  Immiscible fluid displacement in porous media is fundamental for many
environmental processes, including infiltration of water in soils, groundwater
remediation, enhanced recovery of hydrocarbons and carbon geosequestration.
Microstructural heterogeneity, in particular of particle sizes, can
significantly impact immiscible displacement. For instance, it may lead to
unstable flow and preferential displacement patterns. We present a systematic,
quantitative pore-scale study of the impact of spatial correlations in particle
sizes on the drainage of a partially-wetting fluid. We perform pore-network
simulations with varying flow rates and different degrees of spatial
correlation, complemented with microfluidic experiments. Simulated and
experimental displacement patterns show that spatial correlation leads to more
preferential invasion, with reduced trapping of the defending fluid, especially
at low flow rates. Numerically, we find that increasing the correlation length
reduces the fluid-fluid interfacial area and the trapping of the defending
fluid, and increases the invasion pattern asymmetry and selectivity. Our
experiments, conducted for low capillary numbers, support these findings. Our
results delineate the significant effect of spatial correlations on fluid
displacement in porous media, of relevance to a wide range of natural and
engineered processes.

Fluid Dynamics;Soft Condensed Matter