Wettability effects on invasion stability
Fluid displacement in porous media is often unstable, in the form of thin, convoluted fingers that penetrate only a small portion of the pore space. Instability associated with is unfavourable viscosity ratio--when the invading fluid is less viscous than the one displaced, e.g. gas displacing liquid--has been thoroughly investigated since the seminal Saffman-Taylor experiment (1958)
Yet, the fundamental effect of wettability (the relative affinity of the fluids to the solid), that is the remarkable difference between imbibition and drainage, exposed in the classic experiments of Stokes and coworkers (1986)
, remain largely unexplained.
Stokes et al. showed that if the invading, less viscous fluid is non-wetting (drainage), the displacement occurs through preferential flow paths leaving much of the original fluid behind, whereas if the invading fluid is more wetting (imbibition), the front becomes more compact with a more efficient displacement. We explain these classical yet intriguing observations by computer simulations, showing that wettability changes the microscopic mechanisms of pore invasion, which in turn changes the global pattern, an effect which gets weaker as the flow rate increases.
|High Ca, strong drainage
||High Ca, weak imbibition ||Low Ca, strong drainage
||Low Ca, weak imbibition
|Viscous fingering: destabilization of the entire interface, where high defending fluid pressure in the “gulfs” between fingers allows only the finger tips to advance (screening)
||Capillary fingering driven by disorder. Quasi-static (IP-like) advancement by invasion into the largest accesible pore along the interface
||Compact displacement resulting from dominance of non-local, cooperative pore-filling events ("overlaps")
For further information, see:
Phys. Rev. Lett. 2015 for a presentation of our novel pore-scale model, explaining the transition between the different displacement regimes.
Nature Sci. Rep. 2016 showing how wettability effects interacts with those of rate and pore size heterogeneity.
PNAS 2019 for a comprehensive comparison of pore-scale models for multiphase flow in porous media.