Pore‐scale Study of Ion Transport Mechanisms in Inhomogeneously Charged Nanoporous Rocks: Impacts of Interface Properties on Macroscopic Transport

The electrokinetic transport mechanisms of multispecies ions through 3-D nanoporous rocks with chemical reaction at the solid-aqueous solution interfaces are investigated. We systematically study the multiphysics transport phenomena by considering either inhomogeneous (local surface charge based on local pH and ion concentrations) or prescribed homogeneous surface charge at solid-aqueous solution interface while the pores are screened via electric double layers. We develop a lattice Boltzmann numerical framework to solve the set of governing equations (Poisson-Nernst-Planck plus Navier-Stokes). Our modeling results reveal that the averaged local electric potential of the nanoporous rock is significantly underestimated (about 83%) when a homogeneous surface charge is prescribed based on the bulk solution properties. It is shown that increasing the porosity of the nanoporous media considerably increases the absolute values and inhomogeneity of the surface charge, which means that while the electric double layers screened the pores, increasing the porosity enhances the ion selectivity of the porous medium. When the scenario with inhomogeneous charge is taken into account, the predicted electroosmotic permeability and tortuosity are higher in comparison with the prescribed homogeneous case. Moreover, we have studied the electrostatic tortuosity, coupling coefficient, and the effective excess charge density of the nanoporous rocks. The results demonstrate that ignoring the inhomogeneity of surface charges may cause erroneous prediction of the ion transport through porous rocks with chemically active surfaces. Plain Language Summary How can we study the subsurface geological properties? How can we understand the contaminant deposition rate at underground? How can we provide a better understanding of the diffusion and dispersion phenomena in tight rocks? To answer these questions, one needs to consider the transport of multispecies ions through the complex geometries like unstructured porous media. However, these transport mechanisms are highly coupled multiphysicochemical phenomena, which are influenced by the local properties of the solid-aqueous solution interface such as locally acquired surface charge. In the present contribution, we scale down our vision into the pore-scale to understand what is happening inside these complex mazes when we apply external body forces (i.e., applied an electric field and pressure gradient). The modeling results revealed several substantial facts that a great underestimation for the local electric potential of the porous rocks will happen when someone ignores the locally acquired surface charge due to local solution properties. In addition, it has been shown that the movement of the ions and water flow should be more tortuous when we consider the electric charge inhomogeneity compared with a prescribed homogenous surface charge. Measuring the electrical and streaming conductivity of the nanoporous rocks, which play a key role in studying the underground geological properties, demonstrates higher amounts for inhomogeneously surface charge.