Synthesis of micritic carbonate analogs: Effect on velocity-pressure sensitivity and dissolution

In this study, we investigate the effect of micrite content on the acoustic properties of well-controlled microstructures created in carbonate analogs. In particular, we examine the effect of micrite on acoustic velocities, sensitivity of velocity to pressure, and their response to dissolution upon saturation with a reactive fluid. Samples are created by mixing coarse (i.e., sand size) and very fine (i.e., micrite size) calcite grains in different amounts and then coldpressing the mixture under pressure to obtain solid, synthetic samples. Bench-top measurements of porosity, permeability and acoustic velocity were performed on the samples. Acoustic velocities were also measured as a function of confining pressure before and after the injection of CO₂-rich water. Results indicate micrite content to respectively increase the magnitude of P- and S- wave velocity and decrease its sensitivity to pressure. Thus, the micro-porous micrite aggregates seem to have a stiffening effect on the rock microstructure. This is probably due the sub-round (i.e., stiff) pore structure within micrite aggregates. Furthermore, the high-surface area of micrite aggregates promotes dissolution when the rock is saturated with acidic water. Chemical analysis of Ca⁺² concentration in the outlet fluid after injection shows larger dissolution in micrite-rich samples. Post-injection velocities in micrite-rich samples seem to decrease in magnitude as a result of dissolution.