Ultrasonic
velocity measurements (Fig. 2A) show that Vp and Vs increase with
σm, and the younger samples (FB12 and GB13) generally
have lower wave speeds. Vp to Vs ratios vary between 1.6 and 1.9, with
the least consolidated and youngest sample (FB12) exhibiting the highest
values. After saturation, sample GB13 Vp increased by
~250 m/s on average while Vs decreased by
~100 m/s on average, increasing the Vp to Vs ratio from
~1.65 to ~1.95.
Figure 2. A) The left-top panel reports ultrasonic velocities measured
on the four dry samples as a function of mean stress
(σM). The right-top panel shows ultrasonic Vp and Vs for
sample GB13 when dry and saturated with water. The dashed line indicates
the theoretical dry velocity from Gassmann fluid substitution (Gassmann,
1951), considering the dry sample velocities, density, and porosity. The
inset shows examples of P and S waveforms recorded for the dry sample at
the conditions indicated by the circled dots. The bottom panels report
the Vp / Vs ratios for the laboratory data and the fitting curves
reported in the panels above. B) Permeability for samples FB12, MT07,
and MO02 as a function of effective stress (σ’). An example of pore
pressure (Pp) equilibration and fitting curves (dashed lines) are shown
in the ”Pp equilibration” inset. During (a) sample FB12 was compacted by
step increasing and by holding σM for several hours; an
example of partial compaction is shown in the ”FB12 creep” inset. The
loss of porosity of sample FB12 due to compaction is shown in the inset
”FB12 compaction”. The blue and red curves show the permeability of
sample MT07 after fracturing (S1 and Fig. 3A) and during healing in two
consecutive cycles (kT1 and kT2). During kT1 (b), the sample was kept at
a confining pressure of 70 MPa for more than 48 hours. KT2 was acquired
~30 days after cycle 1. The end of KT2 was stage S2,
where the sample was CT-scanned (Fig. 3B) and exposed to a
>97% relative humidity atmosphere for 72 hours, and then
stage S3, where we CT-scanned (Fig. 3C) and remeasured permeability.
Before compaction, sample FB12 permeability ranged between 200 and 400
µD. Then, we raised P c twice to 70 MPa, causing
the permeability to decrease by a factor of two and porosity by 3%
(i.e., at σm~7 MPa, porosity varied from
17.3 to 14.2%). In the following two cycles, whereP c reached 200 MPa, porosity further decreased to
13.9%, and the permeability declined by almost an order of magnitude.
Concurrently, the ultrasonic Vp increased from 2.6 km/s to 4 km/s.
Samples MO02 and MT07, when intact, have permeabilities below 100 nD,
regardless of σ’. The permeability of the fractured MT07 evolved between
stages S1, S2, and S3. After S1 and during the first permeability
measurement cycle (kT1), the permeability dropped from 2 µD to 87 nD.
After exposing the sample to σ’ ~65 MPa for more than 48
hours (Fig 4B b), we performed a permeability test, and found that the
permeability further decreased to 24 nD. The permeability remained
~2 orders of magnitude lower than the permeability at
S1, i.e., around 30 nD, when σ’ was reduced. The second permeability
testing cycle (kT2) started 39 days after the end of kT1. The increase
of σ’ caused the permeability to drop to 9 nD. During the following
decrease of σ’, the permeability resembled pre-fracturing values. A
final test was performed at σ’ 4.5 MPa and permeability dropped to 300
nD, seven times lower than the initial value measured at σ’ 5.6 MPa.
After exposing the sample to humidity (S3) for 72 hours, the
permeability, measured at σ’ 3.7 MPa, decreased from the S2 value by
roughly five times.