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.