Magnetic resonance imaging of the development of fronts during spontaneous imbibition

► We image spontaneous imbibition in oil saturated cores at different conditions. ► Brine invasion started at localized points and grew hemi-spherically outwards. ► A defined front formed after invasion of a third of the length of a standard core. ► Core heterogeneity dominated over the effect of gravity where the front is developed. ► Front velocity is explained by capillary pressure gradient within heterogeneous cores.