In situ saturation development during spontaneous imbibition

The in situ saturation development for reservoir core plugs was monitored during spontaneous imbibition using magnetic resonance imaging (MRI). The images monitored the movement of the imbibition front and the saturation change across the front as a function of initial water saturation. At zero initial water saturation, the water-front exhibited a very steep saturation gradient and after the front passed through a given volume no additional oil production was detected from that volume. As initial water saturation was increased the front moved more rapidly, showed a reduced saturation gradient and significant oil was produced after the front passed through a given volume. At 33% pore volume initial water saturation, in a highly water-wet plug, and at 15% pore volume initial water, in a less water-wet plug, no water front was observed; the oil saturation uniformly decreased everywhere in the core during imbibition. These results suggest two concurrent mechanisms of water influx: (1) plug flow water displacement of oil and (2) a long-range process, possibly a thickening of pre-existing capillary water films on the surface. For initial water saturation between 0% and 22%, the resistance to the influx of water initially limited the rate of oil production and formed the water front. When the plug reached a uniform water saturation, or at high initial water saturation (33%), the resistance to water transport decreased so that the transport of the oil out of the plug became the rate-limiting component.