Hydro-Thermodynamically Driven Fluid Mixing Across Phases in Porous Media

Fluid mixing and its interplay with hydrodynamic instabilities, particularly viscous fingering, as well as flow channeling through heterogeneous media, have been traditionally studied for fully (im)miscible conditions in which a (two-) single-phase system is generally represented by two components, e.g. a solvent and a solute, with (zero) infinite mutual solubility. However, many problems of interest at subsurface conditions, e.g. gas injection/migration in hydrocarbon reservoirs, involve multiple species transfer. While in previous studies fluid properties are assumed to be either constant or to follow simple mixing rules depending only on concentration, multicomponent fluid properties behave non-linearly, through an equation of state, as a function of temperature, pressure, and fluid compositions. Moreover, depending on the minimum miscibility pressure, a two-phase region with finite, non-zero mutual solubility may develop, e.g. in a partially-miscible system. Fingering and mixing mechanisms in this regime have gained less attention in previous studies and have never been compared to those in single-phase problems. Here we study mixing of fluids with partial mutual solubility, which is induced by viscous flow fingering, channeling, and species transport within and between multicomponent phases. In particular, we uncover the non-linear mixing dynamics of 1) a finite-size rectangular slug of a less viscous fluid attenuated by a carrier fluid during rectilinear displacement, and 2) a semi-infinite volume of injected fluid diluting in a medium that is saturated with a more viscous ambient fluid. The carrier/ambient fluid is a multicomponent hydrocarbon mixture which can be fully or partially miscible with a less viscous fluid such as carbon dioxide. We perform accurate, high resolution numerical simulations that are thermodynamically-consistent in order to capture small-scale fingering patterns as well as the complex phase behavior exhibited by mixtures. The results provide a broad perspective into how multiphase multicomponent flow can alter the characteristics of fluid mixing in porous media.