Interplay between Pore and Solid Tortuosities of Synthetic Rocks

Abstract:

To face the climate change, underground reservoirs are promising candidates to sequester greenhouse gas such as CO2 or balance the intermittency of renewable energy sources by storing H2. The estimation of the hydraulic properties of the host rock appears as pivotal, to predict the migration of injected fluids and associated multiphysical solicitations at the reservoir scale. Thanks to the advancement of imaging techniques, the estimation of hydraulic properties based on microscale simulations has become common practice. However, the correlation of hydraulic properties with geometric measures of the microstructure remains mostly elusive.

A widely used set of morphometers are the Minkowski functionals, which are able to predict permeability to some degree [1], but struggle on complex microstructures containing microporosity and surface roughness [2]. In particular, Minkowski functionals fail to capture transport-relevant topology and connectivity. Interestingly, these morphometers appear sufficient to predict the mechanical performances of common porous materials [3, 4].

In this work, we investigate the interplay of additional descriptors, namely pore tortuosity and solid tortuosity, and their impact on hydraulic and mechanical properties. Various studies have highlighted the significance of pore tortuosity for more precise permeability estimation [5], conventionally inferred from electrical resistivity measurements. Similarly, the connectivity of the solid matrix can be interpreted by the thermal conductivity of the dry material.

This interplay is first investigated through numerical analysis of digital synthetic rocks. To do so, various microstructures are generated controlling the porosity and exploring different generation algorithms. Subsequently, the pore and the solid tortuosities are determined with geometrical and physical analysis. In particular, a Fast Fourier Method is employed to estimate the thermal, hydraulic and mechanical properties of the synthetic microstructures [6].

This numerical approach is then complemented by an experimental campaign conducted on synthetic rocks. These samples are obtained by thermal sintering of glass beads, allowing a relative control on the microstructure. Once the samples are produced, electrical resistivity and thermal conductivity are measured with low-cost experiments to estimate the pore and solid tortuosities. In the same vein, cyclic uniaxial compression tests are conducted on the synthetic rocks to determine the mechanical behavior of the materials.

Bibliography

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