1 Modelling Task 8 EBS Task Force Meeting 16, Lund, 28 November 2012 Dr. David Holton [email protected] Dr. Steven Baxter [email protected]

1

Modelling Task 8EBS Task Force Meeting 16,

Lund, 28 November 2012

Dr. David Holton

[email protected]

Dr. Steven Baxter

[email protected]

2

Motivation

• The Nuclear Decommissioning Authority (NDA) are studying and considering the safe disposal of high level wastes within the UK.– A range of potential concepts are being considered, including the

use of bentonite as part of an EBS to surround disposal canisters.

• Participation in the EBS Task Force offers the NDA the unique opportunity to further the UK’s capabilities.

• Modelling Task8 has provided NDA the opportunity to:– verify and validate techniques and processes developed for

modelling bedrock and the bentonite interface;– demonstrate confidence in modelling the resaturation process; and– develop methodologies to represent the interaction between the

groundwater flow from the rock, and resaturation of the bentonite.

3

Task 8

Task 8C• Task 8C1

– Create a local scale model of the BRIE / TASO site.• Boundary conditions from large scale model.

– Use specified deformation zones (3).– Predict inflows to deposition holes (~0.07).– Support field experiment - importance of fractures and rock matrix.

• Task 8C2– Evaluate the effects of fractures on the wetting of the bentonite.– This is the Base Case for future calibration and learning from

prediction exercise.

4

Model domain & boundary conditions

5

Tools

6

Fracture orientation

Set Orientation

trend Plunge Fischer

concentration P32 P32 0.5m < L < 10m

1 280 20 10 1.10 0.85

2 20 10 15 2.00 1.55

3 120 50 10 0.75 0.58

All Sets 3.85 2.99

0

1

2

3

4

5

KO0020 KO0018 KO0017 KO0015 KO0014 Overall P32

Borehole

P10

,co

rr o

r P

32 [

m-1

]

7

Transmissivity / length correlation

1.E-16

1.E-15

1.E-14

1.E-13

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

0.001 0.01 0.1 1 10 100Length [m]

Tran

smis

sivi

ty [m

2 /s]

Ävrö granite (core)

Quarts monzodiorite (core)

Small features from Matrix Experiment

Background features from True BS

Transmissivity = 7·10-11 · Length 1.7

8

Fracture model

Model domain

Major structures Major structures + background fractures

wfracture_02wfracture_01

NNW4wfracture_01

wfracture_02

9

Transmissivity local to the tunnel

Fractures coloured by transmissivity for a vertical slice through the boreholes, and a horizontal slice at z = -418m.

wfracture_01

NNW4

10

Pressures local to the tunnel

Fractures coloured by pressure for a vertical slice through the boreholes, and a horizontal slice at z = -418m.

11

Inflows to probe boreholes

0.1

1

10

100

KO0020 KO0018 KO0017 KO0015 KO0014

Borehole

Infl

ow

[m

l/m

in]

BoreholeAverage Inflow

(ml/min)Realisation 2 Inflow

(ml/min)Measured Inflow

(ml/min)

KO0020 4.6 0.0 -

KO0018 7.1 0.7 -

KO0017 4.8 1.0 0.5

KO0015 6.6 0.7 -

KO0014 6.4 3.1 1

12

Inflows for “realisation 2”

Inflow locations to each of the probe boreholes

Fracture connected to the boreholes, coloured by transmissivity

13

Pressure recovery for “realisation 2”

The upper 1m of boreholes are packed off.

BoreholeAverage Pressure

(bar)

Realisation 2 Pressure

(bar)

Measured Pressure

(bar)

KO0020 7.0 0.0 -

KO0018 5.2 8.1 -

KO0017 5.1 5.6 6

KO0015 4.3 2.6 -

KO0014 4.3 4.0 3

14

Expansion of KO0017 and KO0018

• Average inflows increase to the larger diameter holes:– Flows are consistent with log(r) behaviour.– A skin may exist in the vicinity of the borehole wall, and therefore the

scaling may be different.– Flows are channelised, and hence there is the possibility of

intersecting new channels.– Larger diameter hole intersects more fractures.

15


Borehole

Av. inflows for all probe

holes

Av. inflows for expanded KO0017 &

KO0018 % increase

(ml/min) (ml/min)

KO0018 7.1 10.0 41.74%

KO0017 4.8 5.5 15.33%0.1

1

10

100

1 2 3 4 5 6 7 8 9 10

Realisation

Infl

ow

[m

l/min

]

Inflows to KO0017G01

Inflows to KO0018G01

0.1

1

10

100

1 2 3 4 5 6 7 8 9 10

Realisation

Infl

ow

[m

l/min

] Inflow to probe boreholes

Inflow to expanded deposition hole

16


Expansion of boreholes KO0017 and KO0018 to 0.3m diameter

17

Effective permeability

• Use CONNECTFLOW to calculate the effective permeability for individual grid blocks

CONNECTFLOW Model TOUGH2 permeability

18

Upscaling

keff = (kmax kint kmin)1/3

The geometric mean of the principle components of the permeability tensor

Upscaled conductivities, with

fracture traces overlain.

19

TOUGH2 – open borehole conditions

KO0017G01 KO0018G01

• Inflows to open boreholes KO0017G01 and KO0018G01.– Locations consistent with CONNECTFLOW calculations.

20

Resaturation of bentonite

• TOUGH2 used to model the resaturation front through the emplaced bentonite in the 5 deposition holes.

• Note: – 7 orders of magnitude variability in permeability.– Initially gave some convergence difficulties, especially with

heterogeneous wetting associated with a fractured host rock.

• Following slides illustrate results for realisation 2 of the stochastic fracture network.

21

Parameters

• Fracture permeability and porosity from the upscaling.

• Assume a matrix permeability for granite ~ 10-21m2, porosity 0.5%.

• Bentonite ~ 6.4 x 10-21m2, porosity 44%.

22

Modelling bentonite resaturation

• van Genuchten capillary pressure function used for Bentonite and fractured rock:

Fractured Rock Bentonite

• Relative permeability functions (a: aqueous phase, g: gas phase):– van Genuchten used for the fractured rock:

– Fatt and Klikoff cubic law used for the bentonite:

,~

1~

1,~

11~ 21

21

SSkSSk rgra

,1~ 11

SPP ocap ,

1

~

ar

ara

S

SSS

,56.0.10039.21 4

oP

,3.0.10080.11 7

oP

,~

1,~ 33 SkSk rgra

23

Evolution of the bentonite

• Bentonite is emplaced in all 5 boreholes– The evolution of liquid saturation is highly heterogeneous.

T = 0yrs

Liquid Saturation Liquid Pressure [MPa]

24



T = 0.1yrs


25



T = 1yrs


26



T = 10yrs


27




T = 100yrs

28

Resaturation rate of the bentonite

• For the heterogeneous description of the bedrock.– Saturation is characteristically heterogeneous for the low

permeability rock matrix (10-21 m2)– Minimum & maximum values taken for 5 realisations of the model.

• Homogeneous descriptions of the bedrock are shown to resaturate much more quickly.

Time to 99% Saturation KO0020G01 KO0018G01 KO0017G01 KO0015G01 KO0014G01

Minimum 18.7 10.9 12 7.4 9.3

Realisation 2 42.2 75.4 29.2 16.7 18.9

Maximum 42.2 75.4 40.3 25.8 18.9

Time to 95% saturation KO0020G01 KO0018G01 KO0017G01 KO0015G01 KO0014G01

Heterogeneous 32.8 62.3 25.2 12.1 16.7

Homogeneous 0.6 0.6 0.6 0.6 0.6

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Rock matrix effects

T = 10yrs

The low permeability rock matrix is

potentially slightly

desaturated by the

bentonite

30

Altering rock matrix permeability

• The default rock matrix permeability is 1 10-21m2.– Sensitivity analysis by considering rock matrix permeabilities of:

1 10-20m2, and 1 10-19m2.– Deposition hole KO0015G01 considered in isolation.

Base Case: 1 10-21m2 Variant: 1 10-20m2 Variant 1 10-19m2

T = 0yrs T = 0yrs T = 0yrs

31



1 10-20m2, and 1 10-19m2.– Deposition hole KO0015G01 considered in isolation

T = 0.1yrs T = 0.1yrs T = 0.1yrs


32





T = 1.0yrs T = 1.0yrs T = 1.0yrs

33





T = 10yrs T = 10yrs T = 10yrs

34


• Time evolutions for the centre of the bentonite at z = -417m.

Gas Saturation - KO0015G01

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12 14 16 18 20

Time (yrs)

Gas

Sat

ura

tio

n (

%)

Base Case: RM = 1E-21

Variant: RM =1E-20

Variant: RM =1E-19Rock Matrix Variant

Time (years)

95% Saturation 99% Saturation

Base CasePermeability = 10-21 m2 11.7 13.5

Permeability = 10-20 m2 2.2 4.8

Permeability = 10-19 m2 0.6 4.4

35

Conclusions

• Have illustrated a methodology to model the heterogeneity in the rock.

• Default properties show highly heterogeneous wetting and resaturation times.

• Resaturation depends on intersecting fractures, but also on the rock matrix.

• Surprisingly a deposition hole can have the same effective equivalent hydraulic conductivity but can resaturate at a different rate (depends on matrix conductivity of the granite).

36

Potential future Task 8 activities

• Task 8D – e.g. updating the fracture network, bentonite installation, etc.

• Sensitivities (e.g. matrix permeability of the granite).• Larger models (although smaller models were useful).• Capillary pressure / relative permeability functions (robustness).• The stochastic fracture generation.

– Further realisations of the upscaled fracture network (what does the resaturation look like on “average”).

– Transmissivity relationship.– Calibration of the fracture network (does this help?).

• Mechanical coupling?• Bentonite installation.

– Including a bottom plate, central tube and upper rubber seal.

Documents

1 Modelling Task 8 EBS Task Force Meeting 16, Lund, 28 November 2012 Dr. David Holton [email protected] Dr. Steven Baxter [email protected]