Magnetic resonance imaging of the development of fronts during spontaneous imbibition

Magnetic resonance imaging (MRI) was used to monitor the displacement of refined oil by spontaneous imbibition of brine into fully oil-saturated cylindrical cores with either one end open (OEO) or two ends open (TEO) to flow. The rocks were outcrop Whitestone Upper Zone (UZ) limestone, Berea and a Bentheim sandstone/Rordal chalk composite. In all cases, the wetting state was very strongly water-wet (VSWW). For OEO imbibition, high resolution imaging showed that brine invasion started at localized points and grew hemi-spherically outwards. The invaded regions increasingly overlapped until, after invasion of about one-third of the length of a standard core plug, a well-defined imbibition front formed that progessed as a remarkably sharp front at a velocity proportional to the square root of time. This behavior was usually observed for different boundary conditions and a range of permeabilities. For imbibition tests on a single core with TEO, imbibition started from the same core face no matter whether the core was horizontal, vertical or also vertical but inverted, showing that core heterogeneity dominated over the effect of gravity. The effect of heterogeneity was investigated further by imaging imbibition fronts in a TEO composite core formed from chalk butted against sandstone to provide a large difference in pore size. The fronts in the composite core moved at different times and velocities. Results are explained in terms of the capillary pressure gradients that develop within the composite core and, in particular, the capillary pressure at the open face(s). None of the behavior observed for a wide range of conditions can be modeled by the standard method which assumes relative permeabilities and capillary pressure relationships to give self similar fronts that expand as the front advances. The development of distinct fronts supports the modeling of imbibition, after the initial stages, as a piston-like displacement process.