2019 | Dual porosity system (pore and fracture) permeability in volcanic tuffs by computed tomography

Active caldera systems are sources of both volcanic and seismic hazard, and also potentially significant geothermal resources. The aims of this proposed research are: (i) To constrain permeability of fracture systems within actively-deforming parts of calderas, which will impact how potential geothermal fluids will circulate. (ii) To map this permeability onto a natural system, based on active seismicity catalogues.

We focus on the active Campi Flegrei Caldera, around Napoli, Italy (De Siena et al., 2017). Surface samples of the dominant rock type, the Campi Flegrei tuff, have already been experimentally deformed in a triaxial rock deformation apparatus at Portsmouth University, UK, at a range of confining pressures, fluid pressures, and strain rates. The samples failed brittlely and display a range of Mode I (pure dilational) and Mode II (shear) fractures. Formation of such fractures should, respectively, generate isotropic and double-couple seismic events so we should be able to determine which sort of fractures are likely to be seen at depth beneath Campi Flegrei – and where – from active seismicity catalogues. We expect substantial microscopic fractures – invisible in hand specimen – were also generated. Additionally, the samples will have primary (pre-deformation) porosity. The porosity induced is a function of fluid-rock interaction, which is particularly likely at grain/microscopic scale because the volumetric changes and thus induced stresses that would accompany reaction of these poorly-crystalline (thus highly reactive) glasses may be significant.

To constrain the porosity and permeability of these systems, we will acquire helical microCT scans of the deformed samples with an effective voxel size of 25-30 micrometers, as well as of undeformed tuff. Avizo software will be employed to process (filter, crop) the microCT scans, then segment the porosity and separate it into fracture and pore spaces. Fig. 1 demonstrates this will not be a simple task because the fractures are complex, and the host rock comprises an agglomeration of vesicular pumice fragments. Through visual examination of these high-resolution CT scans, criteria will be established to differentiate primary porosity from porosity induced during the rock deformation experiments. GeoDict software will then be employed to calculate permeability of the segmented porosity systems. We will also examine the relationship between microscale fractures and host mineralogy (and its potential variation due to reaction) by statistical analysis of the CT value (a proxy for mineralogy) of fractured materials.

De Siena, L., Chiodini, G., Vilardo, G., et al. (2017). Source and dynamics of a volcanic caldera unrest: Campi Flegrei, 1983–84. Scientific Reports, 7(1), 8099. Tajcmanova, L., Vrijmeod, H., Moulas, E. 2015. Grain-scale pressure variations in metamorphic rocks: implications for the interpretation of petrographic observations. Lithos 216-217, 338-351.