Résumé
Reservoirs can undergo inelastic compaction due to production induced effective pressure changes. Contrasts in the rocks matrix compressibility, introduced by the collapse of the pore space, may lead to significant frequency dependent dispersion and attenuation of seismic waves due to fluid pressure diffusion at the micro to mesoscopic scale. Understanding how compaction influences these rock properties and their frequency dependence may contribute to improving the monitoring of reservoirs, for example through 4D seismics. We present the results of a hydrostatic compaction experiment on a Chauvigny limestone saturated with glycerin. During the experiment we measured the samples permeability, as well as the dynamic bulk modulus at seismic and ultrasonic frequencies, using the forced oscillation method and P- and S-wave travel times, respectively. The bulk modulus measured, at an effective pressure of 2.5 MPa, on the intact and saturated sample exhibited no dispersion of the bulk modulus between 1 Hz and 1 MHz. Furthermore, the results were consistent with Gassmann's prediction for an undrained and mechanically isotropic sample. With increasing effective pressure, the onset of inelastic compaction (P* approximately 13 MPa) is observed in the decrease of the P- and S-wave velocities, porosity and permeability. The bulk modulus measured during intermediate stages of compaction and after returning to the initial effective pressure, again did not exhibit dispersion between seismic and ultrasonic frequencies. The absence of dispersion may indicate that the inelastic compaction was mainly associated with plastic pore collapse and not the introduction of cracks in the rock matrix. The interpretation of the results is complicated by the fact that compaction possibly did not occur homogeneously throughout the sample. However, the experiment is the first to study the effects of inelastic compaction on the broadband viscoelastic response of reservoir rocks and provides valuable information for future experiments.