing hydrate dissociation or formation

A coupled geomechanical numerical simulator based on the hydrate simulator TOUGH-HYDRATE and the geomechanical simulator FLAC3D

Geomechanical Properties ?

• Data on geomechanical properties of hydrate-bearing sediments are very limited; laboratory methods challenging

• Currently, geomechanical properties are taken from laboratory experiments on hydrate-bearing Toyoura Sand (Masui et al., 2005, 2008)

• Assumed elasto-plastic, Mohr-Coulomb model and modified elastic and strength properties for pore-filling hydrate (and ice)

0

2

4

6

8

10

0 10 20 30 40 50 60 70

Hydrate saturation Sh (%)

Tri

axia

l com

pre

ssiv

e st

ren

gth

(M

Pa)

● Natural core sample○ Reconstituted sample■ Synthetic GH sample

0

200

400

600

800

1000

0 10 20 30 40 50 60 70Hydrate saturation Sh (%)

Ela

stic

mod

ulu

s Ｅ

50 (

MN

/m2)

● Natural core sample○ Reconstituted sample■ Synthetic GH sample

(Masui et al. OTC2008)

Numerical Test of HBS Mechanical Behavior During Methane Production

• Pressure, temperature, and stress conditions correspondent to an oceanic HBS

• Simulate constant rate production for 15 days

0

10

20

30

SHYD

(%)

0

20

40

60

SICE

(%)

30

10

50

0

10

20

30

SHYD

(%)

0

20

40

60

SICE

(%)

30

10

50

Numerical Test of HBS Mechanical Behavior During Methane Production: At 15 days

Q = 0.1 kg/s

Stress 10 MPa

Hydrate Saturation Ice Saturation


Q = 0.1 kg/s

Stress 10 MPa

Bulk Modulus Cohesion

0.0

0.2

0.3

0.4

K (GPA )

0.6

0.8

1.4

C (MPa)

1.0

1.2

0.0

0.2

0.3

0.4

K (GPA )

0.6

0.8

1.4

C (MPa)

1.0

1.2


Q = 0.1 kg/s

Stress 10 MPa

Volumetric Strain Vertical Displacement

-1.3

-1.4

-1.2

-1.1

v

(% )

-0.2

0.0

uz

(m)

-0.1

-1.3

-1.4

-1.2

-1.1

v

(% )

-0.2

0.0

uz

(m)

-0.1

General Lessons Learned, Challenges and Needa (1/3)

•Significant advances has been made in thermo-hydro-mechanics of geological systems, both

• In understanding and insight

• in modeling techniques

•A number of challenges has been met through studies under a number of international projects

• Research cooperation at a deep and detailed level

• Use of alternative approaches, multiple conceptual models, and different simplification methods

• advanced insight and understanding

• Advanced models and test against field data

General Lessons Learned, Challenges and Needs (2/3)

•A number of challenges remain ahead as we get into new problems such as those related to CO2 injection storage and methane production from gas hydrates. Examples are

• Constitutive relationships, probably mainly through comprehensive laboratory investigations on core samples --- a major investment is needed

• Efficiency in handling highly non-linear problems

• Efficiency in handling complex geologic geometry

• Large scale realistic field studies with extensive monitoring and measurements are needed

General Lessons Learned, Challenges and Needs (3/3)

• Limitations to the current approaches include

• How to improve laboratory measurements

• How to handle heterogeneities

• How to upscale THM from small scale measurements to larger scales

• How to predict far into the future

• How to calculate prediction uncertainty ranges

•Need new ideas and approaches

• convert the limitations to challenges and then

• work on challenges to obtain solutions!!!

Work continues:

To see order out of disorder from multiple THM-C couplings!

Fin

Thank you for your attention

Documents

Coupled Thermo-Hydro-Mechanics in Fractured Rocks: Modelling Status and Data Needs Chin-Fu Tsang Lawrence Berkeley National Laboratory USA Imperial College