Nondilute effects reshape miscible displacement in porous media

Miscible displacement of nondilute fluid pairs in subsurface porous media plays a key role in CO2 subsurface sequestration and many other applications. We conduct microfluidic experiments in a near-fracture porous geometry so that nondilute effects are isolated from other disruptive behaviors. We show that although Darcy-Fickian coupling works well for dilute fluid systems, it completely fails to predict nondilute fluid-fluid displacement. Compared to that predicted by Darcy-Fickian models, nondilute fluid-fluid displacement demonstrates much faster and "diffusive" mixing, an unexpected wedgelike mixing front, and abnormal convection perpendicular to the major concentration. Simply correcting Fick's law cannot reproduce these phenomena. Maxwell-Stefan drifting and Korteweg stress may rationalize experimental observations, but major challenges emerge in micro-scopic modeling and upscaling. We provide this work as a benchmark and call for future study to incorporate appropriate nondilute effect terms at different scales into the modeling of CO2 sequestration and other relevant applications.