  We investigate experimentally the impact of heterogeneity on the capillary
pressure hysteresis in fluid invasion of model porous media. We focus on
`symmetric' heterogeneity, where the contact angles the fluid interface makes
with the oil-wet ($\theta_1$) and the water-wet ($\theta_2$) beads add up to
$\pi$. While enhanced heterogeneity is usually known to increase hysteresis
phenomena, we find that hysteresis is greatly reduced when heterogeneities in
wettability are introduced. On the contrary, geometric heterogeneity (like
bi-disperse particle size) does not lead to such effect. We provide a
qualitative explanation of this surprising result, resting on rather general
geometric arguments.
