The effect of micrite content on the acoustic velocity of carbonate rocks

We have demonstrated the effect of micrite content on the acoustic properties of well-controlled microstructures that are created in the laboratory to closely mimic those of carbonate rocks. In particular, we examined the effect of micrite content on the acoustic velocity, sensitivity of velocity to pressure, and changes in velocity resulting from dissolution upon saturation with a reactive fluid. We followed Dunham's classification and fabricated the samples by mixing coarse (sand-sized) and very fine (micrite-sized) calcite grains in different ratios and then cold compressing the mixture. The acoustic velocities were measured under benchtop conditions and as functions of confining pressure before and after the injection of a CO2 aqueous solution. The benchtop measurements indicated that the addition of micrite made the frame of the carbonate samples stiffer. Because the sensitivity of velocity to pressure decreases as the content of micrite increases, we hypothesized a stiffer pore structure in micritericher fabrics. Moreover, the content of micrite seems also to affect the change in elastic moduli upon dissolution. Micriterich samples experience a drop in elastic moduli after fluid injection, with respect to the moduli measured under dry conditions. This was interpreted as being likely due to dissolution, which weakens the rock frame. Such an effect seems to overcome the stiffening that results from dispersion mechanisms under high-frequency conditions.