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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.