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Fluid flow on different scales is of interest for several Earth science disciplines like petrophysics, hydrogeology and volcanology. To parameterize fluid flow in large-scale numerical simulations (e.g. groundwater and volcanic systems), flow properties on the microscale need to be considered. For this purpose experimental and numerical investigations of flow through porous media over a wide range of porosities are necessary. In the present study we sinter glass bead media with various porosities and measure the permeability experimentally. The microstructure, namely effective porosity and effective specific surface, is investigated using image processing. We determine flow properties like tortuosity and permeability using numerical simulations. We test different parameterizations for isotropic low-porosity media on their potential to predict permeability by comparing their estimations to computed and experimentally measured values. [ABSTRACT FROM AUTHOR]

The flow of fluids through porous media such as groundwater flow or magma migration is a key process in geological sciences. Flow is controlled by the permeability of the rock; thus, an accurate determination and prediction of its value is of crucial importance. For this reason, permeability has been measured across different scales. As laboratory measurements exhibit a range of limitations, the numerical prediction of permeability at conditions where laboratory experiments struggle has become an important method to complement laboratory approaches. At high resolutions, this prediction becomes computationally very expensive, which makes it crucial to develop methods that maximize accuracy. In recent years, the flow of non-Newtonian fluids through porous media has gained additional importance due to, e.g., the use of nanofluids for enhanced oil recovery. Numerical methods to predict fluid flow in these cases are therefore required. Here, we employ the open-source finite difference solver LaMEM (Lithosphere and Mantle Evolution Model) to numerically predict the permeability of porous media at low Reynolds numbers for both Newtonian and non-Newtonian fluids. We employ a stencil rescaling method to better describe the solid–fluid interface. The accuracy of the code is verified by comparing numerical solutions to analytical ones for a set of simplified model setups. Results show that stencil rescaling significantly increases the accuracy at no additional computational cost. Finally, we use our modeling framework to predict the permeability of a Fontainebleau sandstone and demonstrate numerical convergence. Results show very good agreement with experimental estimates as well as with previous studies. We also demonstrate the ability of the code to simulate the flow of power-law fluids through porous media. As in the Newtonian case, results show good agreement with analytical solutions. [ABSTRACT FROM AUTHOR]

Rocks in the Earth are not homogeneous, but consist of different mineralogical phases with different rheological properties. When deformed, these heterogeneous rocks therefore also exhibit heterogeneous deformation, which depends on the rheological contrast between the different phases and their distribution within the rock. The effective properties of such heterogeneous mixtures have received a significant amount of attention in the past [Treagus, 2002; Madi et al., 2005; Jessell et al., 2009; Dabrowski and Schmid, 2012; Kaercher et al., 2016], but it has not yet been possible to link random weak phase topologies to the effective rock rheology.Here we use a numerical approach to gain insight into the relationship between phase distribution topology and the effective rheology/ deformation of two-phase rocks. To this end, we prescribe the distribution of weak phases is using random fields and deform the resulting structures in simple shear. The usage of random fields allows us to prescribe a certain topology of the weak phase and to investigate its effect on bulk properties. Adding a weak phase has several effects: First, the internal strain rate, stress and pressure fields become strongly heterogeneous, thus at times resulting in unexpected behavior and localization of deformation. Second, the bulk rock is weakened. The amount of weakening strongly depends on the topology of the weak phase as well as on its rheology. We performed a large number of simulations for different viscosity contrasts, volume fractions and weak phase topologies to obtain the desired amount of data needed for statistical analysis of bulk rock deformation properties. We then link the statistical properties of the weak phase distribution to the effective rheology using an analytical solution.Dabrowski, M., and D. W. Schmid (2012), A two-phase composite in simple shear: Effective mechanical anisotropy development and localization potential, Journal of Geophysical Research, 117, B08406, doi:10.1029/2012JB009183.Jessell, M. W., P. D. Bons, A. Griera, L. A. Evans, and C. J. L. Wilson (2009), A tale of two viscosities, Journal of Structural Geology, 31(7), 719–736, doi:10.1016/j.jsg.2009.04.010.Kaercher, P., L. Miyagi, W. Kanitpanyacharoen, E. Zepeda-Alarcon, Y. Wang, D. Parkinson, R. A. Lebensohn, F. De Carlo, and H.-R. Wenk (2016), Two-phase deformation of lower mantle mineral analogs, Earth Plan. Sc. Lett., 456, 134–145, doi:10.1016/j.epsl.2016.09.030.Madi, K., S. Forest, P. Cordier, and M. Boussuge (2005), Numerical study of creep in two-phase aggregates with a large rheology contrast: implications for the lower mantle, Earth Planetary Science Letters, 237(1-2), 223–238, doi:10.1016/j.epsl.2005.06.027.Treagus, S. H. (2002), Modelling the bulk viscosity of two-phase mixtures in terms of clast shape, Journal ofStructural Geology, 24(1), 57–76, doi:10.1016/S0191-8141(01)00049-9. [ABSTRACT FROM AUTHOR]

Abstract EGU 2019Grain size reduction as a weakening mechanism in 3D numerical models:Is it sufficient to initiate and stabilise transform faults?Transform faults are conservative plate boundaries since they do not produce or consume any crust. Instead, they accommodate motion via strike-slip movement. In nature, these highly localised, ductile shear zones show a great amount of grain size reduction up to ultra-mylonites. Various studies concluded that grain size reduction can lead to a progressive localisation of strain (Jaroslow et al., 1996, Jin et al., 1998, Warren and Hirth, 2006).In numerical models, transform faults are difficult to reproduce, as mantle convection simulations tend to produce poloidal rather than toroidal motion. In order to obtain toroidal motion in numerical models a strain weakening mechanism is necessary. So far, in transform fault modelling ad hoc strain weakening has been applied to reproduce the observed weakening phenomena (e.g. Hieronymus, 2004, Choi et al., 2008, Gerya, 2010a), but without including a physics-based weakening mechanism. However, also numerical studies suggest grain size reduction to be a potential candidate for weakening (e.g. Kameyama et al., 1997). In order to utilise a more realistic weakening we incorporate grain size reduction due to dynamic recrystallisation in the ductile regime. We investigate the effect of grain size reduction occurring at transform faults using a 3D-numerical model with a composite rheology (diffusion and dislocation creep). Viscosity is therefore pressure-, temperature- and grain size-dependent. Instead of imposing ad hoc strain weakening, we let the model evolve self-consistently and allow for a gradually developing strain weakening and strain localisation due to grain size reduction.Our results show that weakening purely by grain size reduction can localise strain strong sufficiently to initiate a transform fault. However the stability and longevity of the transform faults highly depends on the amount of grain damage, the thickness of the brittle crust and to a smaller extent on the velocity of the spreading centres. Our most stable transform faults arise when the crust is thick and when we use a temperature-dependent grain damage parameter. Yet, so far all transform faults are only stable for a maximum of around 1.5 Myr. [ABSTRACT FROM AUTHOR]