Pore‐Scale Investigation of the Electrical Resistivity of Saturated Porous Media: Flow Patterns and Porosity Efficiency.

The electrical resistivity‐porosity relationship of porous media is critical for reliable formation evaluation. Archie's equation is empirical, and uses only porosity as pore‐space property, neglecting the spatial variability of the pore space; its fitting parameters may have unclear physical meaning and be nonconstant over a wider porosity range or in spatially complex rocks. We use microfluidics augmented with pore‐network modeling to investigate the effects of pore‐space properties and their evolution processes on electrical resistivity‐porosity relationships. Both a flow‐pattern parameter and a measure of porosity efficiency are implemented to quantify the spatial variability of the flow field and the conduction efficiency of porous media. Results indicate that both pore‐size distribution and pore‐space evolution impact the electrical behavior considerably. In cases of unimodal pore‐size distributions, a larger pore‐size variation or a higher porosity reduction in small pores results in higher values of Archie's porosity exponent, m; the flow pattern becomes more heterogeneous, and the efficiency of porosity to conduct electricity decreases. For cases of bimodal pore‐size distributions, the formation factor‐porosity relationship is nonlinear in log–log plots; the flow behavior is primarily affected by the fraction and connectivity of large pores. Results suggest that using porosity alone as pore‐space characteristic is inadequate to describe the electrical behavior of complex porous media. Petrophysical classification based on flow patterns and porosity efficiency is an effective alternative to differentiate the results. We introduce the electrical quality index as an effective parameter for petrophysical classification, which is verified with core data for both Fontainebleau sandstones and carbonates. Plain Language Summary: The electrical current conducts mainly through the pore fluid in a saturated porous medium. The formation factor—ratio of the electrical resistivity of saturated porous media to the electrical resistivity of the pore fluid—contains information of the pore space in subsurface rocks valuable for petroleum engineers and earth scientists. The widely used Archie's equation relates porosity and formation factor. However, Archie's equation is empirical, uses only porosity to represent a complex pore space, and ignores how the electrical current travels in the pore space. We study effects of pore‐space properties on electrical resistivity in porous media using well‐designed pore space characterized by statistical pore‐size distributions. The electrical resistivity of the pore space is measured experimentally in micromodels—a pore space fabricated on glass chips, and simulated in pore networks—a virtual lattice of pores. Results show that the electrical flow field is heterogeneous even at pore scale, and both pore‐size distribution and diagenetic processes impact the electrical behavior considerably. We propose parameters to quantify the spatial variability of the flow field and the conduction efficiency of the pore space. These parameters provide additional information of the quality of the pore space in conducting flow and can be used in petrophysical rock classifications. Key Points: Effects of pore geometry and its evolution on electrical resistivity is investigated by microfluidic experiments and pore‐network modelingThe use of porosity alone as pore‐space property in Archie's equation is inadequate to describe the electrical behavior in porous mediaParameters are introduced to quantify the flow heterogeneity and conduction efficiency in the pore space and for electrical rock typing [ABSTRACT FROM AUTHOR]