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Comments of the German Association for Repository Research (DAEF)

Till Popp, Wolfgang MinkleyInstitute for Geomechanics GmbH (IfG)

Washington, DCSeptember 7-9, 2016

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„Deformation assisted fluid percolation in salt“ (Ghanbarzadeh et al., 2015)

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Ghanbarzadeh et al., 2016 - What is the explosive?

The study confirms already existing observations of Lewis & Hollnes (1996), but …

However, deformation may be able to overcome this threshold and allow fluid flow.The observed hydrocarbon distributions in rock salt require that percolation occurred at porosities considerably below the static threshold due to deformation-assisted percolation.

Therefore, the design of nuclear waste repositories in salt should guard against deformation-driven fluid percolation. In general, static percolation thresholds may not always limit fluid flow in deforming environments.

The low permeability of static rock salt is due to a percolation threshold.

Sophisticated analysis is needed to proof if the integrity of the geological barrier salt is really violated according to this thesis

Why are Hydrocarbons inside salt?

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Percolation threshold – Evaluation of salt barrier integrity

Percolation threshold is a mathematical concept related to percolation theory, which is the formation of long-range connectivity in random systems. Below the threshold a giant connected component does not exist; while above it, there exists a giant component of the order of system size.https://en.wikipedia.org/wiki/Percolation_threshold

Permeability in a salt barrier can be only induced under special mechanical or hydraulical conditions which result from the same micro-physical process, i.e. the percolation of flow paths along grain boundaries after exceeding a threshold.

This corresponds:

(1) at deviatoric conditions with the dilatancy boundary and

(2) at increased fluid pressures with the minimum stresses

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Geomechanical proof of the salt barrier integrity

Cap rock

GOK

drifts

Underlying bed

clayanhydrite

salt

potash

f (I2)

Dilatancyboundary

Dilatancy field

Compaction field

I1

Cap rock

GOK

drifts

Underlying bed

clayanhydrite

salt

potash

Detail A

pL = gL ∙ h

s3 s3

Hypotheticalgradient

pL = Fluid pressures3 = minimum stress

Generally accepted! Part of the integrity analysis, as

requested by the GERMAN SAFETY REQUIREMENTS

(BMU, 2010)

Dilatancy criterion

Minimum stress criterion

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A 3rd threshold ? The dihedral angle q for the salt-brine system

PT-dependent change of the wetting properties between brine and salt :• Change of the wetting angle –

development of connected pore channels along triple junctions (Lewis & Holness, 1996).

• Increase of the permeability up to 10-16 m2 (Schleder et al., 2007).

q > 60°Rock salt is tight

Increase of permeability is possible

q < 60° Rocksalt is permeable

Well known theory, but neglected because the pT-conditions are not relevant for a salt repository (e.g. VSG)

Repository

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The material tested vs. natural salt

Table salt: Grain sizes: 200 – 400 µm Composition: analytical grade halite

(99.9% pure)For each experiment, about 150 mg of

halite and 7-15 mg of distilled water used

Þ Water will dissolve saltÞ 4,5 – 9 wt.-% correspond to

7 – 16 Vol.-% = brine filled porosity of the condensed material

Natural rock salt (e.g. bedded salt Harlingen)Grain sizes: < 1 mm ... 10 mm … 1dmComposition: > 90% Halite, anhydrite, clay, accessoric mineralsÞ Water content, usually 0,1 – 1 wt.-%

20 mm

The grain scale distributions and the water content are not realistic!

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The experimental approach – Undrained Hydrostatic Experiment

Hydro-thermal autoclaves with ovens

pmax = 400 MPa

External Furnace Tmax ≤ 850°C

The teflon capsule is positioned inside a platinum tube (5 mm outer diameter) in a

Pressure vessel: 20 to 100 MPa; 100 to 275°C Quenched to room

conditions within 1 minute, i.e. putting the autoclave in water

Teflon capsule and covered with a Teflon lid Grain boundary opening due to

unloading effects respectively thermal shrinking

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The texture investigation method - Pore-Scale Imaging

Equilibrated salt-brine structure, reconstructed from X-ray micro-tomographic images of synthetic salt sample scanned by 1.1µm resolution. Each side of the cube corresponds to 660µm. left: Salt grain separation using watershed algorithm. right: medial-axis or skeleton of pore space. (source: https://sites.google.com/a/utexas.edu/ghsoheil/research)

P =

100

MP

a an

d T

= 27

5°C

P =

200

MP

a an

d T

= 10

0°C

University of Texas High-Resolution X-ray Computed Tomography Facility

Zeiss (formerly Xradia) microXCT 400 scanner:

3D resolution to ca. 1.1 μm pixel Image analysis:

Reduction of noise level Converting grayscale image

data into segmented images Filtering the segmented data Quantification and post

processing Pore space topology and

connectivity Estimate of the Dihedral Angle

It’s an indirect method which quality depends on the data evaluation …

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The original data sets from Lewis and Hollnes (1996)

The pore property changes described by the dihedral angle are small !!

With respect to experimental artefacts due to grain boundary opening .. What is the reliability of the estimate

of the Dihedral Angle?

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The static pore-scale theory - influence of porosity /water content?

Under consideration of natural salt properties the observed phenomena are probably not realistic, mainly due to the unusual high water content realized in the experiments

(source: https://sites.google.com/a/utexas.edu/ghsoheil/research)

Modeling of Textural Equilibrium using synthetical 3D-networks indicates that independently from the dihedral angle connectivity of pores depends on porosity (respectively the amount of brine)

Soheil Ghanbarzadeh

dihedral angle q

Poro

sity

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Experiment vs. Nature: salt from drill holes

q > 65°

60° < q < 65°

q < 60°

Petrophysical observations in wells - Occurence of HC in salt

Low rock resistivities and occurence of HC may give hints for fluid-connected pore space due to percolation

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UV-LightArtifical light

2 m 2 m

Occurence of hydrocarbons in the salt dome Gorleben (1)

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Occurence of hydrocarbons in the salt dome Gorleben (2)

Conclusion in Greenpeace (2010) – Hydrocarbons below a salt deposit contradict against a repository because the fluids will migrate through the salt no long-term safety

Presentation of J. Hammer (BGR)

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Observations on ARA salt cores from the OMAN Salt basin

Schoenherr, J., J.L. Urai, P. Kukla, R. Littke, Z. Schleder, J.-M. Larroque, M. Newall, N. Al-bry, H. Al-Siyabi, and Z. Rawahi (2007): Limits to the sealing capacity of rocksalt: A case study of the Infra-Cambrian Ara Salt from the South Oman Salt Basin: AAPG Bulletin, v. 91/11, p. 1541-1557.

Salt is impregnated by oil / bitumen contact between carbonate stringers / salt

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Cartoon of the mechanism of diffuse dilatancy of ARA salt

Stage 1: Diffuse dilation of salt

• Evidence for decompaction and local damage during uplift

• Over-pressurisation of Stringer fluids

violation of the minimal stress criterion

Pressure-driven percolation

Stage 2: Re-sealing• After the oil pressure dropped to

values equal to s3 in the rock salt (initial situation)

The micro-scale deformation mechanisms (dislocation creep and fluid-assisted grain-boundary-migration recrystallization) results in re-sealing of the fluids

from Schoenherr et al., 2007

Stage 1:

Stage 2:

Violation minimum stress criterion Pressure-driven

percolation

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Is the PT-dependent dihedral angle q – threshold real?

IfG Kármán pressure cell: s3 – max = 1000 bar T up to 120°C Hydrostatic / deformational

conditions Sample size (natural salt)

Length 200 mm Diameter 100 mm

Permeability testing with Gas, brine and oil

Preliminary test results are available

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Preliminary lab test results at T = 97°C and increasing confinement

No gas flow was detected! Resolution better than 10-20 m2.

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„Deformation assisted fluid percolation in salt“ (Ghanbarzadeh et al., 2016)

ConclusionsMy personal opinion,it is a well written and interesting paper but the authors wanted to amplify the public interest by including aspects of nuclear waste storage.However, there are some remarks regarding the conclusions:• Sample size and water content are not appropriate to natural salt• The thesis, that salt becomes permeable if the dihedral angle becomes

lower than 60° is only based on theoretical models.• Porosity resp. fluid content is an important parameter for the porespace

network, but not discussed by the authors in the paper.• The occurence of hydrocarbons is not always an indication of permeability

(autochthonous origin is possible)• It is a well known fact that salt can become permeable so that fluids can

migrate through it (especially during the diagnesis or salt dome uplift), but the dilatancy and minimal stress criterion are sufficient to explain the acting processes.

As summary, from our point of view, the integrity of salt as host rock for storage of radioactive waste is out of question.