多孔質岩石の空隙構造と比抵抗の関係解明に向けたデジタル岩石物理の活用

抄録

Rock resistivity is essential for interpreting subsurface resistivity structures obtained from electromagnetic surveys. Using rock physics models, we can estimate water content from resistivity; however, the predicted results are strongly dependent on the choice of model with the assumption of the internal pore microstructure. In this study, we evaluated the relationship between resistivity and pore microstructure of clay-free sandstones based on Digital Rock Physics, with a particular focus on tortuosity. The simulation results demonstrated an increase in resistivity and its anisotropy 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 tortuosity, resulting in higher resistivity and anisotropy. The resistivity of high porosity data could be fitted by Archie’s equation with empirical parameters, whereas the resistivity of low porosity data could not. This suggests the difficulty of applying Archie’s equation to low porosity rocks. On the other hand, using the calculated tortuosity, the equivalent channel model reproduced the resistivity over a wide range of porosities. Our results suggest that tortuosity can be a key factor in demonstrating electrical properties.