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[SEM14-02] Estimating electrical resistivity and tortuosity of sandstone using digital rock physics
Keywords:rock resistivity, tortuosity, digital rock physics, effective medium theory
Electrical resistivity is sensitive to the presence of fluids. It can therefore be a good indicator of the distribution of geofluids in the upper crust. Using rock physics models, we can estimate water content from resistivity data retrieved from geoelectromagnetic observations. Such rock physics models are numerously proposed by assuming different equivalent pore geometries. However, the predicted results are strongly dependent on the choice of model with some empirical parameters related to the assumed pore structure.
Digital Rock Physics (DRP) enables us to calculate effective (macroscopic) rock properties from three-dimensional images of rocks, usually obtained from microfocus X-ray computed tomography. Although previous studies have used DRP to estimate the electrical resistivity of rocks, no studies have focused on the effects of internal pore microstructures on resistivity.
In the present study, as a first step to investigate the relationship between resistivity and pore microstructure, we evaluate the resistivity and tortuosity based on the simulated local electric current for different types of digitized porous rocks. The local field of electric currents was simulated from the potential difference between the inlet and outlet boundaries via the finite element method. In this analysis, fluid and solid, respectively, were modeled with conductivities of 5 S/m and 10–5 S/m. Digital rock models were collected from different CT images having wide ranges of porosities (~7–26%).
The simulation results demonstrated an increase in resistivity with decreasing porosity. The tortuosity values calculated from the local electric current further explain the evolutions in resistivity. This suggests that the smaller pore volumes (i.e., porosities) prevent pore connectivity and enhance the tortuosity, producing higher resistivity. The equivalent channel model of using estimated tortuosity reproduces the resistivity with high accuracy, which further supports that tortuosity can be a key factor in demonstrating electrical properties.
Digital Rock Physics (DRP) enables us to calculate effective (macroscopic) rock properties from three-dimensional images of rocks, usually obtained from microfocus X-ray computed tomography. Although previous studies have used DRP to estimate the electrical resistivity of rocks, no studies have focused on the effects of internal pore microstructures on resistivity.
In the present study, as a first step to investigate the relationship between resistivity and pore microstructure, we evaluate the resistivity and tortuosity based on the simulated local electric current for different types of digitized porous rocks. The local field of electric currents was simulated from the potential difference between the inlet and outlet boundaries via the finite element method. In this analysis, fluid and solid, respectively, were modeled with conductivities of 5 S/m and 10–5 S/m. Digital rock models were collected from different CT images having wide ranges of porosities (~7–26%).
The simulation results demonstrated an increase in resistivity with decreasing porosity. The tortuosity values calculated from the local electric current further explain the evolutions in resistivity. This suggests that the smaller pore volumes (i.e., porosities) prevent pore connectivity and enhance the tortuosity, producing higher resistivity. The equivalent channel model of using estimated tortuosity reproduces the resistivity with high accuracy, which further supports that tortuosity can be a key factor in demonstrating electrical properties.