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

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

7 Transmissivity / length correlation

8 Fracture model Model domain Major structures Major structures + background fractures wfracture_02 wfracture_01 NNW4 wfracture_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 Borehole Average Inflow (ml/min) Realisation 2 Inflow (ml/min) Measured Inflow (ml/min) KO KO KO KO KO

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. Borehole Average Pressure (bar) Realisation 2 Pressure (bar) Measured Pressure (bar) KO KO KO KO KO

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 Expansion of KO0017 and KO0018 Borehole Av. inflows for all probe holes Av. inflows for expanded KO0017 & KO0018 % increase (ml/min) KO % KO % Inflows to KO0017G01 Inflows to KO0018G01 Inflow to probe boreholes Inflow to expanded deposition hole

16 Expansion of KO0017 and KO0018 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 k eff = (k max k int k min) 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 ~ m 2, porosity 0.5%. Bentonite ~ 6.4 x m 2, 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:

23 Evolution of the bentonite Bentonite is emplaced in all 5 boreholes –The evolution of liquid saturation is highly heterogeneous. T = 0yrs Liquid SaturationLiquid Pressure [MPa]

24 Evolution of the bentonite Bentonite is emplaced in all 5 boreholes –The evolution of liquid saturation is highly heterogeneous. T = 0.1yrs Liquid SaturationLiquid Pressure [MPa]

25 Evolution of the bentonite Bentonite is emplaced in all 5 boreholes –The evolution of liquid saturation is highly heterogeneous. T = 1yrs Liquid SaturationLiquid Pressure [MPa]

26 Evolution of the bentonite Bentonite is emplaced in all 5 boreholes –The evolution of liquid saturation is highly heterogeneous. T = 10yrs Liquid SaturationLiquid Pressure [MPa]

27 Evolution of the bentonite Bentonite is emplaced in all 5 boreholes –The evolution of liquid saturation is highly heterogeneous. Liquid SaturationLiquid Pressure [MPa] 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 ( m 2 ) –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% SaturationKO0020G01KO0018G01KO0017G01KO0015G01KO0014G01 Minimum Realisation Maximum Time to 95% saturationKO0020G01KO0018G01KO0017G01KO0015G01KO0014G01 Heterogeneous Homogeneous0.6

29 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 m 2. –Sensitivity analysis by considering rock matrix permeabilities of: m 2, and m 2. –Deposition hole KO0015G01 considered in isolation. Base Case: m 2 Variant: m 2 Variant m 2 T = 0yrs

31 Altering rock matrix permeability The default rock matrix permeability is m 2. –Sensitivity analysis by considering rock matrix permeabilities of: m 2, and m 2. –Deposition hole KO0015G01 considered in isolation T = 0.1yrs Base Case: m 2 Variant: m 2 Variant m 2

32 Altering rock matrix permeability The default rock matrix permeability is m 2. –Sensitivity analysis by considering rock matrix permeabilities of: m 2, and m 2. –Deposition hole KO0015G01 considered in isolation Base Case: m 2 Variant: m 2 Variant m 2 T = 1.0yrs

33 Altering rock matrix permeability The default rock matrix permeability is m 2. –Sensitivity analysis by considering rock matrix permeabilities of: m 2, and m 2. –Deposition hole KO0015G01 considered in isolation Base Case: m 2 Variant: m 2 Variant m 2 T = 10yrs

34 Altering rock matrix permeability Time evolutions for the centre of the bentonite at z = -417m. Rock Matrix Variant Time (years) 95% Saturation99% Saturation Base Case Permeability = m Permeability = m Permeability = m

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.