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

Abstract

Capillary rise experiments (spontaneous imbibition tests) were conducted in the laboratory with ultrasonic and X‐ray monitoring on the Sherwood sandstone and the Majella grainstone. The aim was to provide a direct comparison between the variation in seismic attributes (amplitude, velocity, spectral content, and energy) and the actual fluid distribution in the rock. Two pairs of ultrasonic P wave sensors located at different heights on a cylindrical rock specimen recorded every 5 s the waveforms when capillary forces make water rise up into the rock from the bottom in contact with a water tank. Simultaneously, computerized tomography scan images of a vertical cross section were also recorded. Two important results were found. (i) The amplitude of the first P wave arrival is impacted by the upward moving fluid front before the P wave velocity is, while the fluid front has not yet reached the sensors level. In contrast, the P wave velocity decreases when the fluid front reaches the Fresnel clearance zone. The spectral analysis of the waveforms shows that the peak frequency amplitude is continuously decreasing without noticeable frequency shift. (ii) A methodology based on the calculation of the analytical signal and instantaneous phase was designed to decompose each waveform into discrete wavelets associated with direct or reflected waves. The energy carried by the wavelets is very sensitive to the fluid substitution process: the coda wavelets related to reflections on the bottom end face of the specimen are impacted as soon as imbibition starts and can be used as a precursor for the arriving fluid.

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

“…This experiment was done on a sample cored from the same block as the SH‐hor sample studied by David et al . [] but in a direction parallel to bedding. Therefore, on CT images the bedding appears to be vertical.…”

“…In Figure b, parameters d A and d V are plotted versus R F : with very few exceptions this plot confirms that the P wave amplitude is impacted when the capillary front is below the Fresnel clearance zone, whereas it is located above the Fresnel clearance zone when velocity is impacted. Therefore, our former observations derived from ultrasonic monitoring of imbibition experiments on a limited number of samples [ David et al ., , ] are confirmed with this extended data set including 14 different carbonate and sandstone samples.…”

“…The first results of this kind of experiments on four rock samples were published by David et al . [, ]. Here we present unpublished results which allow us to extend our data set to 14 different rock samples that experienced similar spontaneous imbibition tests with imaging and ultrasonic monitoring (Table ).…”

“…Furthermore, this amplitude drop occurs when the fluid front appears to be located well below the zone probed by the two ultrasonic source/receiver pairs. A similar behavior was reported in a companion paper of the same special volume [ David et al ., ] on two additional rocks, the Majella grainstone and the Sherwood sandstone. Whereas the relationship between the fluid front position and the onset of velocity decrease could be explained by taking into account the size of the Fresnel clearance zone [ Spetzler and Snieder , ], no clear explanation could be given for the decoupled evolution of P wave amplitude and velocity.…”

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.…”