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

摘要

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 cold-pressing 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 CO_2-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.