Elastic Softening of Limestone Upon Decarbonation With Episodic CO 2 Release

Reactive transport under confining stress through porous media leads to complex, coupled feedback mechanisms between chemical and physical alteration. We exposed four intact limestone cores to hydrothermal conditions conducive to the wollastonite‐producing decarbonation reaction. We allowed for the episodic release of pore pressure to mimic pulsing flow in crustal systems and mitigate the accumulation of carbon dioxide (CO 2 ). Energy‐dispersive X‐ray spectroscopy results show that all four reacted samples developed a new calc‐silicate phase during the reaction. Alteration from the fluid‐mediated reaction was pervasive but nonuniform, resulting in the development of a small volume of highly compliant pore space with grain contact softening. Lower initial velocity sensitivity to confining pressure and higher initial pore connectivity lead to more development of soft porosity. The relevance of the results lies in enhancing seismic monitoring capabilities of potentially seismogenic areas—large decreases in observed seismic velocity in limestone undergoing decarbonation can only be accounted for if the nonnegligible change in the elastic properties of the rock frame are understood. Since the potential for seismicity of a subsurface system depends both on the elastic stiffness and stress state of the rocks, rocks undergoing reactive transport cannot be modeled as though their elastic moduli are time independent. Accurate modeling of the subsurface behavior depends on understanding not just the particular rock reactions taking place but also the initial pore network and velocity sensitivity to confining pressure of those rocks, and how the elastic properties of that rock evolve.