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OPG’s Deep Geologic Repository for Low & Intermediate Level Waste

Phase I Hydrogeologic Modelling

Jonathan Sykes, Eric Sykes, Stefano Normani

Yong Yin and Young-Jin Park

Department of Civil and Environmental Engineering

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Outline1.

Applicability of the computational model (FRAC3DVS-

OPG)

2.

Development of the numerical model

3.

Density-dependent flow

4.

Abnormal heads

5.

Potential for a transmissive

PreCambrian

6.

Paleo-climate simulations: Bruce DGR Geosynthesis modelling

vs. other studies

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1.

Applicability of the computational model (FRAC3DVS-OPG)

•

The NWMO has supported significant improvements of the code FRAC3DVS (Therrien

et al., 2004)

–

Density-dependent flow –

Sub-gridding and Sub-timing

–

Mean Life Expectancy as a performance measure

–

Inclusion of one-dimensional vertical stress term

•

The model has been verified and undergone QA

4

–

p is pressure–

k is intrinsic permeability

–

ρ

is concentration and pressure dependent fluid density

–

μ

is the concentration dependant fluid viscosity

–

Ss

is the specific storage–

ς

is

the

one-dimensional

loading

( )t

StpS

xgzpgk

xzz

ssj

ij

i ∂∂

−∂∂

=⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂+∂

∂∂ σζρ

μρ

Formulation for paleo-climate analyses -vertical strain and incompressible poresareally homogeneous load

1.

Applicability of the computational model (FRAC3DVS-OPG)

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2. Development of the numerical model

•

The Phase 1 regional and site-scale numerical modelling

was performed to

gain a quantitative understanding of groundwater system evolution and resilience to change

•

The modelling

was performed as part of a multi-disciplinary approach that integrated information from the Phase 1 –

Geologic,

–

Hydrogeochemical, and

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2. Development of the numerical model

•

There were two modelling

stages:–

Preliminary modelling

–

Modelling

using the GLL00 geologic framework

•

What was known at the beginning of the study:–

Cambrian is overpressured

(e.g. Texaco

6) –

cause is unknown; there are hypotheses in literature

–

Hypothesized structural traps–

No knowledge that the Ordovician at the B

DGR

d

d

7

2.

Development of the numerical model•

The attributes of the modelling

reported

in the Phase 1 report are as follows:–

Based on 3-D GLL00 geologic framework model

–

Uses the bathymetry for Lake Huron and Georgian Bay plus DEM and bedrock outcrop/subcrop

data

–

The regional-scale is spatially extensive:•

Surface extent includes outcrop for the Niagaran

•

It allows the issue of the northern and southern extent of the Cambrian to be investigated using different conceptualizations of the lateral boundary conditions

•

Surface

extent

corresponds

to

surface

water

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2.

Development of the numerical model•

It honours

the parameters and data

from the Phase 1 site characterization program

•

Paleo-climate analyses are investigated at the regional scale

•

The cause of the abnormal heads can be investigated at the regional scale–

Over-pressured Cambrian

–

Under-pressured Ordovician

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Spatial domain, DEM and rivers

18 000 km2

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Top of Cambrian

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Top of Niagaran

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Regional-scale and site-scale grids

32 slices

27,728 nodesper slice

Blocks:900.9m by 762.8m

Area:19.078 km by18.918 km

Blocks:127 m by112.6 m

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2.

Development of the numerical model: Solution procedure:

•

Initial condition:–

Density-independent steady-state flow ….plus

–

Initial TDS distribution….allow to–

Reach pseudo-equilibrium for assigned boundary conditions and parameters that reflect present system state

•

Alter state: e.g. paleo-climate –

Permafrost depth

–

Ice load•

Geo-mechanical model: vertical strain only

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2.

Development of the numerical model

•

The design of the modelling

program considered base case, sensitivity, paleohydrogeologic

and

‘what if’

simulations exploring the occurrence and maintenance of observed anomalous over-

and

under-pressure head conditions in the Cambrian and Ordovician sediments.

•

Investigate parameter sensitivity–

Parameter constraints define the response surface

–

Perturb Cambrian, Ordovician and Silurian hydraulic conductivities

–

Estimate sensitivity for MLE

•

Investigate model sensitivity–

Recharge

boundary

condition

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2.

Development of the numerical model

•

Parameters include:–

Hydraulic conductivities

–

Porosities–

Storage coefficients

–

Transport parameters–

Mechanical parameters (E, ν)

–

Constitutive relationships (e.g. density and viscosity)

–

Boundary conditions–

Initial conditions

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Parameters, boundary conditions and initial conditions for regional-scale analyses

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3. Density-dependent flow

•

Dense fluids can have a significant impact on basin flow (Park, Sudicky

and Sykes, 2009)

–

driving forces from surface boundary conditions may not be strong enough to cause shallow fresh groundwater to penetrate into the deep brine region

–

the existence of brine may be an indicator of a hydrogeologically

stable environment

•

In the Michigan Basin: 1.0 ≤ ρ ≤ 1.2•

The fluid viscosity can be significantly greater than 1 cP

for high TDS Ca-Cl

brine

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3. Density-dependent flow

•

The formulation of the equations for density- dependent flow follows that of Frind, E.O., 1982.

Simulation of long-term transient density- dependent transport in groundwater. Advances in Water Resources 5. 73-88.

•

Methodology –

two options were investigated:–

Initial TDS distribution plus halite and evaporite

source term

–

Initial TDS distribution that was allowed to equilibrate to equivalent freshwater heads

•

Equivalent freshwater head cannot be used to interpret vertical gradients –

environmental heads (Lusczynski, 1961) must be used.

•

To

obtain

a

solution

for

density-dependent

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Total dissolved solids distribution

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Base

case equivalent freshwater heads:

pseudo-equilibrium time is 1,000,000 years

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Base

case equivalent freshwater heads:

pseudo-equilibrium time is 1,000,000 years

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Base-case environmental heads: pseudo-equilibrium time is 1,000,000

years

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Base-case environmental heads

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4.

Abnormal heads •

The abnormal heads are dealt with extensively in the Phase 1 report.

•

The Phase 1 modelling

philosophy was to honour

the DGR borehole data

(and explore the sensitivity of the state variables to the parameters)

•

The cause of the abnormal pressures could not be revealed by adjusting parameters

•

The cause of the abnormal pressures can

only

be

investigated

by

exploring

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4.

Abnormal heads•

A requirement of the abnormal pressures of the Cambrian is overlying, extensive, low vertical hydraulic conductivity strata.

•

Hypothesis:–

The low pressures in the Ordovician may be the result of stress relief as a result of significant removal of mass through erosion or deglaciation, that was at a rate that is greater than that of water influx to these low permeability units from the over and under-lying units with higher pressure; the pressure distribution will be

ill

l

i

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4.

Abnormal heads •

A triage approach is followed:–

Phase 1 investigated fully saturated flow only

–

Paleo-climate analyses do not explain the abnormal pressures (this conclusion to be further developed in Phase 2)

–

Mechanical properties for the Ordovician relative to that of the Cambrian have not (to date) supported a mechanical explanation for the abnormal pressures

–

There may be Basin influences –

A

conclusion

of

the

Phase

1

study:

It

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Basin Influences: Phase 2 work

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Environmental heads at DGR- 1/DGR-2

•

Surface: 185.84 mASL

•

Data: March 3, 2008

•

Pressure measurements

•

TDS estimates

•

Abnormal pressure:–

Cambrian

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Site-scale modelling of environmental heads in DGR-1/DGR-2: pressure support for Niagaran but

not the CambrianAnisotropy 10:1

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Site-scale modelling of environmental heads in DGR-1/DGR-2: pressure support for Niagaran but

not the CambrianAnisotropy 100:1

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5.

Potential for a Transmissive Precambrian•

It can be hypothesized that the upper zone of the Precambrian is fractured

•

The zone could provide a preferential pathway

•

Analyses from the Phase 1 study provide support that a preferential pathway does not occur:–

the zone would be inconsistent with the abnormal pressures in the Cambrian

–

The preliminary modelling

study included a transmissive

zone above the Precambrian

throughout the domain –

Cambrian pressures are

too

low

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Continuous Cambrian (Scenario 6)

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Horizontal boundary condition: Scenario 16

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Mean Life Expectancies for Regional-Scale Scenarios

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6

. Paleo

climate simulations: Bruce

DGR Geosynthesis

modelling

vs. other studies

•

The paleo-climate modelling

has used a three- dimensional regional geologic framework with 31

layers•

Parameters are from DGR site characterization study

•

Uses Peltier

simulation

•

Lemieux and Sudicky–

Continental scale, simplified geometry (1 material layer for Michigan Basin)

–

Large grid blocks and hence large longitudinal dispersivity

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6

. Paleo

climate simulations: Bruce

DGR Geosynthesis

modelling

vs. other studies

•

Bense

and Person (2009)–

Simplified geometry (4 aquifers and 3 semi-

confining layers for hypothetical intracratonic basin)

P

biliti

d

ifi t

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6

. Paleo

climate simulations: Bruce

DGR Geosynthesis

modelling

vs. other studies

•

The ice sheet model is significantly different –

it would not agree with ocean level data

Peltier

Bense

and Person

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Key findings from the Scenario analyses of the Phase 1 study

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Salient conclusions:•

Diffusion is the dominant transport mechanism in the Ordovician sediments.

• There are multiple natural bedrock barriers both within the intermediate and deep groundwater zones. –

The low permeability of the Ordovician units and resulting low estimates of pore water velocity: diffusion dominant

–

The low permeability of the Lower Silurian units –

The low permeability of the Salina units

–

The long travel path in the Niagaran

• Simulation of anomalous vertical hydraulic head distributions within the Ordovician and Cambrian sediments at DGR-1 and DGR-2 indicate that groundwater movement is converging on the

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Salient conclusions (continued):•

The origin of the anomalously low hydraulic heads observed in the Ordovician sediments is unlikely due to glacial events as a consequence of the predicted loading-unloading cycle.

•

Extensive low permeability strata overlying the Cambrian Formation is required for the maintenance of the observed hydraulic overpressures. Analyses indicate that to preserve the hydraulic overpressure for 1 Ma vertical hydraulic conductivities of 1 x 10-14

m/sec or less are required. This is consistent with the Phase 2 hydraulic testing results.

•

Paleohydrogeologic

simulations for a glaciation

scenario indicate that basal meltwaters

would not penetrate below the Salina Formation. Simulations further indicate that while ice-loading will influence hydraulic head distributions, transport processes remain diffusion dominant within the Ordovician sediments.

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Questions . . .