The rock physics and geochemistry of carbonates exposed to reactive brines

When carbonate-rich rocks are brought into contact with an acidic brine their mechanical and acoustic responses depend on many factors including pH, porosity, permeability, effective stress, and time. These complexities hinder the understanding of processes such as hydrothermal fluid circulation, seismicity, and deep burial diagenesis. The present study addresses how different lithofacies exposed to the same reactive brine undergo varying degrees of transformation and whether it is possible to remotely detect these phenomena in the Earth. Results are presented from fluid injections carried out on a large and varied set of calcareous rocks under hydrostatic stress. The output brine was analyzed for dissolved mineral concentrations and the rock porosity, permeability, axial strain, ultrasonic velocity, and images from electron microscopy were contrasted before and after injection. Microbialites were found to be the lithofacies most vulnerable to changes in their transport properties. However, all samples irreversibly compacted with the greatest strain in the most porous and permeable cores. The most extreme structural changes discovered through imaging were the welding of microporous zones, grain sliding, and the fracturing of various phases. Observations are consistent with a chemically enhanced weakening of the rock frame that generated compliant pores. The associated decrease in velocity of the dry rock can be approximated with linear relations that depend on both porosity and effective stress.