Ultrasonic monitoring of spontaneous imbibition experiments: Precursory moisture diffusion effects ahead of water front

Abstract

Fluid substitution processes have been investigated in the laboratory on 14 carbonate and siliciclastic reservoir rock analogues through spontaneous imbibition experiments on vertical cylindrical specimens with simultaneous ultrasonic monitoring and imaging. The motivation of our study was to identify the seismic attributes of fluid substitution in reservoir rocks and to link them to physical processes. It is shown that (i) the P wave velocity either decreases or increases when the capillary front reaches the Fresnel clearance zone, (ii) the P wave amplitude is systematically impacted earlier than the velocity is, (iii) this precursory amplitude decrease occurs when the imbibition front is located outside of the Fresnel zone, and (iv) the relative variation of the P wave amplitude is always much larger than that of the P wave velocity. These results suggest that moisture diffuses into the pore space ahead of the water front. This postulate is further supported by a quantitative analysis of the time evolution of the observed P wave amplitudes. In a sense, P wave amplitude acts as a precursor of the arrival of the capillary front. This phenomenon is used to estimate the effective diffusivity of moisture in the tested rocks. The effective moisture diffusivity estimated from the ultrasonic data is strongly correlated with permeability: a power law with exponent 0.96 predicts permeability from ultrasonic monitoring within a factor 3 without noticeable bias. When the effective diffusivity is high, moisture diffusion affects ultrasonic P wave attributes even before the imbibition starts and impacts the P wave reflectivity as evidenced by the variations recorded in the waveform coda.

Comment on “Ultrasonic Monitoring of Spontaneous Imbibition Experiments: Precursory Moisture Diffusion Effects Ahead of Water Front” by David et al. (2017)

“…This report was motivated by the following recent papers on spontaneous water imbibition experiments: David, Barnes, et al () and David, Sarout, et al (). In particular, these papers (see also David et al ; Dautriat et al ) present experiments in which moving water front was monitored using ultrasonic transducers (active acoustic monitoring).…”

“…It was noticed that the amplitude of P waves dropped significantly before any changes in P wave velocity was detected. David, Sarout, et al () related this early change in amplitude to diffusion of moisture (water vapor) ahead of the water front. But why does the moisture diffusion affect only P wave amplitude but not the velocity?…”

Ultrasonic monitoring of spontaneous imbibition experiments: Acoustic signature of fluid migration

“…Comparatively to these earlier studies, our results suggest that water adsorption on mineral grains has more impact on wave amplitude than it has on wave velocity. An extended data compilation providing clues that moisture diffusion accounts for the early amplitude drop that is presented in a companion paper in this special volume [ David et al ., ].…”

“…Our results on the Sherwood sandstone and the Majella grainstone confirm our previous findings on the Saint‐Maximin grainstone and the Savonnières limestone: the amplitude of the first P wave arrival is impacted by the upward moving fluid front before the P wave velocity is, when the fluid front is still located well below the ultrasonic transducers plane. At this stage we suggest that amplitude is first impacted by water vapor adsorption, an issue which is further discussed in the companion paper in the same special volume [ David et al ., ]. The P wave velocity drop is triggered by the fluid front when it reaches the Fresnel clearance zone, an ellipsoidal volume including the source‐receiver transducers and the direct raypath between them.…”

Evolution in Seismic Properties During Low and Intermediate Water Saturation: Competing Mechanisms During Water Imbibition?

“…Through an interaction between capillary, gravity, and wettability driving forces, fluids such as water can be spontaneously sucked in by a porous rock and migrate through its pore network, leading in some cases to large saturation at equilibrium (large times). Recent spontaneous imbibition experiments evidenced a strong reduction in ultrasonic wave amplitudes, with little changes in wave velocities (David, Barnes, et al, ; David, Sarout, et al, ). Those observations were attributed to diffusion of moisture migrating faster and ahead of the front of imbibing liquid water, thus leading to attenuation although not leading to strong elastic softening of grains contacts.…”