Abstract
Understanding seismic attenuation in carbonate rocks is critical for improving reservoir characterization and fluid monitoring during hydrocarbon exploration. This study investigated the behavior of P-wave attenuation (1/Q (p)) during fluid substitution from saltwater to oil in coquina samples from the Morro do Chaves Formation, an analogue of Brazilian pre-salt reservoirs. Laboratory experiments were conducted at an ultrasonic frequency (1.3 MHz) by using the spectral ratio method to quantify attenuation. The complex pore structure of coquina, which includes interparticle, moldic, vuggy, and microfracture porosities, significantly influences wave dissipation mechanisms such as squirt flow and Biot-type relaxations. Four samples with different porosity-permeability relationships were analyzed, and attenuation measurements were correlated with saltwater saturation (S (w)) under controlled conditions. The results reveal a clear inverse relationship between 1/Q (p) and S (w), with empirical models showing strong correlations (R (2) > 0.8). Although the experiments were conducted at high frequencies, the observed trends provide insight into frequency-dependent attenuation and fluid mobility effects relevant to field-scale seismic applications. The lack of comparable studies of carbonate rocks with such heterogeneity reinforces the novelty of this work. These findings contribute to our understanding of attenuation mechanisms in porous carbonates and support the development of predictive models for use in reservoir-scale monitoring.