Elastic Softening of Limestone Upon Decarbonation With Episodic CO2 Release

• Published in: Journal of geophysical research. Solid earth Vol. 123; no. 9; pp. 7404 - 7420
• Main Authors: Head, Dulcie, Vanorio, Tiziana, Clark, Anthony C.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-09-2018
• Subjects: Analytical methods; Carbon dioxide; carbonate; Confining; Contact pressure; Decarbonation; Elastic properties; elastic softening; Energy dispersive X ray spectroscopy; Geophysics; Grain; Limestone; microstructure; Modelling; Modulus of elasticity; Organic chemistry; Pore pressure; Porosity; Porous media; Pressure; Properties; reactive transport; Rock; Rocks; Seismic activity; Seismic velocities; Seismicity; Sensitivity; Silicates; Softening; Spectroscopy; Stiffness; Stone; Transport; Velocity; Wollastonite; X-ray spectroscopy
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2018JB015733

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 (CO2). 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.
Key Points
Higher initial pore network connectivity heightens the elastic property changes due to decarbonation under confining pressure in limestone
Large rise in the pressure sensitivity of velocity after decarbonation is accompanied by pervasive compliant porosity observed with SEM
Assessment of reaction‐induced frame weakening improves seismic survey interpretation, including remote detection of pore pressure changes