ReSiDue: A coupled rock physics and geochemical approach to understanding how rock-fluid interactions change permeability in volcanic systems

Reactions between aqueous fluids and rocks result in secondary mineral formation – alteration – that can block permeable pathways in the Earth’s crust and change the hydraulic properties of geological systems. The propensity, extent, and timescales of rock alteration are therefore important controls on crustal permeability. However, understanding the intricate relationships that result in rock alteration and changes in permeability - including dissolution, transport, and redistribution of chemical compounds - represents a technical challenge and remains a key unsolved problem in the Earth sciences. As a result, we do not fully understand how these processes modify the structure of permeable channels and over what timescales they may hamper crustal fluid flow, limiting, for example, our ability to effectively model the efficiency of geothermal reservoirs or forecast volcanic eruptions. The SNSF-funded ReSiDue project – Redistribution of Silica and Deposition under a volcanic edifice - addresses how the redistribution of the most common compound in the Earth’s crust - silica (SiO₂) - changes the permeability of rocks under volcanically relevant conditions. Silica alteration is ubiquitous in volcanic systems and results in changes to rock structure and thus hydraulic properties. However, the processes that lead to silica alteration remain understudied, especially at high temperatures, because they involve complex feedbacks between fluid-rock interactions and petrophysics. Using a mix of detailed petrophysical, microstructural and geochemical characterization, and water-rock interaction experiments, we are quantifying 1) the physical and chemical conditions promoting silica alteration in volcanic edifices, 2) how fluid-flow pathways in rocks change over time as a result of silica alteration, 3) how these changes modify permeable flow, and 4) on what timescales these processes are active. The newly operational High temperature Reactive flOw permeabiLity Device – HAROLD – allows for in situ monitoring of permeability evolution over time, during reactive flow. This apparatus can operate at temperatures up to 500°C and confining and pore fluid pressures of 100 MPa and 20 MPa, respectively, allowing us to target conditions relevant to volcanic edifices and shallow geothermal reservoirs. This systematic understanding of how silica alteration changes the permeability of volcanic rocks will help refine volcanic outgassing models, with potential application to risk assessment and hazard mitigation efforts. However, beyond volcanology, this project sets the infrastructural, experimental and analytical foundation needed to more broadly study the relationships between rock-fluid interactions and crustal fluid flow, including in geothermal systems.