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Another key factor controlling the sealing behaviour of a damaged clay rock is its swelling potential.
Clay minerals in the rock matrix will expand into interstices when wetted with water. After a
water injection phase of about two months, the gas permeability was reduced by two orders of magnitude
from 1·10 -16 m 2 to 1·10 -18 m 2 . Because of the large sample size and the relatively short water
injection time, the fractures in the sample might be only partly resaturated and resealed by swelling
clay and water. Therefore, the fractures might be easily reopened by relatively low injection pressures.
An additional stress increase from 3 to 6 MPa reduced the permeability significantly below
10 -20 m 2 .
Ka
Figure 3.2: Sealing of fractures in claystone (Opalinus Clay) by recompaction (core size: diameter
260 mm, length 600 mm).
3.2 EDZ in rock salt
Permeability K (m 2 )
1E-13
1E-14
1E-15
1E-16
1E-17
1E-18
1E-19
1E-20
0 2 4 6 8 10 12 14 16 18 20
Radial stress σ 2 = σ 3 (MPa)
During recent years, R&D in rock salt has concentrated on in-situ investigation of the excavation
damaged zone (EDZ). Three related projects were performed in the Asse salt mine. The first was
the ALOHA project in which the extent and hydraulic properties of an EDZ under the floor of a
drift were investigated [10]. The second was the EC-funded BAMBUS II project [11] which concentrated
on EDZ anisotropy and self-sealing; the third project ADDIGAS [12] within the ECfunded
NF-PRO project was completed in 2007 and investigated the effectiveness of EDZ removal
and the subsequent evolution of a new EDZ.
Depending on stress conditions and history, the EDZ can extend one to two metres into the rock salt.
Permeability can increase by several orders of magnitude up to the range of 10 -14 to 10 -13 m 2 , usually being
below 10 -20 m 2 . Figure 3.3 shows a compilation of permeability measurement results from BAM-
BUS II and ADDIGAS.
209
axial stress = 19 MPa
loading
unloading
Opalinus Clay at Mont Terri
Large sample: D=260mm, L=600mm
permeability at p=0.7MPa
permeability at p=0.5MPa
permeability at p=0.3MPa