Continuous Recording of Viscoelastic Relaxation Processes at a Constant Ultrasonic Frequency Due To Wave‐Induced Fluid Flow in a Microporous Carbonate Rock. Issue 19 (27th September 2021)

• Record Type: Journal Article
• Authors: Geremia, Davide; David, Christian
• Is Part Of: Geophysical research letters. Volume 48:Issue 19(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-09-27
• Subjects: dispersion -- attenuation -- patchy saturation -- ultrasonic monitoring -- carbonate rocks -- P‐wave; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL095244
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: In a set of water injection, imbibition and drying tests on the Obourg Chalk, relaxation processes induced by variations in water saturation were continuously recorded at a constant ultrasonic frequency, with a peak in attenuation concomitant to a velocity increase from a lower to an upper bound. This behavior is well described by standard viscoelastic models, considering a continuously changing relaxation frequency which at water saturation above 80% matches the frequency of our ultrasonic pulse generator. Using a patchy saturation model, we show that the heterogeneity in fluid distribution is responsible for this relaxation phenomenon which could be observed at the ultrasonic frequency, thanks to the very fine scale of the rock microstructure combining low permeability and high porosity. We believe that our detailed data set can feed new models or help improve existing ones for a better understanding of wave‐induced fluid flow processes in reservoir rocks. Plain Language Summary: Relaxation processes were continuously recorded during experiments in which the fluid content in a porous rock was changing with time due to injection, evaporation, or capillary processes. Significant variations in velocity and amplitude of compressive waves were observed and could be accounted for by models involving heterogeneous distributions of fluids at the microscopic scale. Such continuous recording of a relaxation transition, as predicted for example by the viscoelasticity theory, was made possible in the range of frequency used in our experiments because of the very fine grain and pore size in the tested rock. Our data set will help geophysicists to improve the modeling of wave‐induced fluid flow in reservoir rocks. Key Points: Viscoelastic relaxation processes were continuously recorded versus time at fixed ultrasonic frequency in fluid substitution tests on chalk Increasing the water saturation above 80% induces a transition from relaxed to unrelaxed regime Johnson's model for patchy‐saturated porous media accounts well for the observed P wave velocity and attenuation variations with saturation